Firing Tips
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Bisque firing schedule-electronic kilns
This is a typical firing schedule for a bisque firing in a electronic kiln.
Segment 1: 80 degrees / hour to 250 degrees F
Segment 2: 250 degrees / hour to 1000 degrees F
Segment 3: 150 degrees / hour to 1300 degrees F
Segment 4: 180 degrees / hour to 1685 degrees F
Segment 5: 80 degrees / hour to 1928 degrees F.
Technique
Pit Firing
Early cultures found clay in the ground and must have discovered its plastic and fired qualities by accident, probably discovering some burnt clay in a camp fire. This very basic firing then evolved into the pit-firing. Pit-firing involves placing unfired or bisque fired pottery in a pit in the ground, then covering the pottery with suitable burning materials, e.g. dried grasses and branches. Depending on the amount of work to be fired, a pit of the appropriate size is dug. A bed of dry leaves and twigs and possibly coal, which will burn slowly, is placed at the bottom of the pit and the pottery placed on top of this. The work is then covered with more leaves and twigs and dung, if available, building up a mound over the pieces. Once the stacking process is finished, the pile can be lit around the edges and left to smolder for several hours, if not until the next day. Towards the end of the burning process, it is possible to bury the pit in earth or sand, which will cut off the oxygen supply and create a strong reducing atmosphere inside the mound. Not all clays are suitable for such a firing, especially the more refined types available from suppliers. Additions of grog 'open up' the clay and make it more resistant to heat shock. Clays dug directly from the earth may be suitable 'as is', or might profit from additions of grog or volcanic ash, which also resists severe temperature differences. If using a commercial clay, get a clay suitable for raku firings. The best color results can be achieved with iron bearing, or red clays.
Oxidized firing
In an oxidation firing, ceramic wares are heated to a certain temperature. The heated ceramics draw oxygen from the kiln chamber and from the flue burner ports, spy holes and any other holes or cracks in the kiln. The oxygen combines with carbonaceous materials in the glazes and ceramic body, turning these into ash. Metal pigments in glazes, such as iron, will oxidize, giving the glaze a particular color. Oxidation firings can be done in most kilns, except those which are not designed for it, e.g. a black firing kiln, where sugar is introduced into the kiln which volatilizes and creates carbon, which is trapped in the clay body, creating a black surface. To do an oxidation firing, leave the burner ports and any damper open, so enough air exchange can happen in the kiln. In electric kilns, oxidation is the norm.
Use sand to stop glaze running onto shelves
How to fire mid range stoneware (cone 6). kiln wash flakes and sometimes mars a piece in glaze firing. Therefore, use a layer of sand (the same kind that is sold in plastic bags at KMart for kids' sandboxes. Spread the layer very carefully on the shelf after it has been positioned, approximately 1/8 of an inch thick. If you suspect that the glaze will run then use a heavier layer. Since sand will not melt until cone 28 there is no problem. The benefits are that the sand will absorb any glaze which may run off the pot and therefore the piece can be removed from the shelf without damage to the shelf or the pot. Occasionally some sand adheres to the vessel this is easily hand ground with a stone or machine ground with my Makita grinder. It is important to remember to put the sand on the shelves after they are in the kiln and keep it 1/2 " away from the edge of the shelf in order to avoid contamination. I have not had any problems with sand particles getting in my glazed pieces and I do have a venting system. The secondary benefit is that pieces with large flat bottoms never warp! I also use the same technique for bisque firing all platters.
Substitute for hot waxing
A good substitute for hot waxing or wax resist is Mop-n-Glo (floor wax). Not only is it cheaper, but it will prolong the life of your elements in an electric kiln. The fumes are also not as toxic and you don't get the smoke from burn off! (L.M.)
Shivering
Shivering: Sometimes a piece of glaze will crack off, normally near a rim or at edges. Some clay may be attached to the glaze piece that cracks off. This occurs because stress has built up between the clay and glaze that can't be absorbed. It is often caused by over-sponging which takes away the fine clay particles and leaves behind the groggier clay particles which are not elastic enough to absorb the stress.
To get like firings every time
To get like firings every time, you must place your cone in the automatic kiln sitter in exactly the same way each time you fire your kiln
How to do a Pit-Firing
How to do a Pit-Firing
This is the sort of thing you can easily do in your own backyard (subject to local council regulations of course). It's a simple firing technique used in many ancient cultures across the globe and popular with potters today.
Here's How:
Dig a pit of the appropriate size, depending on the amount of work to be fired.
Place a bed of dry leaves and twigs and possibly coal, which will burn slowly, at the bottom of the pit
Place the pottery on top of this.
Carefully sprinkle oxides and carbonates around the pieces (particularly copper carbonate), which volatilize and result in flashes of color appearing on the fired work.
Cover the work with more leaves, twigs and dung (if available), building up a mound over the pieces.
Once the stacking process is finished, light the pile around the edges and leave to smolder for several hours, or until the next day.
Towards the end of the burning process, bury the pit in earth or sand, which will cut off the oxygen supply and create a strong reducing atmosphere inside the mound.
Allow the kiln to cool overnight and open the next day.
Remove excess scum with a wire brush under a running tap.
Tips:
Additions of grog or volcanic ash 'open up' the clay and make it more resistant to heat shock.
The best color results can be achieved with iron bearing, or red clays.
Bisque firing the work first helps to prevent shattering and cracking.
Popular Cone Values
Popular Cone Values for Most Clays
For Bisquing Earthenwares Cone 04
For Glazing Earthenwares Cone 05
For Bisquing Stonewares and Porcelains Cone 06 to Cone 04
For Glazing Stonewares and Porcelains Cone 4-10 -See Clay Body Description For Specific Cone Range.
For Bisque
Make sure your pieces are dry before loading them into the kiln. Pieces that are still wet will feel cool when placed on your cheek. Wet ware can blow up in the kiln, as the water inside the clay expands.
Large flatware
Items such as large plates benefit by being in the center of the shelf for even heating. This helps reduce warpage.
Keeping a firing log
It is very useful to keep a firing log. Start your log entry by describing how the kiln is loaded (types and sizes of items, density of packing.) You will find that the more densely the kiln is loaded, the longer it will take the kiln to reach the appropriate temperature. You will learn how to adjust for this with your particular kiln. Typically you will add a longer soak at the top temperature when the kiln is more densely packed.
Protecting the base
You can protect the bottom of your kiln by placing a full shelf there and coating it with kiln wash.
Bisque firing
Bisque firing
For a bisque fire in particular, you need to drive off the water that is left in the pot. If you fire too fast, the steam will cause the piece to explode. (This is true even if the piece is very dry, because there is still moisture inside the clay molecules.) So it is important to fire bisque slowly.
If your pieces are not completely dry, you may want to candle them first. Candling is done on a manual kiln by turning the bottom switch on low and holding it there for several hours (6-10). With an electronic kiln, you would program the kiln to remain at around 150 degrees F for this time.
Glaze positioning
Leave at least 1/2" between all pieces when loading your kiln. Remember that the piece will expand during the firing cycle before it contracts and shrinks.
Expansion and contraction
Expansion and contraction of the kiln during firing will eventually cause the case of the kiln to loosen. To prevent this, the clamps on the jacket, lid and bottom of the kiln should be tightened occasionally when the kiln is warm.
firing jewelry
A bisque tile can serve the same purpose as a shelf when firing jewelry or other light pieces
OH&S/Be prepared
Every object in the raku area should be treated as if were capable of causing severe burns instantly on contact with your skin. This includes bricks, kiln shelves, tongs, pots, peep-hole plugs, reduction containers (garbage cans), water buckets, and all parts of the kiln. Before you touch anything, confirm that it's not hot. Anything that is hot (especially bricks or kiln shelves removed from a hot kiln) should be kept out of the way of passers-by and preferably cooled by a squirt from the hose or a dunk in a bucket of water.
Loading Electric
Since all heat rises, the bottom of a kiln is the hardest part to heat. Because of this, as much of the bottom of the kiln should be exposed to radiation from the sidewalls as possible. Two of the element grooves should be between the bottom of the kiln and the first shelf. The first layer in the kiln should be small, light pieces, with ample room for air circulation around their bases. Placing small, low pieces on the bottom of the kiln uses your furniture more efficiently as tall pieces may be placed on the top shelf and other posts and shelves do not have to go over the tall pieces. The most important item that goes into your kiln is the cone. It merits careful placement and sufficient space in planning your load.
Firing time is important
A slow firing is always preferable to a fast firing.
The heavier a kiln is loaded the longer the firing cycle.
Dunting:Thermal Shock
Dunting can occur months or even years after firing. For example, a pot might split right in half after 3 months. This is likely the result of thermal shock. In this case the clay and glaze expand at different rates when exposed to temperature variation, and this change causes the object to crack. To be more specific, the body has contracts more than the glaze. If the glaze is weaker it will shiver (see above). If the clay is weaker the object will crack.
History
Black Firing Amongst the 'primitive' firing techniques you will find the technique of Black Firing. This method involves heating a primitive gas-brick kiln to about 1000oC (these days with gas) and then adding copious amounts of sugar, which then volatilize and impregnate the clay with carbon, giving it a matt black surface. Glazes may also be used with this method, which can result in some interesting effects. The required temperature is usually reached in about five hours. The gas is then shut off and flue and any cracks sealed. Sugar is pushed into the burner port and volatilizes. Then the burner port is quickly sealed with bricks and fire clay. The kiln cools slowly and can be opened the next day.
Applying kiln wash
Make sure you have a good layer of kiln wash on the shelf. This is to facilitate the removal of glaze drips. Do not apply kiln wash to the sides of the kiln, or underside of the kiln shelves. The wash is likely to flake off and land on your glazed pieces.
General Raku firing
Learning to fire raku well is an exercise in patience, control, timing, and, observation. The results depend not only on the course of the firing in the kiln, but especially on the timing of the smoking as the piece cools. Some of the important factors are the temperature of the pot as it comes out of the kiln, how long it takes to get the piece in to smoke, the weight of the piece, the size of the container and the combustible material used for smoking. There are many special effects glazes each of which can vary quite a bit depending on the firing. The three main types of raku glazes commonly used are crackle glazes, lusters, and copper matts.
refire a glazed piece
If you plan to refire a glazed piece and need to stilt it, coat the bottom with a light coat of clear glaze. This will give the bottom a texture and keep it from slipping of stilts.
Opening the kiln
Do not open a kiln in the middle of a firing for any reason. let it cool gradually and you will not ruin any ware.
Drilling holes
To drill a hole in a fired ceramic item, it is important to use a diamond head drill bit. Stuff the item with material, this reduces the shock to the ware, drill slowly and firmly, as any hammering of the drill could crack the piece.
Approx cone temps for bodies
Cone 022 10850F Lusters
018 1238 "
06 1840 Bisque, low fire glaze, raku
01 2030 Earthenware
1 2057 "
5 2156 Stoneware
10 2300 Stoneware, porcelain
12 2390 Porcelain
The numbers go higher and lower, but this is the range most commonly used.
How to
The usual method of creating a reducing atmosphere is to close off the flue or chimney. This can be done by laying a piece of kiln shelf or kiln brick across the top of the flue/chimney, or if it is a larger chimney, by pushing in the damper (a piece of tile which can be pushed into a slot in the chimney, which is at an accessible height, reducing the air flow). Reduction firings are generally not recommended for electric kilns, although it is possible to achieve reducing effects by introducing combustible materials (e.g. twigs) into the kiln chamber through the spy hole. But the carbon monoxide tends to attack the kiln elements (electrical coils), which then deteriorate quickly.
Not reaching temperature
If you are having trouble getting your kiln to reach temperature, or the firing is taking extra long, the first thing to check is the power. If your kiln is too far from the breaker box, you may be getting voltage drops. Or if it is a hot, summer day when everyone is running their air conditioning, the voltage on your line is probably low. Speak to your electricty supplier about the best times to fire for optimum efficiency.
Over glazes on porcelain
You can fire overglazes directly on porcelain bisque but you may have to use a hotter cone. These overglazes can be applied directly and fired in the 017-015 range.
firing greenware
When firing greenware, it can touch other greenware as well as the kiln shelves. Kiln washed shelves are not necessary when firing only greenware.
Description
In a reduction firing, the oxygen supply to the kiln chamber is restricted, resulting in a saturation of free carbons in the kiln atmosphere, mostly in the form of carbon dioxide and carbon monoxide. In a strong reduction firing, flames will lick out of the flue and through other openings or cracks in the kiln walls. This is because the flame needs oxygen to burn, so it will try to get it from wherever it can. The reduction cycle generally begins between 1600o-1900o F (900o - 1000o C). Typical reduction glazes are 'copper reds' and 'shinos'.
Firing reds
The red family of glazes is easily contaminated and requires more venting and a faster firing schedule.
Glaze and body fit
Glaze and body fit?
Since the glaze and the body on which it is fired (bisque) are made from different materials, it is important that they expand and shrink a like amount when heated and cooled. If they don't, then the fired glaze can be stretched to the point where it can crack (crazing), or it can be pushed together on to itself to a point where shivering or crawling occurs.
Loading the Electric kiln
It is important to remember that one element groove must be between every shelf with the exception of half shelves.
Stacking for bisque
Items may be stacked on and inside one another for bisque firings. They won't stick together. However, you can cause problems by doing this.
* The carbons may not burn out completely from an area that is covered by another piece, and this may cause defects during the glaze firing.
* Some items may break if they are not allowed to expand and contract freely. So if stacking two bowls for example, make sure there is plenty of room between them. Remember that items will shrink during firing.
* Stacking may cause more uneven temperatures throughout the kiln.
* Some people think the tighter they stuff the bisque load, the better. And you may have success with this method. But other people find that they do better when pieces are given space. My advice is that you can pack a bisque load tighter than a glaze load, but don't overdo it.
Recognising Different cracks
If you have a crack, find the point where it is largest (widest). This will be the point where the crack started, and can help you understand what happened. Cracks in the rim usually were caused by stresses in the raw stage. Cracks in the base usually occur in the firing. Another way to determine the cause of a crack is to look at the surrounding glaze. If the glaze at the end of the crack is sharp, it cracked in the later stages of firing, probably during cooling. If the glaze is round at the edge of the crack, the crack probably occurred early in firing and the glaze had time to heal over. Remember that most cracks occur from stresses that occur during drying, even if they don't show up until later. But some cracks occur because of firing and cooling, so it is usually best to fire and cool slowly.
Approx. firing times
Bisque or low fire glaze firings (04-06) usually require 5-7 hrs firing time. Porcelain and stoneware high firings take from 6-10 hrs.
Firing Temperatures
Firing Temperature.
The speed at which the temperature climbs during a firing is critical, especially during the first 600c of a biscuit firing, as it is during this period that most of the physical and chemical changes occur. Once 600c has been reached, the clay has changed to a permanent ceramic material. Continuation of the firing to a higher temperature increases both the strength and durability of the ware. Gloss firings do not usually require the initial slow firing rate needed for biscuit, but they offer an opportunity to vary the glaze through using a range of atmospheres and techniques, such as raku, oxidation and reduction.
Firing raku lustres
Lustres need a very heavy reduction while the glaze is still hot, so it's best to move them as quickly as possible from the kiln to the smoking container. A small container holds less oxygen, so using the smallest container possible will help to obtain a heavy reduction. Because the piece comes in contact with the combustibles while the glaze is still soft, lustres usually show some scarring in the glaze surface.
Raku Process
Raku Pottery is earth derived...the firing process is unique and daring, and in the eyes of the Zen Masters, the process truly reflects the most fundamental rhythm of enlightened life.
Read this entire article Raku Process
Seal bisque
Did you know that milk will help to seal bisque? If you have a vase that has been properly glazed but still leaks a bit when water stands in it a long time, let some milk stand in it for a couple of days it will seal the pores and prevent leaking.
Loading: Around the Grooves
Be sure that at least one element groove is between the top shelf and the top of the kiln.
The first layer in the kiln should be small, light pieces.
Firing lustres
If you should happen to get broken, wiggly lines on your piece after a gold firing you may be over-firing.
Firing down for high fire glazes
Mid to High fire glazes often look better if they are cooled slowly. For this reason 3" brick is preferable for high firing. However, it is possible to slow down the cooling by "firing down". With a manual kiln, when you would normally turn the kiln off, instead turn the switches down to medium. With an electronic kiln, you will want to program this ahead of time. As an example, your last segments could allow rapid cooling to 1950 degrees F, a 30 minute hold at that temperature, then slow cooling at a rate of 150 degrees per hour down to 1100 degrees F. At that point the kiln would turn off.
loading a new glaze
If firing a new glaze you are unsure of, put the whole piece on a piece of bisque you have pre-made. Then if glaze runs, it will run onto this piece instead of ruining your kiln shelves.
Changes in firing
All clays and many minerals contain water which does not leave the body until above 700°F. Organic (carbon) materials need to be oxidized (burned out). Other minerals, such as calcite, break down and give off a carbon dioxide gas. Minierals such as flint (silica) undergo a sudden expansion on heating to 1060°F and contraction during cooling.
Limit timer
A ‘limit timer' is the term used for the timing device that may be installed on a kiln to shut off the power at a predetermined and set time.
Do not use stilts with porcelain
Stilts cannot be used to support porcelain bisque as they become embedded in the porcelain when heated to high temperatures.
lid of your kiln does gap
If the lid of your kiln does gap in front, loosen the hinge screws a half turn when the kiln is hott. This allows the lid to reposition itself thus compensating for heat expansion. After repositioning, retighten the hinge screws.
Mixing bisque and glaze in firing
Do not place greenware and glazed ware in the same load. The gasses emitted by greenware may cause discolouration of the glaze.
If it is necessary to mix glaze and greenware pieces when firing a load, the glazed pieces should be packed in the lower part of the kiln with the greenware above.
Temperatures
Glazes and clays mature at different temperatures and may vary from company to company. Always check the container or package for the recommended firing cone.
Mixing oxidation and reduction
Sometimes a firing cycle for a particular glaze (e.g. a 'copper red') will include various stages of oxidation and reduction. Re-oxidation brightens up a glaze, although the effect will usually still be different from a pure oxidation firing. Re-oxidation is often done towards the end of a firing cycle, in order to eliminate excessive carbon trapping, which could result in dullness.
Read the labels
Some manufacturers have special requirements for firing the glazes they make. Be sure to check the bottles of each colour for the proper cone.
Loading: Glaze
Avoid crowding glaze pieces, as some have the tendency to ‘flash' or ‘spit' during firing and may leave a spot of unwanted colour on another of your pieces.
Kiln Sitter tips
Since a kiln sitter works by gravity, it is important for the kiln to be level.
Kiln sitters can drift and need to be calibrated periodically.
People often find that they need to put a slightly higher cone number in the kiln sitter to get the kiln to fire to the correct temperature. For example, to achieve a cone 6 inside the kiln, they must use a cone 7 in the kiln sitter. You will experiment with this for your individual kiln. The firing log will assist you.
Note: a kiln sitter does not assist in turning up the kiln, just in turning it off.
A little kiln wash dabbed on the top of new cone supports greatly reduces the chance of a cone sticking to the new metal (which may cause your kiln to over or under fire).
Pinholes in glaze
Pinholes in glaze can be caused by under-fired bisque, applying glaze on greenware or firing too rapidly. Sometimes these holes are in the greenware and should be rubbed out before firing. If pinholes appear in the glaze, fill the holes with glaze and refire.
Prevent glaze on stilts
To avoid getting a big pile of glaze on stilts, try coating them with kiln wash.
Raku Body
When making pieces for raku firing, consider the stress the process puts on the work. The thermal shock will crack large pieces made from most fine textured clays, so it's best to use a body containing sand or grog. Long Beach is good for medium sized pieces, and B-mix with sand will work if you need a white clay to use with crackle. Large pieces should be made of a coarse raku or sculpture body. Any piece that's at risk of cracking or exploding in a stoneware firing is doubly at risk when raku fired, so be sure to construct raku pieces well. While the clay is still in the bag, consider how to design the piece so it can be sensibly removed from the kiln with tongs. Avoid extremely fragile pieces or very heavy thrown pieces.
Black Firing Tips and Tricks
Black Firing Tips and Tricks Read this great article on Blackfiring by one of the masters of this technique, Salvatori Lolicato: Black-firing - Black Firing Tips and tricks
Textured glaze
If you have a glaze that is supposed to have a texture after firing, and it does not, you may not be applying enough of the glaze or are over-firing.
Maturing ceramic bodies
Most ceramic bodies have only one exact point at which they will mature properly. There is no way that you can control or guarantee the kiln being turned off at just the right point, except by using the proper cone and watching it closely to protect your ware.
Glaze firing Reduction
Glaze firing Reduction: Reduction firings are normally carried out in gas, wood or oil-fired kilns, as the burning of carbonaceous material is required to produce a reducing atmosphere. The carbon from the burning material combines with the oxygen present in the atmosphere; when carbon in the material being burnt exceeds the oxygen available in the atmosphere, incomplete burning occurs and carbon and carbon monoxide are formed. These then take oxygen from any available source, including the oxides in the clay and glazes present in the kiln. When this extraction of oxygen happens, clay and glazes become “reduced” and this can affect the colours obtained from the oxides. For example, copper oxide can produce red.
Exposure to smoke
Try to avoid smoke when firing raku. The volume of smoke can be greatly reduced by placing one piece in each can and quickly covering it. When using newspaper as a combustible, avoid colored print. The colored inks contain heavy metals that may be carried off in the smoke. People with asthma or other respiratory sensitivities should probably avoid raku altogether.
Burn and fire prevention
For burn and fire prevention, the raku area should be kept clean and organized. Left over combustibles should be kept separate from the firing area and reduction containers. There must always be a charged garden hose and a bucket of water on hand. When pulling pots, there should be a coordinated plan for which pieces are going in which cans, and only one person at a time should be removing pots from the kiln. The cans should be arranged in some logical fashion with the lids all accounted for and ready before any pots come out of the kiln. Anyone not involved should stand back out of the way to watch. A group of people firing together with no idea what they're doing is an accident waiting to happen.
Firing Bright Reds/yellows
Oxygen: Most commercial glazes and underglazes are designed for use in an oxidizing environment (usually in an electric kiln.) Red, orange and yellow are particularly sensitive to the amount of oxygen in the air. All clay has carbon in it, much of which burns out in the early stages of firing. This creates carbon monoxide, which will affect the glaze if it hangs around. So you want plenty of air flow, to remove this carbon monoxide as quickly as possible.
If you fire with a vent, you should be getting enough oxygen in the kiln. If not, make sure the top lid is propped and peephole plugs are out for the first few hours. With manual venting it is best to put colors such as red on the top shelf where they will get more oxygen.
Dunting: Silica inversion
Dunting can occur as you cool through the first silica inversion at 1063 degrees F. At this inversion the body contracts suddenly. The more silica (quartz) in the body, the more contraction. Since different parts of the pot reach this temperature at different times, it doesn't all contract together, and that causes stresses which can crack. Take for example a tall pot. The top will cool much faster than the bottom, because the bottom has the whole temperature of the kiln shelf keeping it warm. So the top will cool faster than the bottom, causing a crack around the bottom wall. Dunting can also occur as you cool through the 439 degree F inversion. A similar thing happens as above.
Grey areas in red glazes
Grey areas in red glazes are caused by not enough glaze. Reds must always be applied on bisque, never greenware.
Kiln shelf re-cycling
Don't throw away any kiln shelves that you accidently break. They will come in handy during special firings of pieces when you know that glaze might run. You may also use them only part way across your kiln, allowing a large or tall piece to be placed on a lower shelf up past your broken shelf.
Removing stilt marks
After glaze firings, the stilts are removed from the ware by breaking the thin film of glaze holding them. Handling with caution, remove the sharp edges. This can be done by grinding off with a carbarundum stone, electric grinder or hand grinder.
Kiln wash: where to apply
Apply kiln wash to the bottom of your kiln and one side (top) of your shelves before your first glaze firing.
kiln sitter troubleshooter
If your kiln sitter shuts off your kiln before maturing pieces, it may be caused by a kiln sitter out of adjustment, a blown fuse or circuit breaker, the wrong cone in the sitter, the cone becoming knocked loose during firing or an incorrectly set kiln-timer
Firing crackle glazes
For best results with crackle glazes, after pulling the piece out of the kiln, cool the pot for a minute before putting it in to smoke. A crackle pot crackles because the glaze (the outer layer) cools before the clay itself. The glaze shrinks before the clay gets a chance to, so it cracks. Blowing on the piece before smoking it will help to promote a good crackle. Red or brown bodies will fire to a pink color with a clear crackle glaze. If you want a white pot, use a white clay body. Opaque white crackle glazes can be made, but they don't usually crackle as well.
Suggested firing guide: Stoneware gloss oxidation
Suggested firing guide: Stoneware gloss oxidation
1280-1300c (cone 10)
150c per hour up to 1280-1300
Use of cones
If you use cones for firing that need to be mounted in a pat of clay and placed in front of a peep hole you might find a cone holder designed for that purpose easier to use.
reduction firing
A reduction firing is an incomplete burning where smoke and soot (the excess carbon) rob the clay body and glazes of part of their oxygen, thus changing their colour and texture. This is primarily a firing technique in pottery.
Holes in lace or fabric which appear after firing
Holes in lace or fabric which appear after firing, can be avoided by first washing fabric thoroughly before dipping into slip. Apply enough slip to the fabric so the clay will be strong enough to hold its own weight. The fabric must burn out before the clay matures, leaving only the clay shell.
Ware and your electric elements
It is important that you do not let any ware touch the elements of the kiln. You may not only damage the elements themselves, but you are sure to damage your ware.
Bumps in porcelain
Bumps in porcelain are usually caused by wet greenware and over-fired porcelain bisque.
Overfiring
If your kiln overfires, you kiln sensing rod on the kiln sitter is probably blocked or it is not in adjustment. The kiln may not be level, something has blocked the sitter or possibly the sitter is defective.
Firing large or flat pieces
If large flat pieces are being fired in your kiln, the edges should be placed between the elements. This may eliminate possible cracking from uneven heating.
Reducing warpage bisquing flatware
Items such as plates and tiles often warp less if bisqued on their side. You can lean them against the side of the kiln.
Suggested firing guide: Earthenware biscuit
Suggested firing guide: Earthenware biscuit 1150c (cone 2) 100c per hour up to 600c, then 150 per hour up to 1150c A relatively high biscuit temperature is used with this clay body to reduce the incidence of crazing in the glaze while retaining the clay's porosity. Other clays may benefit from a lower biscuit temperature (1120-1140c) depending on their maturing temperatures (check the bag).
Refiring on stilts
If you are going to refire a piece of glazed ware, but are having difficulty making the stilts stick to the bottom, try glue. The glue will not effect the piece and fire off.
Firing cast ware
The average cast piece may be fired on a fast firing schedule. Slow firing of heavy pieces reduces danger of breakage and many people feel that slow firing produces better ware.
History
Raku Pottery was developed in Japan over 400 years ago as the Ceremonial Tea Ware of the Zen Buddhist Masters. It was preferred by the Masters because of its humility, its tasteful unpretentiousness, its simple naturalness, and its deliberate avoidance of luxury...all very intrinsic to the Zen philosophy.
Seldom watertight, Raku is actually a very poor choice for a casserole or a flower vase; it is pottery apart from utility or function. Raku must be approached with a different criterion in mind, like a painting or a symphony. According to the Zen Masters, its elusive, subtle, yet vigorous beauty is Raku's only worth. It is valued because it is believed that the Spirit of the Maker is embodied in the form and revealed at the foot, which is traditionally left naked (unglazed). It is believed that if we are alert to ourselves, in contemplating the Raku form, we will recognize in it our own Spirit and Meaning.
Modern raku
Modern raku is a method of firing pottery loosely based on a sixteenth century Japanese technique. The Japanese would fire tea bowls in a small wood-fired kiln and then remove the bowls from the kiln with tongs while still hot and rapidly cool them. The thermal shock would crackle the low temperature lead glaze. Raku ware was preferred for the tea ceremony because of its soft feel and unassuming, quiet aesthetic.
tile setters'
Did you know that ‘tile setters' are racks used to fire tiles to keep tem from warping during firing?
cracking: Even glazing
A thick layer of glaze on the inside of a pot, and a thin or no layer on the outside will cause stress. The result is often a spiral crack up the sides.
If glaze pools on the inside of a pot, tension is created and the pot may crack or split across the base.
Colour Maturity
Colours mature at a lower temperature on ceramic pieces than on porcelain or hard china.
Grinding pots
Sharpening stones can be put to great effect by using them to grind off by hand left over bits of grog or alumina on the feet of pots and bowls or other surfaces.
Description
Woodfiring has an age-old tradition in ceramics. The very first ceramics ever fired were done (probably accidently) in camp fires and rudimentary kilns thousands of years ago. Under woodfiring we now understand firing a brick kiln with wood as fuel. This necessitates a a certain kiln contsruction type, sometimes with several burning chambers and stoking ports for feeding in the wood. As the wood burns, ash is created which deposits itself on pots in the kiln, creating a natural 'wood ash' glaze. These natural glazes are made from silica, potash, calcia and other various ingredients that form naturally in the burnt wood.
Access to a kiln
The glazes will be fired, and thus permanent. But that means you need access to a kiln. One place to check is with contemporary ceramics studios. Often they will fire your own pieces for a fee. Another is to check the Kiln Time-sharing listings at BigCeramicStore.com. There may be a potter in your area who will sell you space in their kiln. Another approach would be to enrol in a community class that has a kiln. And finally, you can buy your own kiln. A small kiln can be had for only a few hundred dollars, and is much more convenient that trying to find space elsewhere. You also have the benefit of total control over the process, so someone else doesn't smudge your design, or drop your piece, for example. These small kilns plug into normal household power, and if you upgrade to a larger kiln later they can be used as test kilns to test glazes.
Avoiding contamination of colours
Sometimes a glaze on one piece will affect the color of the glaze on another piece. This is particularly true if you are firing without a vent. Keep colors such as red separated from other colors for best results.
Glaze firing: Oxidation
Glaze firing Oxidation: This is the firing of a glaze in the presence of oxygen. Electric kilns will, in normal use, produce oxidized glazes. The adequate supply of oxygen during firing enables the clay and glaze materials to maintain their oxygen levels and the oxides present to remain intact, thus oxidation colour responses are achieved from the oxides used. For example, copper oxide will produce greens.
Dunting Description
Dunting is a special type of crack which occurs from stresses caused during firing and cooling. These stresses primarily occur during two critical points of firing called silica inversions which occur at 1063 degrees F (573 degrees C), and 439 degrees F (226 degrees C). At these inversion points, the structure of the silica molecules rearranges. It is important to fire slowly through these two temperatures, and electronic kiln profiles often do this for you automatically while they are heating. Most dunting however is caused in cooling. These cracks appear as long, clean, body cracks with sharp edges. If the ware is glazed, the glaze edges are sharp. They may be vertical, horizontal, or spiral.
Clays
Flashing colors are typical of woodfiring, usually where a reduced atmosphere has been use in the higher temperature ranges, followed by an oxidised cooling. The flashing colors are very dependent on the nature of the clays used, and generally low iron clay bodies fluxed by felspars produce an orange coloured flash....
Read this great article by Owen Rye, prominant Australian Wood-firer: Woodfired clay bodies
Copyright Ceramics Today. Full article available at Ceramics
Today
Smooth sharp or chipped edges on bisque ware
Smooth sharp or chipped edges on bisque ware before you glaze and final fire your work. Unfortunately pots are very vulnerable at the bisque stage and especially if you work in a community lab or you share a studio your bisqued pots are bound to get chipped or broken at some point. Use sandpaper on broken or chipped areas or they will become razor sharp after a layer of glass has adhered to them! Fortunately many hardware stores carry specially cut and reinforced smaller pieces which are perfect for smoothing over the rough bottom or edge of a bisqued pot. It's a good idea to go over rough spots just before you glaze them. Sandpaper is our friend.
Electric Kiln voltage
A standard electric kiln normally requires 220-240 single phase voltage to work properly. It is possible to fire your kiln on a 208 volt source, but firing times may be slower. If your kiln should be wired for 208 volts you cannot use it on 240 volt source.
Moving your kiln
If you must move your kiln be sure to handle it by the outside which is protected with stainless steel. Fire brick is soft and can be damaged easily.
Kiln problems
If you have problems with your kiln, be sure to go back to the place you bought it. Normally they are happy to help you.
Vertical setter
Did you know that a ‘vertical setter' was a rack to hold plates during china firings?
Make a cone holder
If you use a cone holder in firing, you can make one very easily by taking a teaspoon of slip and pouring it on a plaster bat or mold. Stick a cone in it and you have a perfect cone holder.
Silica for porcelain firings
Did you know that ‘silica' or flint is a mineral which will not harden at firing temperatures and is used for separating porcelain pieces during firing?
Loading: Greenware
When loading your kiln, leave ample room between each piece for air circulation.
When loading your kiln, place greenware directly on the shelves or the floor of the kiln. Greenware can be stacked so it touches other greenware.
Applying Kiln wash
When the surfaces of your kiln floor of shelves which are protected by kiln wash become rough, they should be scraped off and a new coat added.
Using High setting on electric kiln
Heat or temperature is equalized when you turn all the switches to the high setting.
New Kiln care
If you have just purchased a new kiln, it will give longer and better service if broken-in carefully. Moisture is present in the kiln from water used in manufacturing and this must be dried out by pre-heating. Check the kiln frequently during the first few firings, and note the colour inside of the kiln so that you can learn to estimate the temperature with accuracy. If chips should appear in the lid of your kiln, they should be repaired immediately to prevent them from becoming larger. The bottom of your kiln should be covered with kiln wash at all times. If glaze drippings accumulate, they should be scraped off and a new coating of kiln wash be applied.
Packing your kiln
Try to place your heaviest load of ware in the layer in the kiln that has the most element grooves included.
Loading overglazed ware
Overglazed ware is loaded into the kiln in the same manner as loading a glaze firing. Ware must be prevented from sticking by the use of stilts, and it must not touch other pieces of ware.
Supporting plates
Plates fired on edges may be supported at the bottom with large spur stilts which can also be used to separate adjoining plates. Tall posts can be used to separate plates from the walls.
Plate holders insure even firing of china painted plates
Plate holders insure even firing of china painted plates
No two kilns are exactly alike
There is nothing mysterious about a kiln and you have no reason to be afraid of it. No two kilns are exactly alike, even if they are the same brand and model.
Before firing
Every time you get ready to fire your kiln, check the inside to be sure it is clean and free of dust. Check the lid and wall brick for loose fragments which might fall on the ware. If possible, vacuum the interior to remove all dust and foreign materials from the elements
Electric Kiln troubleshooting
If it takes longer now for your kiln to fire, It could be caused by a defective switch or element, or may even be the wall receptacle. If your kiln will not heat, the power cord may not be plugged in, fuses or breakers blown, kiln sitter not activated or power cord blown. If when firing, your wall receptacle or plug heats up, check to see if there is a loose connection in the receptacle. If a kiln makes popping noises when firing, it is normal. This is due to the contraction of kiln parts which are enlarged from thermal expansion.
Troubleshooting: no power after firing
If the fuse or circuit breaker in your home fails after firing your kiln, it could be due to an inadequate fuse or breaker, inadequate wiring size or a defective fuse or breaker.
Situating your pyrometer
If you purchase a pyrometer to aid in firing, be sure that you mount it on the wall and not on the kiln, as heat will damage it.
Precautions
If you do not fire porcelain properly, the various colours that porcelain is available in will not be true.
When firing porcelain, your kiln must be very clean. To ensure that it is, you should vacuum it out before a porcelain firing.
Saving Kiln shelves
„h To prevent glaze from running onto your kiln shelves, cut some slices of kiln brick and fire your pots on that. The brick will stop the glaze from running onto the shelf. Excess glaze can be removed with an angle grinder (use caution with this tool!) Shelves will last much longer, if not indefinitely!
Reduction firing
In a reduction firing, the oxygen supply to the kiln chamber is restricted, resulting in a saturation of free carbons in the kiln atmosphere, mostly in the form of carbon dioxide and carbon monoxide. In a strong reduction firing, flames will lick out of the flue and through other openings or cracks in the kiln walls. This is because the flame needs oxygen to burn, so it will try to get it from wherever it can. The reduction cycle generally begins between 1600o-1900o F (900o - 1000o C). Typical reduction glazes are 'copper reds' and 'shinos'.
Burnishing
The technique of burnishing pottery can be traced back to ancient times. Burnishing involves no more than rubbing the clay surface with a smooth tool to produce a mirror-smooth surface. In reality it has a compressing effect on the clay particles. It can be done when the clay surface is leather hard and up until it is almost completely dry. Most clays are suitable for burnishing although the finer the clay the smoother the burnished surface. Suitable tools for burnishing include: Smooth rounded beach pebbles, The convex side of metal spoons and smooth knife handles. After the pot is smooth, draw your design with lead pencil then scratch around design with a knife. Designs can either be geometric or organic. Once the pot has been blackfired it can be left without further treatment or polished with oils to enhance the shine.
Use cones
Use witness cones in the kiln even if you have an electronic controller or kiln sitter. This will show you the temperature that was actually reached inside the kiln (heat-work). See my article: '(Orton Standard)Temperature Chart ' at http://www.ceramics-tips.com/RscArticleV.asp?id=369
New Electric kiln run in
New kilns, or kilns with new elements, should be fired once to Cone 04 without any ware in the kiln. This correctly oxidizes the elements and ensures a longer life. They may smoke a little due to oil left on the element wire during manufacturing.
Avoid thermocouples
If using an electronic kiln, make sure you place the shelves at heights that avoid the thermocouples. The thermocouple should be at least 1-2 inches from a kiln shelf or ware.
Staggering the shelves
Some people believe that if you have half shelves, it is best to stagger their heights to get more even heating. This is not very important in an electric kiln. It is very important in a gas kiln.
How to load the kiln
The bottom layer of shelving should be on 1/2" - 1" posts. Your ware is placed on this first shelf, posts are set around the perimeter, and another layer of shelving placed. Make sure you leave at least 1/2" between the top of your tallest piece and the next shelf, to allow for expansion during firing. Usually for whole shelves you will use 3 or 4 posts. For half shelves, you will use 3 posts per side. Posts should be placed over each other as you rise up through the kiln.
Cracks Description
In general, cracks result from stresses in the clay. There is always some stress in clay because of the fact that it shrinks as it dries and when it is fired, and it also expands and contracts during firing. Sometimes the stress is too much for the clay to handle and it cracks.
Thermal shock
Raku firing also affects the functionality of pieces. Raku is fired at the same temperature as the bisque firing, so raku ware is porous. This is necessary because clay fired to maturity won't withstand the thermal shock of raku. Since the raku glazes craze, a raku vase filled with water will seep a little, and raku is not appropriate for use with food. Many raku glazes depend on heavily reduced copper for their color. While the effects are striking, they don't last forever. Lusters and copper matts gradually fade to green and brown over the course of a few years
Temperature control
At any time we can measure the temperature in the kiln using pyrometers (the digital devices mounted outside the gas kilns). But temperature doesn't give us the whole story regarding how the glazes melt. A more accurate measure is given by using pyrometric cones. Clays and glazes are complex mixtures of minerals. Unlike a simple substance like ice, they don't melt suddenly at a particular temperature. The process of glaze melting requires time for the chemicals to combine and react as well as increasing temperature, so we need a way of measuring both. Pyrometric cones (most often just called cones) are a standardized way of measuring how the heat of the kiln has cumulatively effected the glazes. Cones are composed of much the same stuff as clay and glazes, and each cone number is formulated to deform after being exposed to a particular degree of heat treatment.. The cones are placed in the front of the kiln where they can be seen through the peep-holes in the door while the kiln is firing. When the desired cone bends, the kiln is shut down and allowed to cool. Some cone numbers with approximate corresponding temperatures and firing ranges for different types of work...
Damp greenware
Damp greenware or mended places will cause bumps on the surface of fired ware if you do not allow a piece to dry before firing.
Troubleshooting: crystals appearing
If in firing your lustres, crystals appear, it usually indicates too much heat. However, if a lustre is too old, the same condition may also appear.
Anagama
Anagama
The Anagama kiln will usually consist of of one long firing chamber with a firebox at one end and a flue at the other. Often there will also be smaller stacking ports on the side of the kiln. Traditional Anagama kilns are built on a slope, so that a better updraught can be achieved. Firing time can vary from one day to several weeks. The Anagama kiln is the oldest style of kiln in Japan and has been around since medieval times. The beauty of Anagama style firing lies in the natural ash glazes that can be achieved, and in the excitement of the long firing itself, appreciated by many potters all over the world. There are many different designs for the Anagama kiln, not only in Japan, but on other continents. Not only are there different designs, but also different methods of firing and stacking. No two firings are ever exactly alike, in contrast to let's say, the electric kiln.
Mark your pieces
The worst thing you can do is fire low fire clay or glazes at high fire. The clay and/or glaze will melt all over your kiln and can cause major damage! If you have both low fire and high fire materials in your studio, you might want to mark your pieces differently on the bottom so you don't forget and get them mixed up!
Uneven heat
If you have glaze firing problems because of too much variation, then I recommend the following:
1. Make sure cracks and holes are repaired to keep heat in your kiln.
2. Fire slower during the early part of your firing, before red heat (below 1200°F). This allows heat to soak into the refractory and even out temperatures in the kiln.
3. Consider changing the switching pattern to even out top and bottom temperatures. Switch the bottom to a higher setting before the top or vice versa. Higher settings add more heat.
4. Consider adding an Orton Kiln Vent. These pull hot gases from the top to the bottom of the kiln and cut temperature variations in half. Hoods above the kiln will not help temperature uniformity problems.
Firing Raku Copper matts
Copper matts are the most difficult raku glazes to fire consistently. To get the rainbow colors characteristic of copper matts, the glaze must first be reduced to metallic copper and then re-oxidized slightly. The final effect is similar to the halo created by heating a piece of copper with a torch. There are many different schemes for firing copper matts. The most common is to pull the piece from the kiln hot, smoke it as usual to reduce the copper, then let it re-oxidize while it cools in the container. When removed from the container, the piece is usually quenched in water to stop the re-oxidation.
How to fire
When firing China paints, you should vent your kiln on low and medium heat settings and close the lid when turning to high.
Bisque and gloss
There are two main stages in firing a clay object. The first stage, known as biscuit firing, converts clay into a permanent, porous material with the handling properties necessary for glaze application. The second stage, known as gloss firing, melts the glaze onto the pot.
Testing the kiln atmosphere
Sometimes a kiln may be reducing only slightly and it is difficult to tell, because no flame is visible in the flue. Try this: hold a strip of newspaper in front of a freshly opened spy hole. If the paper just burns or gets sucked in towards the spy hole, the kiln is oxidizing. If the paper is blown out and away from the spy hole, the kiln is reducing. Try different damper settings to get the effect you want. Warning: Reduction firings create carbon monoxide, which is a deadly gas, and must be done in well ventilated areas, preferably outside. Do not try wearing a gas mask with a gas filter as compensation -- the carbon monoxide molecules are so small that a gas filter, - even a good one - will not hold it back. Drowsiness and loss of concentration area signs of beginning carbon monoxide poisoning.
Electric kilns, start on low
The kiln should always be started with your switches on low regardless of the type of firing. This reduces shock and dispels moisture from ware slowly.
Misfitting glazed lids
If some of your lids will not fit after you add glaze and fire, you are not leaving enough room for the coats of glaze.
Firing Larger pieces
Larger pieces that take all of the kiln dimensions should be placed so that the projecting edges are between coils.
Cones: which one
Always use the proper cone. Do not try to get by with one that is close if you are out of the proper one.
Under-firing
Underfiring an overglaze will prevent the glaze from becoming soft enough to receive the colour, thus causing poor adherence.
Soda Firing
Soda Firing
The soda or vapor firing is similar to the salt firing, except that the salt is substituted with sodium carbonate (soda ash) or sodium bicarbonate (baking soda). The sodium (bi)carbonate delivers the necessary sodium for glazing, but the effect is much weaker. Often the interiors of pots do not glaze well, so extra glaze may need to be applied. But as carbon is additionally generated, instead of hydrochloric acid, this is an evironmentally friendly alternative.
Waterproofing
Raku firing does not achieve high enough temperatures to make the clay waterproof. Low fire clay also is not waterproof. Both will leak over time, if you leave water in them. This will probably happen even if you use glaze, because it is very difficult to get a perfect clay/glaze fit which will expand and contract together and not craze. (I have a mug that is a cone 10 stoneware fired at cone 6; it is fine with cold water, but when I put hot water in it the bottom becomes damp. What is happening is that when heated, the glaze probably expands and exposes tiny cracks, the hot water seeps through, and then through the clay. This will weaken the clay over time, and this piece definitely should not go through the dishwasher or in the microwave!)
Suggested firing guide: Stoneware biscuit
Suggested firing guide: Stoneware biscuit 1000c (cone 06) 100c per hour up to 600c, then 130c per hour up to 1000c.
American raku
American raku is rapidly fired, removed from the kiln while hot, and usually smoked afterwards. A still red-hot piece is placed in a container or pit with combustible material and covered. Since the oxygen supply to the resulting fire is cut off, the piece receives a very heavy smoking or "post-firing reduction". Many interesting glaze and surface effects can be obtained using this method. While "modern raku" is related to Japanese raku through the adaptation of the technique, the aesthetic of most contemporary work is totally unrelated to traditional Japanese work.
General
To glaze our pots, we will usually be dipping them in liquid glaze. If we dip greenware in liquid, we run a good chance of it absorbing too much water and falling apart. This is why we bisque fire the work before glazing. The main purpose of the bisque firing is to harden the work in order to glaze it. Bisqued clay (or bisqueware) is porous, so it will absorb water to ensure a sufficient glaze coating. Bisque firing also burns off organic compounds in the clay (molds, bacteria, lignite, carbonates, and sulfates) that can cause problems if they burn off through the glaze. For stoneware firing the bisque is usually fired to about cone 06 (18400F).
checking moisture content when firing bisque
To check the moisture content of bisque ware when firing, hold a glass jar in front of the spy hole. If there is no condensation, the kiln can safely be fired at a faster rate.
Propane torch for effect
For that raku piece that came out less than exciting, trying fanning it with a propane torch to bring out more color.
Refiring crazed ware
If you have a piece that has crazed after firing, you can often correct this by refiring your piece a cone or two hotter than the original firing.
Loading differences for greenware and glaze
There is one basic difference between loading greenware and glaze. Glazed pieces cannot touch anything.
Warped ware
Warped ware can be caused by distortion upon removal of the piece from the mold, being fired too close to the elements or firing a piece in an unnatural position.
Firebrick behaviour
The insulating firebrick expands and contracts with each firing. The expansion and contraction is necessary for the long life of your kiln, but some hair line cracks will appear in the brick while the kiln is cold. Do not be concerned about them, as they close tightly when the bricks expand when heated.
Firing process
If in firing porcelain, a piece of ware had to be supported in the bisque firing, it will stand alone in the glaze firing. The lower temperature will prevent sagging.
Positioning plates
When firing plates, position them between the elements for proper heat distribution.
over-firing porcelain
If you have a piece of porcelain that comes out of the kiln without translucent quality, you may be pouring the piece to thick or be under-firing.
Wear protection
Excess glare from a hot firing chamber can be reduced by using dark glasses or a number five welder's lens when looking through the observation holes.
using cones
Cones are available in different numbers and the user must insure that items being fired are fired to the cone recommended by the manufacturer.
Loading: Flatware
It is best to fire a piece in its natural position, however, large flat items such as wall plaques or clocks should be fired on a flat side to prevent warpage.
When loading your kiln, place your shelves in the kiln carefully so the walls of the kiln will not be bumped and damaged.
Do not jar or shake your kiln after the loading has started since the ware on a shelf could be knocked down or broken.
About Raku
Raku originated in Japan in the 16th century, where raku vessels were and still are used in the traditional tea ceremony. It is a low-fire technique, where bisqued work is quickly heated to red hot temperature and then taken out of the kiln and reduced in wood shavings, newspaper or a similar combustible material. Raku ware is decorated with low-fire glazes, which usually contain a lot of frit. The clays used for this firing technique contain a high percentage of grog, so the work will be able to withstand the high temperature fluctuations. Raku, with its battle with the elements of fire and smoke is an exciting technique, one that is suited to communal firings.
Raku firing tips
Since there are so many firing variables that affect the appearance of raku, it's a good idea to eliminate as many as possible when experimenting with raku. Try concentrating on learning how to fire one glaze at a time, and use one easily duplicated form. Identical pieces heat and cool at the same rate, so you'll be much more likely to see how the other factors tie in if you eliminate the variation in the weight of the pieces. Once the basics are clear, it's fairly simple to branch out to other forms.
Wadding for even shelves
If the kiln floor becomes rough and the shelves are difficult to keep level or steady, try putting small wads of clay under each post until it rests solid.
Bat wash
Bat wash is a slurry of refractory material used to prevent kiln furniture from fusing together or pots fusing to shelves or to their lids when fired in position. The ideal wash is one which is relatively cheap and should be easy to make up into a wash which stays put during setting, and lastly it should easily wash off the foot rims, lids etc of the finished ware.
A mixture of aluminium hydrate two parts, china clay one part will be found excellent for most purposes both oxidized and reduced.
Application
Application: In most cases, it is best to apply red/yellow glazes and underglazes thickly, sometimes as many as 4 coats is required.
Kiln furniture placement
All furniture and ware in the kiln should be placed at least 1” from any element. This ensures even heat flow, and minimises the risk of contamination of the elements by oxides.
S Cracks
One of the most common cracks found in pots is the "s" crack, which occurs at the bottom of a pot, in the shape of an s, usually on thrown pieces. I have heard so many theories about this and how to prevent it, that I am not even getting into it now. Perhaps later I will do a whole tip on the different theories so you can decide for yourself! The one thing everyone agrees on is that you should keep the bottom of the pot as dry as possible while throwing, and compress the bottom during throwing and trimming.
Never bisque a piece that's not completely dry
Never ever bisque fire a piece that's not completely dry. This does not require much explanation. Any piece which isn't completely free of moisture, or bone dry may explode and destroy other pieces in the kiln. If there is moisture in the piece, when you put it next to your cheek it will feel cold.
Handbuilt pieces with thick surfaces will take much longer to dry. When in doubt - wait a day, it certainly won't hurt.
Loading: contamination
Be sure your hands are clean when loading your kiln. Oils or dirt from hands may mix with colours effecting them unwantedly.
Kiln ventilation
If you do not vent your kiln properly when firing, your process will have a cloudy appearance and often an impaired finish.
Thick glaze
Pieces that are heavily glazed should be kept further apart during firing to avoid glaze running from one piece to another.
Glaze won't stick
If you find that your glazes do not seem to stick after firing, you may have overfired your bisque originally.
Troubleshooting:Electric Mid firing cycle failure
If your kiln stops heating during a firing be sure to check your fuses or circuit breakers first. Be sure you know what fuses or switches control your kiln and how to replace them. A blown fuse or tripped circuit breaker is not necessarily an indication of an electrical problem in your kiln or wiring. If, your kiln heats in some switch positions, but not in all, you may have a blown fuse, defective element, switch or element connection which can be corrected by replacing a fuse or resetting a breaker. Be sure to locate defective parts if necessary and replace. If you think you have a problem inside the kiln during the firing cycle turn it off and allow to cool before opening. If you have a hot plug or outlet, one or both may be defective and should be replaced if they are too hot to hold. Do not re-fire until fixed.
Sagging times
The length of firing time for glass sagging is less than ceramics. Place an 015 and 014 cone in your kiln in a pat of clay and fire until the 014 is halfway down, turn off the kiln and cool slowly.
Firing flat pieces
To keep flat pieces from warping in bisque firing, lay them flat on two half shelves with a small space between them.
Lustres and china paints
It is best not to fire lustres with china paints as the turpentine in the china paints may distort your lustres.
History
Pit Firing
Early cultures found clay in the ground and must have discovered its plastic and fired qualities by accident, probably discovering some burnt clay in a camp fire. This very basic firing then evolved into the pit-firing. Pit-firing involves placing unfired or bisque fired pottery in a pit in the ground, then covering the pottery with suitable burning materials, e.g. dried grasses and branches. Depending on the amount of work to be fired, a pit of the appropriate size is dug.
A bed of dry leaves and twigs and possibly coal, which will burn slowly, is placed at the bottom of the pit and the pottery placed on top of this. The work is then covered with more leaves and twigs and dung, if available, building up a mound over the pieces. Once the stacking process is finished, the pile can be lit around the edges and left to smolder for several hours, if not until the next day. Towards the end of the burning process, it is possible to bury the pit in earth or sand, which will cut off the oxygen supply and create a strong reducing atmosphere inside the mound. Not all clays are suitable for such a firing, especially the more refined types available from suppliers. Additions of grog 'open up' the clay and make it more resistant to heat shock. Clays dug directly from the earth may be suitable 'as is', or might profit from additions of grog or volcanic ash, if using a commercial clay, get a clay suitable for raku firings. The best color results can be achieved with iron bearing, or red clays.
OH&S:be prepared and alert
Raku Safety
There is clearly no more dangerous activity available in our studio than raku firing. The major dangers include burns, fire hazards, and smoke. It's imperative to be prepared and alert. Everyone involved in firing raku should be properly clothed. This includes long sleeves and pants, shoes, insulating gloves, and a hat.
Don't over-pack
When loading your kiln, do not pack it too full. It is a bad habit to stack things inside of each other, on top or in any combination to save space. Too much of this and the stress caused may result in cracked bisque.
Loading glaze to stop bleeding
If you have a vent, you don't have to worry about colors bleeding from one piece onto another. If you don't fire with a vent this can be a problem, so you might want to segregate colors while loading
Protecting your kiln during lustre firing
If you do not want the inside of your kiln lid to turn black during a gold or lustre firing, tape a piece of asbestos paper on the underside of the lid. Lap the paper over the outside edges and tape to the top of the lid. The temperature is not hot enough to effect the masking tape on top and the paper will protect the lid.
dry footing'
Glazed pieces may be prevented from sticking to the bottom of the kiln if you remove the glaze. This is called ‘dry footing'.
Overglazes
When loading your kiln with pieces that have lustres/overglazes, be sure to leave plenty of room between pieces and do not cut off ventilation to the bottom pieces by covering them with a full shelf.
Using a blower
You will not get to about 1350o F with a pit firing, and with poor luck you will be lucky to hit 900o F. If you add a bit of air (long pipes and blowers) you can get to 1650o F or higher, in addition to be able to add more fuel with the use of the blower, unlike without a blower. However, if you are making your own hand dug clays, this can be quite a mess, because they will over-fire because the iron will change from a refractory to a flux in reduction.
keep a record
It is a good idea to keep a record of your firing times. Some types of pieces or loads will fire faster or slower than others. If you have a record, you can get results each time by knowing ahead of time how to fire a load.
Kiln Jacket
The jackets on most kilns are made of stainless steel and should be kept clean. You can polish them with normal stainless steel cleaner.
Cooling the kiln
A rule of thumb for removing pieces from the kiln is that you should be able to touch and handle them comfortably in your bare hands. If you cannot, they are too hot.
Broken lines in gold
Broken lines in gold can be caused by over-firing or too heavy an application.
Peep hole plug Description
The plugs that are used in the peepholes are not put in to prevent heat loss, but to stop air from entering the kiln.
Suggested firing guide: Earthenware gloss (Alkaline)
Suggested firing guide: Earthenware gloss (Alkaline)
1060c (cone 04)
130c per hour up to 1060c
Cracking: Overfiring
Overfiring a pot, or firing it multiple times will tend to make it more subject to cracking.
Preventing rims cracking
The way the piece is made can either minimize cracking, or contribute to cracking. For example, if you sit an unfired pot inside down (on its rim), you will put stress on the rim. A crack may not appear right away, but it could show up later as the pot dries more or when it is fired. So it's a good idea when turning pots upside down to place the rim on a piece of soft foam.
Different clays can handle different amounts of stress without cracking.
How Carbon Burns Out
How Carbon Burns Out
During heating (firing) the carbon reacts with oxygen to form carbon dioxide and carbon monoxide gases. The carbon leaves the body as a gas. Binders are burned off at a relative low temperature: 300ºF to 500ºF.
Naturally occurring carbon in clay burns off (becomes gases) at higher temperatures: up to 1200ºF - 1400ºF.
Precipitating burnout
Slow Down Firing
Be sure the kiln is vented adequately so there is sufficient oxygen.
Load the kiln with burn out requirements in mind.
Leave plenty of space between ware and shelves. Do not stack ware. Use tile and plate stackers and invert pieces on top of one another to help conserve space and insure proper burnout.
Measuring Heat Distribution
Differences in heat distribution from top to bottom in the kiln are usually far more noticeable for cooler firings like decals. A 2 or 3 cone difference at 022 may only be a 1 cone difference at cone 6. This is because at higher temperatures radiation heats the kiln more effectively. Slowing the first half of the firing can help heat distribution problems. This also helps by allowing more time for air to enter the kiln and burn out organics and for carbon monoxide to leave the kiln. Use a controller to set heating rates and soaks for more precise firings.
fire lace figurines
When you fire lace figurines, leave the peep hole plugs out and the lid propped open until all smoke and odour are gone.
(Orton Standard)Temperature Chart
Pyrometric Cones (Orton Standard)
Temperature Equivalent Chart
For the entire Orton Temperature chart: (Orton Standard)Temperature Chart
Description
Salt firing refers to the introduction of salt to wears the end of a firing to get a salt glaze effect. Usually done in large wood or gas kilns, salt is introduced into the mature kiln chamber by the pound at the end of a firing. Due to the intense heat, the salt volatilizes and the sodium chloride splits into sodium and chlorine gas. The chlorine combines with moisture to form hydrochloric acid, escapes into the kiln atmosphere and exites via the flue, while the sodium combines with aluminium oxide and silica oxide in the clay, forming a glaze on any exposed surface of the work.
Levelling your kiln
Your floor may not be level but a kiln must be for proper firing. If necessary you can add shims under the legs of the stand to make it balance properly.
Troubleshooting: Black or white speck
A black or white speck in your lustre/overglaze after it has been fired may be caused by dust
Carbon burnout rate
The rate at which this carbon burns out is related to:
The amount of carbon present (that is, the amount of natural contaminants in the body). Some bodies have more contaminants than others, such as red clays. This needs to be considered when planning the firing.
Amount of air available (air provides oxygen for burnout) Air needs to be considered when planning the firing.
This is impacted by several factors. A load that is fired very quickly will not allow enough time for the oxygen to react with the carbon, form gases and leave the ware. If ware is stacked during bisque firing, oxygen may not be able to penetrate all surfaces of or inside all the pieces. Also, if gases are not removed from the kiln and replaces with fresh air, then there may not be sufficient oxygen to burn out the carbon.
Crazing
Crazing: A network of very fine cracks in the glaze is called crazing. It is caused by a mis-match between the clay and glaze. It often will not show up until the pot is cooled, or sometimes even until it has been heated and cooled a few times. Some people believe slow cooling will prevent crazing, but the stresses still exist and eventually the crazing would occur.
Senior Cones
Senior Cones are normally set in a series of three ascending cone values in a clay plaque. The "Target" cone is the second cone in the series. Cone plaques are set opposite a "peep hole" opening in the kiln's interior. The kiln operator monitors the progress of the firing by looking through the peep hole and shutting off the kiln when the target cone bends to a "six o'clock" position.
Kanthal Elements
Did you know that ‘kanthal' type elements are made of a combination of iron, chromium and aluminum.
Kiln care/floor
If the floor of your kiln has some rough spots caused from chipping off glaze drippings you can patch it with a special cement. You may also want to place an extra shelf in the bottom for protection.
Description 1
Common in pottery is the term ‘oxidation', which is the method of firing a kiln in a manner that mixes oxygen with the fuel to produce a clean flame with no soot or smoke and complete combustion. The atmosphere within the kiln contains sufficient oxygen to allow the elements in the clay and glaze to be completely oxidized. This method is primarily attempted using gas kilns.
Kiln wash: Alternatives
Kiln wash is normally used to protect the bottom of a kiln and one side of shelves from glaze drippings. If you prefer, however, you can use asbestos paper. After paper becomes brittle or spotted, you can change to a new piece.
Modeling clay
Terra Cotta clay is the normal modeling clay used in ceramics and matures at cone 06-04. It should be fired on a slower schedule than cast ware.
Ceramic Fibre
Ceramic fibre is alumino-silicate fibre material developed for use in jet engines and now used for high temperature insulation in kilns. It can be bought as blanket, paper, boards, ropes in many pre-formed shapes
Bisque
When you are firing bisque, it is very important that the steam have a way to escape. If you are firing with a kiln vent, the moisture can escape. If you don't have a vent, you must prop the lid open a few inches (with a kiln brick or similar item) during candling and the first few hours of firing. Usually the upper peep hole plug is also removed during this time. After this time the kiln lid is closed. The top peep hole plug remains out during the firing.
Mid firing cycle trouble shooting
If you hear something fall in your kiln while firing or if you think it is working abnormally, turn it off immediately. Wait until the kiln has cooled to the touch and open and inspect.
Kiln wash: not to's
Never apply kiln wash to the sidewalls, lid or bottom of shelves, as it will fall off during firing and ruin your glazed ware.
Drying for firing
Remember! Ceramic greenware must be bone dry before placing in a kiln. Wet greenware will crack in firing.
Kiln warranty
All kilns come with some kind of guarantee. Be sure to fill out the warranty card so if you do have any problems you are covered.
locking brace
Most kilns come with a locking brace to hold the kiln open. Be sure that it is locked in position before leaving it unattended. The lid can fall and break or it can fall on your hand.
Firing a new electric kiln
If, when firing a new kiln, you hear a humming noise, it is normal. This usually happens and will only last for a few firings.
chipped area in your kiln lid
If you get a chipped area in your kiln lid it should be repaired immediately.
automatic kiln sitter
Although an automatic kiln sitter does turn off the kiln, it is a mechanical device and cannot be considered an absolute guarantee against over-firing.
Caring for your kiln sitter
Due to heat, corrosion and mechanical wear, the kiln sitter may get out of adjustment over a period of normal operation. The repeated fall for the weight of the kiln sitter may possibly force the trigger to creep. To insure consistent firing it is recommended that the adjustment of the sitter be checked about every 20 firings.
Pyrometer: Uses
A pyrometer is a temperature indicator meter. It can be used to monitor the heating or cooling rate of your kiln. A pyrometer is very valuable when doing glass work or soaking glazes.
Star shaped stilts/bar stilts
If you have trouble getting glaze pieces to set correctly on those star shaped stilts, try using the bar type. You can put them in rows and there will be more points to support the piece.
Time and temperature
Time and temperature profile during the burn out period. Both time and temperature are important for proper burn out of the carbon. Some carbons require much higher temperatures that others. Oxidation should be completed below red heat (1400ºF). Carbon burns out from the surface first. As more oxygen penetrates the body, them more carbon is reacted to form the CO or CO2 gas and the burn out process continues. If there is sufficient time, temperature and oxygen, then complete burn out occurs. If these conditions do not exist, the resulting incomplete burn out is referred to as black coring (where the center of the piece has black or gray cast).
Firing Reds/yellows
Oxygen: Most commercial glazes are designed for use in an oxidizing environment (usually in an electric kiln.) Red, orange and yellow are particularly sensitive to the amount of oxygen in the air. All clay has carbon in it, much of which burns out in the early stages of firing. This creates carbon monoxide, which will affect the glaze if it hangs around. So you want plenty of air flow, to remove this carbon monoxide as quickly as possible.
If you fire with a vent, you should be getting enough oxygen in the kiln. If not, make sure the top lid is propped and peephole plugs are out for the first few hours. With manual venting it is best to put colors such as red on the top shelf where they will get more oxygen.
Caring for your kiln
Your kiln will give you years of service if you care for it properly. It should not be considered as a household appliance, but rather a piece of hard working machinery.
Reds/Yellows on greenware
Don't use red, orange or yellow glazes or underglazes on greenware. Only use on bisque.
UV protection
You should wear dark glasses with UV protection when looking into a hot kiln.
Suggested firing guide: Stoneware gloss reduction
Suggested firing guide: Stoneware gloss reduction
1280-1300c (cone 10)
150c per hour up to 1060c, then 120c per hour up to 1280-1300c
Preventing cracking: Fast drying
The form of the pot may encourage cracking. For example, sharp corners concentrate stress and are more prone to cracking.
Fast drying will tend to cause more stress than slow drying.
Stoneware
After glazing, our piece is glaze fired in one of the large gas kilns. The glaze firing serves to melt the glaze and to fuse (vitrify) the clay. We'll be firing to cone 10 in a reduction atmosphere. Ideally, at cone 10 even unglazed work will be watertight after the glaze firing. Stoneware refers to work that is fired hot enough to fully fuse the clay. Cone 10 refers to the amount of heat required to melt our glazes.
Element coils
The coils in your elements should not be so close together that they touch each other.
retain the shape of glass
If you want to retain the shape of a glass piece that has been painted, you should not fire it above 022 as the glass will soften and distort above this temperature
Purple spots in gold
Purple spots in gold are usually due to a thin application of gold or too much thinner.
Auto cut off temperature control
It is possible to vary the temperature at which your automatic shut off works by the position of the cone. Positioning the thinner part of a pyramid shaped cone under the sensing rod will make it shut off sooner while the thicker part makes it fire longer.
Kiln instructions
Always read your instruction manual as provided by the manufacturer of your kiln. Many kilns are similar but all have their own characteristics.
Pre-firing
Did you know that ‘pre-firing' is a test firing of a kiln to check the function of a kiln when new?
Kiln ventilation
You must always vent your kiln lid (prop it open) during the firing of gold, lustres and decals. You must allow the gasses to escape during the firing process
Cones/Limit timers
You cannot use a clock to time your firing instead of a cone. Many kilns are equipped with limit timers that will shut them off to prevent over-firing but time does not control accurate firing.
Checking for refiring
You should get into the habit of checking your pieces when removing from the kiln and before you remove stilts, in case they need refiring.
Poor colour in coloured glazes
Poor colour in coloured glazes can be caused by over-firing. This is especially true of red glazes which are usually fired at a lower temperature.
Kiln shelf description
A kiln shelf is a flat slab of fired clay which has been fired to a higher temperature than will be encountered in your kiln.
New kiln care
Your kiln should be cleaned thoroughly before firing when new. This can best be done with a nozzel attachment to a vacuum cleaner or a damp cloth or sponge.
Breakages while loading
If you break a piece while loading your kiln go ahead and repair it but do not attempt to fire it before it is bone dry.
Before firing
Before you turn your kiln on be sure that no two pieces are touching each other, the kiln walls , floor or shelves.
to get the maximum use from your kiln
You need a good assortment of shelves, posts and stilts to get the maximum use from your kiln.
Temperature
Be sure to fire ceramic bisque high enough to eliminate any chance of the fired piece absorbing moisture. Moisture absorbed after the piece has been glazed will cause it to expand and crack the glazed surface, which is called crazing.
fire pieces that go together, together.
Ceramic ware that is used together should be fired together such as a box and its lid.
Maturing ceramic bodies
Most ceramic bodies have only one exact point at which they will mature properly. There is no way that you can control or guarantee the kiln being turned off at just the right point, except by using the proper cone and watching it closely to protect your ware.
Fire composites together
When firing porcelain, two pieces of ware that are to be used together must be fired together. Powdered silica must be added to touch edges to prevent pieces from sticking together in firing.
electrical installation
Your electrical installation for your kiln will be as good as the electrician you use.
Holes in lace or fabric which appear after firing
Holes in lace or fabric which appear after firing can be caused by inadequate application of slip
Protect with silica
When firing porcelain lying flat on a shelf or a box and its lid, the undersides should be protected with silica to prevent parts of the pieces from sticking to each other and to shelves.
Spacing your load in electric kiln
When firing allow at least one element groove between every shelf. If your kiln has a blank spacer ring, let at least two element grooves contribute to the heat of the blank space.
Loading greenware
Load only bone-dry greenware into your kiln.
Wet greenware may crack during firing or explode, resulting in damage to other ware in the kiln.
Ceramic greenware should be dried for at least two days, with larger or thicker pieces requiring even longer before firing.
Temperature.
A kiln capable of reaching a temperature of 2250 degrees (cone 6) is necessary to fire porcelain.
Temperature
Over a typical, ceramic glaze, lustres and metallics are fired to cone 018-019. There are a few exceptions so be sure to check the product you are using.
Description 2
What is an oxidation firing?
In an oxidation firing, ceramic wares are heated to a certain temperature. The heated ceramics draw oxygen from the kiln chamber and from the flue burner ports, spy holes and any other holes or cracks in the kiln. The oxygen combines with carbonaceous materials in the glazes and ceramic body, turning these into ash. Metal pigments in glazes, such as iron, will oxidize, giving the glaze a particular color.
Oxidation firings can be done in most kilns, except those which are not designed for it, e.g. a black firing kiln, where sugar is introduced into the kiln which volatilizes and creates carbon, which is trapped in the clay body, creating a black surface. To do an oxidation firing, leave the burner ports and any damper open, so enough air exchange can happen in the kiln. In electric kilns, oxidation is the norm.
3 common ways to fire an electric kiln
There are 3 common ways to fire an electric kiln.
1. By manually turning the kiln on and up, and watching the cones inside the kiln through a peephole to determine when to turn the kiln off.
2. By manually turning the kiln on and up, and using jr cones in a kiln sitter to turn off the kiln when it reaches temperature.
3. By programming an electronic controller to turn the kiln on, up, and off when appropriate.
sagging glass
If you would like to try sagging glass, one thing to remember is that your shelves and kiln must be level. If not, your glass will not be symmetrical.
Witness cones
Make sure you use witness cones when firing reds, to ensure that the glazes are fired to the proper temperature.
Loading tall pieces
It is usually easiest to put the tallest pieces on top, so you don't have to use very tall kiln posts.
Placing cones
Don't forget to add your witness cones while loading. You will want to do this even if you have a kiln sitter or electronic controller, to see what Cone was actually achieved inside the kiln. If you plan to use the cones to determine when to turn off the kiln, make sure they are visible through the peephole.
Cleaning the Kiln
You should regularly vaccuum out the kiln, including the element grooves. This ensures a dust free firing environment.
Over fired glass
If you have over-fired glass, the edges will have needle points and the finish will be dull
High firing
Did you know that ‘high fired' means that the ceramic body is fired to cone 4 or higher.
Kiln placement
One of the first things to consider in locating your kiln is that it should be convenient to an electrical outlet and is in a well-ventilated area. When setting up your kiln, avoid placing it on rubber tiles, linoleum, or on any surface that might tend to mar if heated. When locating your kiln it is best to place it on an asbestos tile or a cement floor. If you need to place it on a floor that may be heat marred, lay down a few squares of asbestos for it to rest on.
A pyrometer
A pyrometer indicates when it is time to start watching the cone in kilns without automatic kiln sitters. It is not a substitute for a cone
Loading Porcelain Glaze
When loading your kiln with porcelain glazed pieces, the process is similar to loading porcelain bisque. Lightweight ware and small pieces should be placed on the bottom of the kiln for best circulation of air and distribution of heat.
Firing stilted plates
The stilted plate leaves the bottom exposed to radiation from the sidewalls. Place it in the centre.
Carelessness
Firing is your most important step in ceramics. All your hard work on a piece can be ruined and your kiln permanently damaged from careless loading or firing.
observation holes
The observation holes of the kiln allow viewing of the firing chamber and pyrometric cones. They also provide an escape for water vapor and gasses.
Cracking: Thickness variations
Variations in thickness will set up stresses, since the thin areas will dry faster than the thick, and stresses won't be evenly distributed. This is very common when the base of a pot is thicker or thinner than the walls. Sometimes this will show up as a circular crack around the bottom edge of a pot.
Uneven-firing/Gas
If you have trouble with heat spots in your kiln, (uneven temperature distribution) causing overheating in particular areas of your kiln, and you have already acertained it is not the packing, by adjusting the shelf heights, and staggering them, you need to adjust your bagwalls. The bagwall is the firebricks that direct the flame through your kiln, usually consisting of two whole firebricks and a sloped half brick, they are moveable. If your ware is slumping on the backwall of the kiln, I would suggest widening the space between the side walls and the bagwall so the bagwall faces more in to the centre of the kiln. It is a matter of trial and error,
Firing wet glazes
Firing wet glazes may cause pinholes, cracking or bubbling. If this should happen, try refiring .
The bottom of kilns
Sometimes the bottom of kilns do become damaged due to glaze drippings. If in cleaning these drippings you've created holes in the floor of the kiln, you can mix kiln wash to a paste consistency and fill the hole allowing the paste to dry before firing.
Firing Ceramics
If your piece is going to last, be glazed or be of any use, it must be fired. Using kilns, firing, getting access to a kiln can be a complicated issue. Learn about firing your piece, getting your piece fired and ensuring that you are firing your piece or pieces properly at ceramics.lifetips.com.
Preventing steam burns
When quenching hot pieces in water, be aware of the danger of burns from steam. The heavy yellow insulating gloves are useless when wet, so be careful to keep them out of the water. Hollow pieces with small holes should never be submerged in water. If only a small amount of water can get inside the piece, the water can be turned to super-heated steam and expelled at great pressure creating the potential for serious burns.
Things to remember when firing
Keep your kiln away from your home living area. Wherever you locate it, it should be well-ventilated so the fumes that are emitted will not bother other members or the family.
Overglaze/lustre plates
Plates with overglazes on them will fire better when supported by a plate holder or placed on edge to permit even heating.
preventing thermal shock
If you are firing pieces such as cups and plates that will be subject to thermal shock (rapid changes in temperature), you should fire them to cone 04.
Preventing sagging
You can help prevent porcelain cups from going out of shape during the firing by kiln washing the rims and placing one cup on top of the other, rims together and handles opposite for balance.
Second firing
By firing a piece a second time, you can create different effects with your antique, art and some specialty glazes.
Peep holes: uses
Peep holes in your kiln are used for observing the firing process so that you can see when the cone bends and when it is time to turn the kiln off if you do not have an automatic shut off.
Peep holes have a secondary function in that they draw oxygen into the kiln, preventing reduction, and carry off smoke and water vapour.
Stacking and lighting
Additional notes on Pit Firing
Preheat your work ( bisque or green ) around the edge of a fire until the pieces are too HOT to handle by hand.
Quickly stack the kiln over hot coals with at least of 4 to 6 inches of fuel, wood 2 to 3 inches in diameter. Juniper is best, then cat claw, pine is not bad -- never use oak. This wood should be very dry. If you have dung available, you can use that as it will give you a firing to around 1100o F. Dung burns hotter and longer than wood, but it was not used in prehistoric times.
The kiln should be covered with 2 to 4 inch diameter wood and dung to at least twelve inches thick. It will be smoking and flaming a bit at this point, the whole kiln must be stacked and fueled within 3 to 5 minutes, before it bursts into full flame.
NO fuel may be added after the initial stacking and ignition of the fuel, as it will take energy from the burning fire and reduce the temperature.
If you have straw bales available, put sheaths on as a final covering just as the kiln ignites fully, it will help keep in the heat and aid in a bit of soaking (being mostly silica, it forms a bit of a blanket over the kiln.)
This firing will be over in less than an hour. Mica is a good addition to the clay body as a good refractory. Saw dust as grog will aid in 'internal firing'. (Mimbres ceramics may well have had vegetable materials in it.)
Kiln shelf varieties
Shelves are made especially for the kiln in which they will be used and come in a variety of shapes. Half shelves increase kiln space by making it possible to fire tall pieces beside one or more layers of small pieces.
Mica as stilts
If you use pieces of mica under touching parts of dryfooted articles, you can keep the glaze from dripping onto shelves.
Electronic controller tips
Some people wonder if it is ok to fire a kiln during very cold weather. It is, but Skutt in particular recommends warming the Electronic Controller (if you have one) to at least 40 degrees F with a space heater or hair dryer. Your kiln will have to work a little longer to get to temperature.
Kiln sitter: description
A kiln sitter is a mechanical control that will shut off your kiln when a pyrometric cone reaches maturity and bends to a predetermined angle. When the kiln sitter is off, the kiln will no longer fire
kiln sitter troubleshooting
Even though your automatic kiln sitter is adjusted at the factory, it may be jarred out of adjustment during shipping or delivery and should be checked again.
You can have an overfiring if a piece falls against the sensing rod of your automatic kiln sitter and prevents it from falling.
For red glazes
Most of the problems with red glazes occur during the firing process.
The recommended bisque you should use for red glazes is that which has been fired to cone 04 or 05.
You should not fire raw clay bodies with reds.
Soaking in glaze firing
Having a soak (or holding the temperature) can be very useful at the end of firing. A soak may last from 15 minutes to an hour or more. This helps even out the temperatures throughout the kiln, and ensure all the pieces have achieved the right temperature. This is particularly useful if the kiln is densely packed. Soaking for too long can overfire ware, so this must be taken into account.
Saggars
If you are not doing a true prehistoric firing, you may use large tin can saggers with "Kingsford" charcoal in them to create a variety of atmospheres. I have had rotten luck getting a good reduction by covering up the fire with sand and dirt. Sometimes it works and sometimes it doesn't.
Junior Cones
Junior Cones are typically used in Dawson Kiln Sitter shut-off devices. The target cone is placed on the kiln sitter cone supports and under the sensing rod. When the Target cone begins to bend, the sensing rod lowers releasing the trigger weight on the outside of the control box. When the trigger weight falls, it shuts off the kiln. Junior cones can be used in cone plaques in a similar fashion to the senior cones shown above. Junior cones used in this way are more difficult to spy and fire hotter than senior cones of the same cone value.
Reduction
When we burn gas with ample oxygen, the by-products of combustion are carbon dioxide (CO2 ) and water (H2O). These pass through the kiln without much effect. If we fire the kiln with insufficient oxygen by cutting down the air to the burners or by sliding in the damper to restrict the flow of air through the kiln, there isn't enough oxygen in the kiln atmosphere to make all the combustion by-products be H2O or CO2.. Instead we get some hydrogen (H2 ) and some carbon monoxide (CO)--this is called a reducing atmosphere. Hydrogen and carbon monoxide would very much prefer to be water and carbon dioxide, so they attack the oxides in the clay and glazes to get the oxygen they need. Thus the metallic oxides are reduced--they're left with less oxygen and this alters the color of the clay and glazes. In reduction firing copper often tends toward red, and iron bearing clays will turn grey. Most metals can exist in many states of oxidation, and several of those can exist at the same time in a glaze, so in practice the results of reduction firing can be quite unpredictable. Much depends on the degree of reduction--how starved for the oxygen the kiln was when the glazes were melting.
Firing red glazes
Do not load red family glazes, green, yellow, or yellow-green glazes, metallic or lustre glazes in your kiln with greenware.
Allow a minimum of two inches of space around red glazed pieces.
Position in kiln
Porcelain pieces should be kept at least two inches from the side walls or elements of your kiln during firing.
Description
Pyrometric cones: These are used to accurately measure the amount of heat work that has taken place in the kiln.
They are made from precisely measured combinations of clay and flux materials which reliably begin to fuse and bend at a predetermined heat.
Early shut off
If something happens to stop the firing early, such as a power failure, you can simply restart the kiln. If using cones, they will continue to absorb heat and will still fall at approximately the correct temperature. With an electronic kiln, the results will also be close unless the kiln has shut off during the final hour or two of firing. This is because most of the heat work happens during that time. If the kiln shuts off toward the very end of firing, you should look at your witness cones to determine when to turn the kiln off.
Glaze firing without a vent
As in bisque firing, the kiln lid should be propped for the first few hours, or until the kiln reaches 1000 degrees F. In addition, the top peep hole plug should stay open during the whole firing. (These steps are both unnecessary if firing with a vent).
Keeping a firing log
It is very useful to keep a firing log. Start your log entry by describing how the kiln is loaded (types and sizes of items, density of packing.) You will find that the more densely the kiln is loaded, the longer it will take the kiln to reach the appropriate temperature. You will learn how to adjust for this with your particular kiln. Typically you will add a longer soak at the top temperature when the kiln is more densely packed.
nichrome wire'
Did you know that' nichrome wire' is what kiln elements and stilts are made with? This is a special combination of Nichol and Chromium that will withstand the high temperatures of a kiln.
Removing from the kiln
Don't remove fired pieces from your kiln until they have cooled enough to handle comfortably. Removing pieces too soon will cause cracks in ware and singed fingers.
Firing time
It is difficult to predict when your kiln will shut off but you can narrow the time down to an approximate hour. The number of pieces you are firing will vary this a great deal.
purchasing kiln furniture
In purchasing kiln furniture, avoid triangular posts that are taller than 10” or square posts that are taller than 14”.
Pyrometer v cones
A pyrometer should not be used as a substitute for pyrometric cones to determine the maturity of the ware.
Firing Rate
Fire reds/yellows very slowly, especially during the early part of the firing when the organics are still burning out of the clay. Firing slowly ensures that carbon monoxide isn't being created faster than it can be replaced with fresh air.
Glazing to eliminate drips
Of course, glazed areas cannot be put directly on kiln shelves, or they will fuse with the shelf. If doing low fire, place your pieces on firing stilts. If doing mid to high fire, only glaze within 1/2" of the bottom of your piece (sometimes more if you have a runny glaze.) Wax the bottom of the pieces before glazing to make it easier to remove the excess glaze.
Always watch your kiln
Remember, that you do not go away and leave your kiln firing an-attended.
Kiln wall thickness
Kilns with less than three inch side-walls may require one cone hotter temperatures than those with thicker walls to do the same job.
Supports for firing glass.
Supports for glass firing may be made from fire brick cut to the proper shape or you can purchase a mold for that purpose.
Low heat switches
The low heat position of the switches on a kiln provides a means of limiting the rate of temperature increase in the early part of a firing. A fairly slow but even heating rate is important in preventing breakage.
cone troubleshooting
If the rate of temperature rise is slow during firing, a cone may bend at a lower temperature than if the temperature rise is rapid.
Bisquing cast pieces
Bisque should be fired at least one cone hotter than glaze for cast pieces.
Incomplete Burn Out
Incomplete burn out can result in several firing problems including: Bloating of the ware - if the temperature is hot enough, the outside of the piece will seal up before all the gases can escape. As the body becomes plastic due to glass forming, gases trapped inside the body expand with heat and cause bloating and sometimes cracking of the ware.
Glaze defects, such as pinholes - the escaping gases will push through the glaze surface and cause bubbles which pop. If these do not heal, then pinholes will result. Appearance of fired bisque - where carbon burn out is incomplete, the piece will have a grayish cast (white bodies) or may have greenish cast (red bodies). The body will also be more porous and weak.
Check all pieces
Check all pieces after removing from the kiln. Defects can occur and you should note them when you take them out of the kiln and avoid the same mistake in future firings.
UV protection
You should wear dark glasses with UV protection when looking into a hot kiln.
Moving pieces
Be careful placing and moving your pieces around. Instead of grabbing them at their tops, lift them from their bottoms.
Troubleshooting: fading colours
If you fire your over-glazes and china paints too high the colours will fade.
Stopping warpage and melting
The thing to remember when firing glass is that it must be cooled quickly to keep pieces from warping or melting. Even if you do cool the pieces properly, it may be necessary to have supports for a particular glass piece to keep its proper shape.
Troubleshooting: cracking/peeling
China paints must not be applied heavily, as this will cause cracking or peeling when fired.
All china paint colours do not reach maximum colour and brilliantly at the same temperature, even when they are fired on the same ware. It is important to know which colours to fire first to the higher temperatures so not to burn out the lower temperature colours.
overfiring troubleshooter
An overfiring in your kiln can occur if the Kiln sitter is our of adjustment or if the free fall of the weight is impeded.
Soft bisque
Soft bisque is greenware fired to cone 018. You should keep a couple of ‘pencil' shaped pieces of this soft bisque around to fill cracks or holes in your bisque pieces before decorating.
To remove a stuck lid
If the lid of a teapot is stuck after firing, you can try this method: Fill the pot about 1/4 with water then put in the freezer. When the water has frozen, let it thaw slightly and then thump against the lid. If the lid is not fused, it may come off.
Description
A pyrometer is used to measure the progress of firing and determine accurately the best time to turn switches on the kiln to high, low or medium.
Peephole plug fit
It is not necessary for your peephole plugs to fit tightly, as it is good to have some air entering the kiln at all times. Peepholes should be as small as possible to control the air flow, but large enough to see the cone clearly.
Carbon burnout
Carbon burnout: Materials used in ceramics contain naturally occurring impurities that can affect the colour, appearance and maturing temperature of the product. Carbon, found in most clays, is normally considered one of these impurities. Carbon can also be present in the additives and binders which make up clay bodies, slips, decals and lusters.
Sawdust Firing
Sawdust Firing
In sawdust firing the piece is packed in a kiln structure surrounded by sawdust. The sawdust is set alight and allowed to burn until combustion ceases from lack of fuel. As the sawdust burns, rich patterns of carbon smudging are left on the surface of the piece. Pieces fired in sawdust have a natural and direct quality that can be very appealing.
The firing is done in a simple firing container rather than in a true kiln. The container requires only a top and walls with small openings to allow air to reach all parts of the densely packed sawdust and smoke to leave during combustion. A sawdust "kiln" can be made from bricks or a metal trash can with holes pierced in the sides with a sharp tool to allow air to enter.
Pieces intended for the sawdust fire can be painted first with Terra Sigillata, which takes the sawdust well and emphasizes the fire markings.
Firing porcelain
Porcelain becomes non-porous when fired, so it must be treated as you would a glazed piece.
painting glass
When painting glass such as bottles, use the same loading and firing procedure as you would for overglaze
Kiln wash: description
Kiln wash is a non-hardening compound that consists of a combination of dry powder and water, and is applied by painting a fairly thick coating to one side of shelves and the floor of the kiln.
Measuring temperature and heat work
Measuring temperature and heat work.
Temperature is usually measured as degrees centigrade using a pyrometer. With a simple pyrometer and firing chart, you can monitor the firing's rate of climb and its temperature at any given moment. By using programmable kiln controllers, you can pre-set a firing schedule automatically.
Cones register the work done by the heat put into the kiln during a firing. Glazes mature due to “heat work”, not temperature, and therefore cones that measure the cumulative effects of heat (heat work) are the best method of monitoring firings. Pyrometric cones bend when their firing value is reached, so set them where they are visible through a spy-hole.
automatic shut off
If your kiln is equipped with and automatic shut off you should coat the top of the cone support and the bottom of the sensing rod with kiln wash to prevent sticking of cones.
Kiln wedge
Most kilns come with a wedge cut from fire brick for propping open the kiln lid in the initial stages of firing. It may also be purchased separately.
Kiln settings
A kiln will not over-fire when you have all the switches on low because it does not attain enough heat inside. It reaches about 100 degrees.
Kiln care/outside
It is important to keep the outside of kiln clean too. The elimination of dust is of particular importance as it can settle on ware and cause defects.
Adding shelves
When adding a shelf to the kiln, hold it by the edges and hold it carefully so the sidewall of the kiln will not be damaged.
Using medium setting on electric kilns
The medium heat setting on your kiln will build-up the temperature in the kiln, but as it is an uneven heating, it should be used only for equalizing the temperature in a one switch kiln. In two switch kilns, use medium for building up the temperature gradually through the middle range, but always in a sequence that will keep the bottom of the kiln hotter than the top.
Kiln hot spots
Your kiln is hotter near the sides and the side of the ware next to these walls will tend to shrink more than the opposite side. This is particularly so when firing porcelain. This can be used to your advantage when firing figurines that tend to warp in the firing process. Turn the inside of the porcelain figure away from the elements and heat will help to hold it straight.
lid of your kiln
If the lid of your kiln becomes chipped or damaged, simply smooth the surface of the hole with sandpaper and blow or vacuum clean. A seal coat may be spread thinly over the exposed brick to seal it and prevent dusting.
Setting a limit timer
If your kiln is equipped with a limit timer as well as an automatic kiln sitter, you should set the timer for about half and hour beyond your expected rising time.
Electric kiln connection
A kiln will operate properly only if the circuit you use is correctly connected. If you plan to use an existing circuit, be sure that it is large enough and is not used for other purposes while the kiln is firing.
Firing reds
Reds mature at a lower temperature than other colours and are fired after the other colours have been fired. Reds and yellows should not be fired side by side
Cones: Description
Cones come in two sizes. Small cones 1-1/8” long are used in kiln sitters and large cones 2-1'4”long are placed throughout the load, and at the level of the kiln sitter to determine temperature variations and the accuracy of the kiln sitter. The latter are called witness cones. The shorter length and higher density of the small cone makes them stand longer than larger cones when placed on ends. However, when used in the kiln sitter, the small cones bend at the same time as large cones standing.
Kiln sitter convenience
The primary value of a kiln sitter is the convenience of being relieved of watching the cones and produces a uniform firing which can be consistent from one firing to the next.
Total firing time is controlled by the length of time the kiln is left on low. The medium heat position should be used only for balancing the heat between the top and bottom of a kiln.
economical use of posts
For the most economical use of posts and kiln capacity, place ware approximately the same height in each layer
adjust and operate your kiln sitter
You should learn to adjust and operate your kiln sitter before firing. You should recheck this periodically.
Choice of clay
White clays usually are more pure (have less organic content). Therefore, they may give better results.
Kiln placement
Kilns should be placed at least 12-18" from all walls during firing. Remove all combustible materials from the area around the kiln.
Post fire reduction
Pots may be pulled out when still at 400o F and refired in an open area with Creasote Bush bundles in the interior and loose open fire of Juniper or Cat Claw on exterior for nice interior smudges, just like the Mogollon wares of the 1300's.
Gauging temperature by sight
If you look in your kiln through a peep hole, pieces that are near the same temperature will be the same colour. Brighter objects are hotter than darker objects.
Reverse the base
The bottoms of most kilns are reversible and may be turned over if one side gets too damaged.
Loading the kiln
When loading your kiln, do not pack it too full. It is a bad habit to stack things inside of eachother, on top or in any combination to save space. Too much of this and the stress caused may result in cracked bisque.
firing bisque
Fire bisque at least one cone hotter than glaze and even more if glaze can be applied easily.
Glazed ware and bisque ware should not be fired together unless both mature at the same cone. Even then, be sure that the glaze is loaded in the bottom of the kiln.
Firing Schedule
The same firing schedule is used for both porcelain bisque and porcelain glaze. The low heat preiod of your kiln can be shortened by two or three hours for glaze firing.
Electric: Wiring
If you are having your home wired, make the following checks of the job before the electrician leaves. Place all switches on high-heat position and watch the elements with the kiln lid open. The elements should come up to a dull red colour over a period of several minutes. If any element or any pair of elements turns bright red within 20-30 seconds, turn the kiln off and have the electrician check to be sure that the ground neutral wire of the circuit is connected to the proper terminal at the fuse box or circuit breaker box and the neutral terminal of the receptical. Failure to do this can cause damage to the elements or to the switches.
Kiln washing the bottom of your kiln
When you are giving your kiln bottom a new coat of kiln wash, be sure to put cardboard against the sides to protect the elements.
Kiln element care
Kiln elements will become brittle after a few firings, so care should be taken if handling is necessary.
Salt firing OH&S
Due to the escaping hydrochloric acid, which is highly toxic, the utmost care must be taken and a good mask with a gas filter (a dust filter is not good enough!) must be worn. Of course such firings must be done in the open, and not in residential areas. Due to the environmentally unfriendly nature of the salt firing, some people prefer soda firing.
Electric: loose elements
If an element pops loose from a groove, this may be due to the loosening of the pins caused by expansion and contraction of the element during heating and cooling.
Safety
Safety
* You should be near the kiln while it is firing, especially toward the end to make sure it goes off on schedule.
* A limit timer is a safety device which is set to turn the kiln off after an amount of time that you set. For example, if you expect your firing to last 8 hours, you may set the limit timer to 10 hours. At 10 hours the kiln will turn off. This can prevent a major catastrophe if the kiln sitter or electronic controller fails.
Electronic controller
An electronic controller turns up the kiln, and also turns off the kiln. It can even be used to do a controlled cool down. You can use a pre-programmed Cone Fire mode, or program your own individual segments. The pre-programmed modes automatically turn the kiln up when it is safe to fire the pieces quickly, and down when the clay is at a point where it should be fired slowly. To determine when to turn the kiln off, the controller uses temperature charts to approximate when the appropriate heat work is done (a certain cone level is reached.) This can vary somewhat based on things such as the density of the load being fired. So it is still important to monitor your firings at least periodically by using witness cones inside the kiln. You will learn whether you need to make adjustments to achieve the desired cone.
Kiln Jacket Description
The Jacket on your kiln becomes quite warm during firing and is too hot to touch. It helps to hold and reflect heat back into the kiln.
Installing your electric kiln
When installing wiring for hooking up your kiln, be sure that the proper gauge wire is being used to carry the load. If the proper wire is not used your breaker switch or fuses will go when the kiln heats up to maximum temperatures.
thermocouple
A thermocouple is the measuring unit of a pyrometer which is inserted into the firing chamber of the kiln through a hole that is drilled in one of the peep hole plugs.
Stilt Marks
If, after firing you have marks in the glaze where the piece rested on a stilt, you can remove these with a rubbing stone. An electric grinding wheel may also be used.
Bar stilts
Did you know that ‘bars are not places where you drink? They are a variety of stilts in the shape of a bar which can vary in length and have sharp nichrome wire points protruding
Kiln Sitter: What it does
A kiln sitter is an electrical switch which is turned on by hand and turned off by the action of a cone bending under a rod.
When the switch on a kiln sitter is turned off, the power to the kiln is interrupted so that no more heat can go into the kiln.
Firebrick behaviour
The insulating firebrick expands and contracts with each firing. The expansion and contraction is necessary for the long life of your kiln, but some hair line cracks will appear in the brick while the kiln is cold. Do not be concerned about them, as they close tightly when the bricks expand when heated.
Opening the fired kiln
Don't open the kiln until it is below 150-250 degrees F, or thermal shock may hurt the ware and/or the kiln elements. You should be able to touch the pieces before you unload them.
Hand modeled pieces
Hand modeled pieces should be fired alone and on a slow firing schedule.
Ceramic glaze
Ceramic glaze will fire like ceramic bisque, except that it usually fires to a lower cone and less time will be required.
Ensure the right clay, glaze, fire
The worst thing you can do is fire low fire clay or glazes at high fire. The clay and/or glaze will melt all over your kiln and can cause major damage! If you have both low fire and high fire materials in your studio, you might want to mark your pieces differently on the bottom so you don't forget and get them mixed up!
Ventillation
A kiln must be vented until all the china paint oil and other organic materials have been burned out
Mini shelves
If you have half shelves, it is perfectly acceptable to have them staggered if it fits the size of your pieces. You can even use pieces of broken shelves to add additional mini-shelves.
Before plugging in your kiln
If you are about to plug in your kiln either for the first time or after moving it, be sure that all switches are off before doing so.
volatilization
In the firing process, ‘volatilization' means the action produced within a firing where some materials are changed from a solid to a gaseous state, such as water turning to steam.
Checking new Electric kiln
You should always inspect a new kiln when it arrives. Vibration and jarring in shipping sometimes causes elements in the kiln to become dislodged.
Glass slumping and ceramic firing
You can fire glass and ceramics at the same time as long as you use the proper temperatures for each item.
Loading around elements
The heating elements must be at a greater temperature than the rest of the kiln in order for heat to be transferred. This causes the area around the elements to be considerable hotter and no piece should be closer than 1” to a coil.
Caring for kiln elements
The elements in your kiln are designed for a long life and are capable of many firings. The life span can be shortened by contact with materials such as bits of bisque, glaze, cones, metal or kiln wash, keep elements clean.
Kiln Sitter
A kiln sitter works as follows: a jr cone of the appropriate number is set inside the kiln sitter box, the cone is held by a retaining bar and a moving rod. When the cone bends (because it has absorbed the correct amount of heat), the rod falls. The rod activates the control mechanism which turns off the current to the coils.
Kiln sitters
Kiln sitters are called automatic shut-off, but it should not be left unattended. An uncontrollable accident such as greenware falling against the end of the kiln sitter tube may cause an over-firing which could damage your kiln. If this happens you should be near at hand to shut it off manually, within an hour of estimated firing time.
Removing stilt marks
You can remove sharp points of glaze left by stilts by rubbing with a carborundum stone.
Kiln lid gap
A gap between the lid and the bottom of the kiln during firing is caused by the distortion of the kiln from thermal expansion.
the top of the kiln
The top of almost every kiln is coated with a thin, high-temperature coating. This makes the lid more durable and prevents dusting of the brick particles on the pieces in the firing chamber
Suggested firing guide: Earthenware gloss (Lead)
Suggested firing guide: Earthenware gloss (Lead)
1080c (cone 04b)
130c per hour up to 1080c
Hand-modeling
If you have students that like to hand model or want to try it yourself, handle the firing of these pieces the same way as your normal ceramic firing.
slumping glass
Glass sagging involves placing of a piece of glass such as a sheet or a bottle in a mold and heating it until the glass softens and takes the shape of the mold.
When sagging glass in a mold be sure to dust the mold with whiting or some recognized separation agent or the glass will stick to the mold.
Things to learn
Special knowledge of woodfiring is necessary -which wood burns best? What different types of wood will result in special ash glaze effects? Where in the kiln should pots be placed for certain effects? (This is an individual matter for each and every kiln and firing cycle). How long will a firing take? (It may actually be a matter of days or even weeks.) What firing cycles are necessary? If you want to know more, check out these tips on how to build a woodfiring kiln.
Cleaning the Stainless steel
Stainless steel will discolour from repeated firings, but can be cleaned
Common sense
Keep curtains, paper walls, inflammable materials and children away from your kiln.
Manual Kiln bisque schedule
This is a typical firing schedule for a bisque firing in a manual kiln.
Bottom switch on low for several hours if necessary (candling).
Turn on all switches to low for 3-4 hours.
Turn all switches to medium for 3-4 hours.
Turn all switches to high until kiln has reached temperature.
(Note, if your kiln has multiple sections, you may turn them up individually if you want slower heating.)
Measuring Heatwork
Heatwork is another critical factor in the colour development of decals. Fading, Shifting and dullness are signs of too much or too little heatwork. This is also true when decals rub off after firing. (White or blank spots or burned off areas are generally related to application, not firing.) Use witness cones to measure heatwork and to check the heat distribution in the kiln. Firing to a temperature or firing to a Kiln-Sitter cone may not give the same results as found with a witness cone next to the ware.
Proof Your Greenware in Your Kitchen Oven
Since excess moisture can cause a piece to explode in the high temperatures of a kiln, it's a good idea to pre-bake it in your kitchen oven. This will assist in the drying process in a gentle way, so that you can be confident of your greenware being absolutely bone dry when you put it into your kiln.
Just start it off at your oven's lowest temperature, and then add another fifty degrees every twenty minutes up to 350 degrees. Hold it at 350 for half an hour, then turn the oven off and let everything cool. Once it's cool enough not to burn you, you can transfer it directly into the kiln.
the most accurate indicator of proper firing time.
Since cones are made from similar material to the ware being fired in a kiln and are subjected to the same heat treatment as the ware being fired, it is the most accurate indicator of proper firing time.
Mounting cones
If you use a cone mounted in a piece of clay to fire your kiln, it should be mounted at an 8 degree incline from the vertical.
What is a kiln
High-temperature furnace used commercially for drying timber, roasting metal ores, or for making cement, bricks and pottery. Oil- or gas-fired kilns are used to bake ceramics at up to 1,760ºC/3,200ºF; electric kilns do not generally reach such high temperatures.
Cone placement
If your kiln is not equipped with an automatic shut off and you do not desire to have one, place your cones directly in front of the peep hole. The cones can be inserted in a pat of clay.
Firing Temperature
Porcelain is usually fired to cone 5 or 6 and requires a slow firing schedule.
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| Susan Sayour |
