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After the mold has been drained of all its excess slip, the mold has to sit and dry. The mold pulls the water out of the clay and dries the piece out.
If you leave a piece in the mold too long, it will dry out too much and crack.
If you don't leave it in long enough, it won't let go of the mold and will probably tear or it will collapse under its own weight.
When I pour, I run a fan across the mold to help dry things out. Six to eight hours later, I go back and take the pieces out.
Larger pieces take longer to dry -- as much as a couple of days.
You can tell if a piece is ready to come out of the mold if it has pulled away from the sides of the pour hole.
When it is time to open the mold, remove the bands from around the mold and gently pull the mold apart. If the piece is dry enough, it should release immediately.
Try to pull the mold halves straight away from the piece since the clay is still soft and can be easily scarred or marred. The piece should handled carefully since it so soft.
When the piece comes out of the mold it will be a dark gray. Set the piece up and let it finish drying until it turns a light gray.
After the piece has dried to a light gray, it is ready to clean and fire in the kiln.
Stir your slip thoroughly with a clean stick, but without introducing air bubbles into the liquid. If you have a large bucket, pour an amount into a smaller container, which will hold enough slip to fill your mold. Pour the slip through a sieve into your dry mold. (Straining the slip eliminates any lumps from getting through, which would otherwise stick to the wall of your pot.) You will be able to observe the slip adhering to the mold and thickening. At this stage it is possible to gently knock the sides of the mold (let's say with the ball of your hand or with a rubber mallet) to free any air bubbles which may be trapped in the clay. These will rise to the surface.
As the clay wall thickens, water is absorbed by the plaster and the level of the slip will drop. Continue pouring small amounts of slip into the mold to top up the level to the top edge. When you think that the correct wall thickness is reached, pour all the slip out of your mold back into your bucket. It is possible to time the period the slip stays in the mold, but this will vary depending on the moisture content of the mold. The molds moisture content will increase with every cast, thus actually lengthening the time of the cast. After a number of casts, depending on the thickness of the mold and its original moisture content, it will become impossible to proceed, and the mold will have to be dried out. After prolonged use, a white substance may crystallize from the mold on drying -- this is a normal reaction due to the additives in the slip and need not be of concern.
Depending on the thickness of your clay walls and the moisture content of the mold, the ceramic object will be dry enough to be removed after several hours. As the clay will shrink, whereas the plaster will not, the ceramic form will shrink from the mold and 'pop out'. Be gentle when removing the still moist clay object from the mold, as deforming might still be possible. Fast drying methods (e.g. microwaving) usually won't work, as the slip has little green strength and is prone to cracking. Having said this, you may then proceed to decorate and fire the work as you like.
Cool Trick! You can make a nice slump mold by centering a tub of plaster in its liquid stage on your wheel and spinning it at constant speed till it sets up. Spinning a liquid in a tub produces a parabolic surface and the faster it spins, the deeper the curvature is. If the speed is fast enough for the edge of the parabola to climb over the edge it will spill plaster all over the place! Try this with a tub of water until you are confident you won't spill it over the edges.
Another way to use plaster is to pour it into and around existing objects. Every time you pour, you will be making a reverse of the object you are pouring into or around. You can pour plaster on top of leatherhard clay. For example, say you carved a relief design into a slab of clay, and you want a mold of it so you can make exact duplicates of that design. First you need to surround your slab with something to hold the plaster.
1. Bisque: You can make press molds out of clay and bisque them. These work quite nicely and clay doesn't stick to them. Carve the inverse of your desired design into leather hard clay. For example, if you want raised lettering, carve the letters out of the press mold and the inverse will be raised. Remember to make your design about 10% larger than desired, to account for shrinkage.
Having a problem breaking greenware when you are cleaning? You can prevent this by creating ‘soft bisque'. This is done by firing your greenware pieces to cone 018 before you start to clean. The bisque will still be soft enough to clean, yet hard enough to allow you to handle without easily being broken.
First, you need a mold made of plaster. The plaster mold absorbs the water from the slip.
Fill the mold with slip. As the mold absorbs the water, the level of the slip in the mold will go down requiring you to top off the mold.
OK. Here's the trick to it all: knowing when to dump the excess slip.
If you leave a mold filled with slip too long, the ceramic piece will be much too thick and heavy.
If you don't leave it in long enough, it will be too delicate and squash and tear as it comes out of the mold.
For my small figures and pieces, I like to leave the slip in the mold for about five minutes before I dump out the excess slip.
On the bigger items, I watch the mold for an eighth of a inch skin to form around the lip of the pour holes before I dump the slip.
Now, a few pieces call for the piece to be poured solid. These include things like plates, bowls, and other utility items. These molds you just keep filling until they stop sucking up slip.
How to Slipcast Ceramics
Slipcasting is a widespread ceramic technique, suitable to the manufacturing of multiples. A plaster mold is required.
If your mold consists of several pieces, tie them together with thick rubber bands or something suitable to the molds size.
Stir your slip thoroughly with a clean stick, but without introducing air bubbles into the liquid.
If you have a very large bucket of slip, pour an amount into a smaller container, which will hold enough slip to fill your mold.
Pour the slip through a sieve into your dry mold. (Straining the slip eliminates any lumps from getting through, which would otherwise stick to the wall of your pot.)
You will be able to observe the slip adhering to the mold and thickening. At this stage it is possible to gently knock the sides of the mold (let's say with the ball of your hand or with a rubber mallet) to free any air bubbles which may be trapped in the clay. These will rise to the surface.
As the clay wall thickens, water is absorbed by the plaster and the level of the slip will drop. Continue pouring small amounts of slip into the mold to top up the level to the top edge.
When you think that the correct wall thickness is reached, pour all the slip out of your mold back into your bucket. It is possible to time the period the slip stays in the mold, but this will vary in subsequent casts, depending on the moisture content of the mold.
Depending on the thickness of your clay walls and the moisture content of the mold, the ceramic object will be dry enough to remove after several hours.
As the clay will shrink, whereas the plaster will not, the ceramic form will shrink from the mold and 'pop out', if the form allows it to.
Make sure you have enough slip to fill your mold.
Have an extra empty bucket handy.
Clay and plaster do not mix -- make sure not to get plaster into your slip!
If you work with ceramics very long and like to pour molds, at some time you will spill slip on the floor. Wiping up the mess is a difficult task. One method of cleaning is to throw a handful of shredded paper on the spill. The paper will absorb the slip and when dry you can remove the spill. You might also use a large mold and place it right in the middle of the spill. Let the capillary action of the plaster work by absorbing the moisture from the slip and the clay will stick right to your mold when you pick it up
Cone 6 Porcelain Clay Slip
Ball clay 20.0
Nepheline Syenite 48.0
Water 36.0% of dry amt
Soda Ash 0.05-.1% of dry amt
Sodium Silicate 0.05-.2% of dry amt
Water 36% of dry amt
Darvan No. 7 0.5% of dry amt
This recipe is much less plastic than ceramic slip but has a faster casting rate. It will deflocculate to 1.8 specific gravity also and will fire to produce vitrified ware that is extremely strong and durable. It has a long firing range and can produce ware of excellent translucency with a clean ball clay.
Plaster makes great slump and hump molds because the clay doesn't stick. And you can make them in a variety of ways. Remember to use mold release on the object, before pouring the plaster.
You can pour plaster into objects you have around the house such as bowls and platters.
You can pour plaster into a bisque item.
Mix. You want to make sure that you aren't adding air to your plaster, or you will get bubbles which will cause problems later. So if you use an electric mixer make sure to keep the blade deep in the plaster. Or, just take your hand (a glove is useful) and place it at the bottom of the bucket, and slowly move your hand back and forth across the bottom of the bucket. This slowly wets all the particles. When you can draw a line in the plaster and it doesn't immediately flatten back out, the plaster is ready to pour.
Before casting a multiple-piece mold, take it apart and examine it so you will know which piece should be removed first after the slip has been poured and the greenware is ready to be removed. To help you remember the disassembling order in a multiple-piece mold, use a waterproof marker to number the pieces. If there is something special that you must know about a particular before casting it, such as “drain with straw”, mark the outside of the mold with a waterproof marker. Once you have examined a new mold for any breakage and are ready to pour. Look it over one more time to see if it has any special problems you will have to cope with such as a small pour hole, undercuts, tricky draining etc.
When using Murphy's Oil Soap or soft soap sponge on the soap with a sponge full of hot water, rinse the sponge in hot water, and rub again. Repeat this process at least 5 times. You will see the water start to bead up on the plaster surface. Once this happens, repeat twice more for insurance. Some more unusual methods of mold release are using shaving cream which is said to leave a lovely waxy film when dry. Or using a thin clay slip; the plaster absorbs the water and leaves a film of clay as a barrier.
To slipcast a ceramic object we need so-called 'slip', which is liquid clay. Slip comes in various flavors, as do other clays: earthenware, midfire & stoneware. Slip is not just powdered clay thinned down with water, but rather a liquid clay with special additives. These additives, such as sodium silicate, keep the slip liquified with as little water content as possible. This has the effect that the slip will be fairly viscous after standing in the bucket for a while. After some vigorous stirring it will become quite thin.
The purpose of mold release is to form a barrier between objects so you can separate them later. There are commercial mold release agents available, but people often use Vaseline, liquid hand soap, Pam Cooking Spray, Crisco, Vegetable Oil, liquid soap, or Murphy's Oil Soap. Vaseline is thick and stiff, and thus tends to leave brush marks behind.
You will need potting plaster for molds,(it is stronger and finer), for a bucket of plaster you will need to fill the bucket to just under 2/3 water, then, wearing a mask, as plaster is toxic, consistantly shake in handfuls/containerfuls of plaster evenly over the water the plaster will fall and settle. When the plaster is forming islands, ie, lumps of grey surrounded by water), shake the plaster onto the water until all is absorbed and the top of the bucket is covered in plaster. Wait until all plaster has gone grey (there should by this time be no water), then stir with your hand, fingers slightly splayed, and move your wrist from side to side, in order to mix the plaster, (this eliminates air bubbles). When all the plaster has been dissolved it is ready to pour. Tap the bucket all around. If you have some air bubbles on top of the mix, either scum off with a paper towel or spray with a mix of water and metho 10%. Once the plaster is disolved, time is a crucial factor. You must get the bucket poured and rinsed within minutes. For this reason organisation is imperative.
The Plaster, once poured will heat, then it will cool, once cool it is ready to take from the mold.
Bisque also makes great slump and hump molds. These can be thrown, or handbuilt, or molded from a found object. You can use found objects (mixing bowls, platters) directly by coating with vegetable oil, PAM, WD-40, etc. or by covering with plastic wrap or newspaper. Usually you will use these as slump molds, as the insides of your bowls and platters have the nice curvature you are looking for.
It is helpful to apply a thin coating all over the surface and allow it to set a little before pouring the final amount necessary for the mold. If the air bubbles come to the top of the thin surface, they will be away from the actual surface you will be using. (This is especially useful if you are pouring upside down, i.e. will use the bottom surface of the mold you are pouring.)
Recognizing Under Deflocculation
If the slip is gelling after a few minutes or livering while mixing, more deflocculant is probably needed. Be careful not to add too much; this is a common mistake and will mean you will have to make more of the powder mix and add it and more water to counterbalance the oversupply of deflocculant. If the slip does not settle out overnight, then you can rest easy, it is not over deflocculated.
If the slip has not thinned after an addition, then there is already enough present. Sometimes a very small addition of water will thin the slip dramatically.
3-D Casting and Tooling
Moulding Plaster (Casting Plaster) - Often referred to as "casting plaster," moulding plaster is a good general purpose plaster where expansion control, hardness and strength are not of major importance.
White Art Plaster - Similar in working properties to Casting Plaster but with a surface hardening agent that reduces paint absorption.
Tuf Cal Plaster - A unique polymer-modified high early-strength fibered plaster. Provides higher green strength and greater chip and impact resistance than White Art or White Hydrocal. Ideal for hollow cast products.
Drystone - A fast setting gypsum cement that provides strong, durable casts without the need to dry the cast pieces. Drystone offers an environmentally sound alternative to resin-based products and casts with reduced breakage compared to conventional plaster and gypsum cement.
Fast Cast - Exterior gypsum cement. Fast setting cement formulated for exterior decorative statuary that can offer increased casting production for more than 300% versus regular Portland cement. Must be used with sand aggregate.
Gardencast - Designed for decorative outdoor statuary. It features low absorption, high strength and efficient casting characteristics.
Hydrocal White - A basic hydrocal gypsum cement used for statuary, glass casting and model making. It can be carved or added to.
Hydrostone - Used extensively for very durable statuary and 3-D casting. Especially suitable where high strength and resistance to water absorption are necessary. Also used for ram press molds.
Ultracal - A low absorption gypsum cement for case molds. Specially made for close tolerance tooling. Provides the greatest hardness, accuracy and freedom from efflorescence of any gypsum cement on the market.
Mix the slip for several hours for best results. It is only by putting energy into the slurry that you can thoroughly wet every particle and extract the best performance. Less deflocculant may be necessary if the slip is mixed for a longer period. This is an advantage because some deflocculants, like sodium silicate, attack your molds and the less that you use, the better. After the slip stands overnight and is mixed for a few minutes the next day, it will usually cast better.
There are many ways to do this. You could put the slab of clay in a box, or you could build a moat around the slab with clay, or you could build a wooden frame around the slab of clay, or use a piece of linoleum or flexible metal rolled into a circle around it. Whatever you use, make sure it is attached well so it doesn't come apart (with string or strong tape), and make a strong connection at the bottom with a roll of clay so the plaster doesn't run out.
Get More Shapes by Modifying Molds
You can modify the profile of a plaster mold after it has been made, and before it is dry. Say you want a rounded bottom hump mold, but only have mixing bowls with flat bottoms. Pour the plaster into the mixing bowl, and when it is has set but is still rather wet, put it on your pottery wheel (flat side down). Center, and trim the bottom into a round shape with a trimming tool.
Large molds may require gallons of slip. Instead of pouring the slip in by hand, try attaching a plastic tap to your bucket(s). Alternatively, a home brewing drum already has a tap attached and is ready-to-go. (It may need to be elevated so that the spout is above the mold opening).
Do not try to remove the seam line from a piece of greenware all in one pass. It is best to go over the line several times until the line is flush with the rest of your piece.
After the bulk of seam lines have been removed with a tool, the next step is to sand the lines with a sander sponge.
The final step in cleaning is to sponge your piece lightly with a damp sponge. This will guarantee smooth greenware and remove excess dust.
The only exception to the sponging technique in cleaning greenware is when you plan an antique wash on the finished bisque. The antiquing colour may concentrate in sponge marks.
If, after cleaning, you see detailed areas that are not clear from either a bad casting or from your cleaning, they should be scribed or carved back into the greenware.
Remember that the bottom is a part of your piece too! The same care should be taken in cleaning this area of greenware.
Be sure to test how well your piece of greenware sets on its base. By cleaning the bottom effectively, you finished piece will not wobble.
A piece of sandpaper attached to a table top is an effective aid in leveling the bottoms of greenware pieces.
After you have completed cleaning a piece of greenware, set it aside for a couple of hours and then go back and inspect it for anything you may have missed.
Things to watch out for when slipcasting:
make sure your mold is dry
make sure you have enough slip to fill your mold
have an extra empty bucket handy
place ample newspaper on your working surface
clay and plaster do not mix! -- make sure not to get plaster into your slip
prevent any foreign matter (e.g. earth) from getting into your slip
be aware that slipcast greenware has little green strength
Clay 25 kg
Water 11.4 litres
N42 Sodium Silicate 130 grams
In a 30 litre container place 11.4 litres of water.
Add Sodium Silicate mixed with 200 mls of water.
Agitate as you add the clay.
Check litre weight is 1750 grams/litre. If litre weight is higher than 1750, add water. If litre weight is lower than 1750, add clay.
Slowly add Dispex mixture (20 grams diluted with 40 mls of water), as required to maintain good mixing fluidity.
Mix for one hour then check if more dispex mixture is needed for pouring fluidity.
Allow slip to mature for 24 hours before remixing, sieving through an 80mesh screen before use.
Makes approximately 20 litres of slip at 1750 grams per litre.
This is the weight per unit of volume of the slip. Specific gravity is defined as the comparison of a liquid's weight with the weight of an equal volume of water. In metric it is even simpler: water weighs one gram per cc (ccs and milliliters are the same). If a liquid weighs 1.8 grams per cc, then it has a specific gravity of 1.8; it is 1.8 times heavier than water. A slip with a specific gravity that is too high is said to be “heavy”. If the viscosity is too high, it is said to be “thick”.
The more water in a slip, the lower its specific gravity will be. The more solids, the higher it will be. As mentioned, slip with too much water will soak the molds more quickly, give slow casts, and result in excessive shrinkage that cracks the ware.
Ideally, a slip should be adjusted to a state of “controlled flocculation” where it is fluid, yet the finer particles remain agglomerated somewhat. Quality slips are intentionally less fluid than they could be. In a totally deflocculated slip the particles are so free to move that they can settle out in a very hard layer. During a cast, the smaller ones will be drawn toward the mold surface causing differences in particle size distribution and drying shrinkage across the thickness of the clay wall, resulting in drying cracks. In thin slips poor mold release can also occur because the fine clay is able to penetrate very small recesses in the mold surface and “hold on”, resisting release. If you have these problems, vinegar can be added to a slip to flocculate it a little. It is quite remarkable to see what a capful of vinegar will do for a bucket of settling slip. If a slurry is in just the right state, it should gel slightly while standing over night since this holds all particles in suspension. Re-mixing will loosen it and the skip should flow freely again.
It is possible to recycle slip, though not to the extent of clay. A limit of 10% used slip to 90% new is maximum. Be sure old slip is totally clean and free of plaster. Mix slip well, I suggest as well as a slip stirrer or mixer, you add some marbles as well. This does a great job.
You can pour plaster around an object such as a commercial tile edge piece, or a light switch plate, or numerous found items. Just remember that the item cannot have undercuts. That is, it can't have areas underneath where the plaster can flow under, or you won't be able to pull the item back out. If you have an item with undercuts, use clay to fill the undercuts. Another useful trick is to take several found objects, push them into a slab of clay (imbedding any undercuts into the clay), then pour the plaster.
Keep adding plaster, sprinkling it in the same manner. After a while the plaster will be visible for a few seconds before it disappears into the water. You are getting close... Keep adding plaster. When you add plaster and can count to 5 before it disappears, you have enough. Let it sit for 1-2 minutes to wet the plaster particles (this helps reduce air bubbles).
Add water to a bucket. It should be between cool and room temperature. You will learn through trial and error how much water to use, but most people tend to underestimate the amount of water needed. Two thirds water to one third plaster is a general rule of thumb. If you want to avoid cleanup, line your bucket with a plastic bag, then discard the whole thing at the end.
Viscosity refers to the mobility of the slip; its “thickness” or “runniness”. A slip that has high viscosity is thick like syrup and one that has low viscosity is fluid. A deflocculant is used to lower the viscosity of a slip and make it more fluid. This lower viscosity, in turn, makes it possible to add more dry material, which again raises the viscosity.
It is possible to use condensed milk to glue the parts together if your broken mold if fairly small and the break clean, without too much chipping. Pieces must fit together smoothly to use this repair method.
It is possible to mend broken molds with glue but you must be careful not to get any glue on the mold. If any does spill onto the face of the casting area, be sure to remove it with fine sandpaper so the pores will not be clogged.
When stirring your slip, use a round dowl. This is less likely to cause air bubbles.
Slip should be about the consistency of heavy cream for proper pouring.
Many people put several layers of newspaper on the table before pouring to make the clean-up easier. Rolling up the top layer usually takes care of any spills.
It is important not to dry your molds too fast. Store them on slated shelves so that the natural air can circulate around them.
When buying a new mold be sure you always open it immediately to inspect the inside for any damage.
When cleaning the inside of a mold, never use anything other than a brush. If you encounter a stubborn piece of clay simply roll some waste clay into a ball and pounce it off.
Since molds work on the principle of absorption, it is important that you avoid touching the inside with your fingers. You do not want to leave any oil residue that may clog pores.
Be sure that you remove all dried clay from the seam area of a mold. It can dry and when you put the mold back together and start pouring again, the dried clay may warp the mold and eventually cause rust spots on your molds.
You may notice a fuzz on your molds when they are being sorted. This fuzz is caused by deflocculents in the slip that have penetrated the mold. You should remove this fuzz as it will harden and plug the pores of the mold.
To make a slip thinner or more liquid I have used Calgon as a deflocculant. The deflocculant helps make the slip more fluid and easier to brush or dip. The percentage of Calgon I use is 0.5% - 2% in dry mix, but if you have a slip that is really thick I would let the slip sit in water over night to help thin it for use.
Measuring Viscosity: The 60 cc veterinarian's syringe is an effective tool for measuring this property. Just pull out the needle and time how long it takes the slip to run out from the 60 cc mark to the 10 cc mark. Calibrate the test on a sample of slip that performs well. If the syringe's hole is less than 2-3 mm, this slip will run out too slow. If that is the case, use the viscometer that comes with the Lehman Slip Test Kit. Thus, for any given viscosity there can be a whole range of possible specific gravities, and for a given specific gravity a slip can have many different viscosities depending on the state of deflocculation. The most common method of preparation is to achieve the specific gravity first using the needed deflocculant, then fine-tune the viscosity using a little more of the same deflocculant. Normally, beginners should try to achieve a specific gravity of 1.75, while experts will be able to work comfortably at 1.8 for most body types.
Various typical recipes for standard low-temperature whiteware and porcelain slips call for quite different amounts of water. This underscores the importance of being able to measure specific gravity and viscosity. Below, I have provided some guidelines on mixing a slip. Keep in mind that slip mixing is actually a three-phase project. The first few times must be dedicated to creating a workable recipe and technique. After this, a period of fine tuning will perfect a process that can be committed to paper for repeating on a routine basis. The notes to follow thus refer to the early stages of learning to mix a particular slip recipe.
The recipes given here are not 'cast in stone'. Use them as starting points. Try to understand the reason for each material's presence so you can adjust the body as needed.
A Typical cone 06-04 Ceramic Slip
Water 45.0% of dry amt
Soda Ash 0.05-0.1% of dry amt
Sodium Silicate 0.2-0.4% of dry amt
Variations of this basic recipe include the use of more talc and less ball clay, the addition of whiting to prevent crazing, the addition of a small amount of barium carbonate to precipitate soluble salts, or the addition of kaolin to improve permeability of water. In general, this recipe is very forgiving and easy to use. But beware, it won't fire white unless you use a white burning talc and ball clay (sometimes not easy to find). Part of the success of the hobby casting market is the tremendously robust nature of this simple recipe and the fact that almost anyone can cast it successfully. However, at the typical cone 06-04 this material fires to produce ware that is very weak and porous; often easily torn apart with your bare hands. Burn to cone 03-02 if possible, but don't go too high as it melts suddenly around cone 6 and becomes brittle long before melting temperature.
Molds for slip casting are made of plaster. This is because the mold must absorb water in order to form a clay body.
Slip refers to the liquid clay. Clay comes in many forms; the texture and chemical content of the slip dictate what type of clay body it forms.
Slips with a very fine texture form porcelain, which is white and extremely delicate and must be supported in the kiln during firing.
On the other end of the spectrum is stoneware, which has a very coarse texture and must be fired at a very high temperature.
Ceramic slip, which is also a clay, is what I work in and have the most experience with. It falls in the middle as far as its texture.
Putting your hand in a bucket of slip is like slipping your hand into a muddy ooze, which feels much like the mud pies we all used to make as children, (only with a much finer granular texture, anyway).
You can cut holes in the canvas where you want the feet to go. That way you can attach the feet while the clay is still wet, and not have to worry about getting it off the sling at exactly the right time, when it is hard enough to hold the shape but still soft enough to add the feet!
Recognizing Over Deflocculation
If the slip does not gel and settles out to a layer on the bottom of the container, then there is too much deflocculant. Too much deflocculant is also indicated by a thin slow cast, a wavy and gritty looking inner surface after draining, poor mold release, and cracking of the ware.
Another thing you might consider is making a form out of loosely crumpled and dampened newspapers. This allows a more loose look. Shape the damp, crumpled newspaper into the form you like, and cover it with plastic. The newspaper will dry and this hump mold will likely be useable for a while. If it starts to come apart you can squirt the newspaper with water and re-shape.
Understanding Slip Casting Bodies
To some potters and sculptors used to the immediacy and tactile character of plastic clay, the liquid clay slip casting process can seem very distant. To others, whose only exposure to casting is the local artware shops where fragile low-fire ceramic frogs and nativity sets are made, the slip casting can appear beneath their dignity.
However, no matter what temperature or type of ware you make, the casting process is not only valuable to mass-produce ware but many shapes are very difficult or impossible to produce any other way............
For this entire article: Understanding Slip Casting Bodies
Put the canvas over a plywood box. Staple the canvas to the outside of the box. Again, the curvatures can be modified by how tightly you pull the canvas. You can staple just two ends, or all four ends for different effects. If you want to make this more versatile, make a version where you can vary the amount of curvature. Attach screws or nails to the outside of the box.
Or, (I'd rather think of a better idea than go to the trouble of making a plywood box), how about using clothes baskets! They come in round or rectangular, and are inexpensive.
One way is to take a piece of cardboard tubing (the stuff that posters come in) and imbed it into the backside of the plaster before it sets. A more common way is to make a plaster foot. Before the mold has set, scratch and roughen up the middle of the plaster, then take a thick coil of clay and place it where you want the foot. Make sure your area is large enough that the mold won't be tippy. Fill the coil with more plaster and let set.
Once you have a slump mold, you can then pour plaster into it to make a hump mold. Because it is plaster on plaster, use a release agent. Likewise, you can make a slump mold from a hump mold by sitting the hump mold on the floor, making a moat and filling the space with plaster.
Terracotta Casting Body
Low fire red bodies can be very strong, I have measured a cone 1 red burning body's strength at 10,000 lbs/square inch, that's as strong as many cone 10 porcelains? Low fire iron bodies turn brown when they vitrify, the red color depends on stopping well short of vitrification. Many terra cotta bodies begin to brown out suddenly above cone 1 and many even lower. Even if the body fires red, if it is burned too hot transparent glazes will flux the surface enough to turn it brown.
You can cast with 100% Redart (a widely available clay in North America) and get a nice product. However it may be too red and could use better working, drying and handling properties. Try 25% of a large particle size kaolin, 25% feldspar, and 50% Redart. If this is deflocculated properly it casts like a dream. Use the above proportions for water, clay, and deflocculant for a starting point.
Don't be to worried about high porosity. Many commercial wares are 6-8% and yet they are very strong. Just make sure the glaze fits (can survive a hot water:ice water test) and strengthens the ware.
While carving, periodically take some clay and test your design by pushing the clay into the surface. Your clay will pick up the loose plaster particles and also allow you to see what the pattern you have carved looks like. Use the same clay a few times, then discard. You do not want to fire this clay, or mix it into your other clay, as plaster mixed in clay is likely to cause an explosion in the kiln.
To make a stamp with a raised profile, it is easiest to first carve your design into clay or plaster. Then form a moat around this with clay and pour plaster over it, making the negative. The advantage is that what you are carving is the same as what will eventually be on your piece (not the negative of it.)
If you are pouring a very large mold, or for some reason didn't mix enough plaster in the first batch, immediately start mixing the second batch. When it is ready you can pour it over the existing plaster. Scratch crisscross lines into the top of the first layer before pouring the second layer to help them adhere together.
Using a cup or scoop, start adding plaster to the water. Sprinkle the plaster out evenly over the whole surface of the water. You don't have to be in a big hurry, but don't work too slowly or the plaster will start to set. Some people prefer to sift the plaster into the water. At first the plaster will sit on the surface of the water very briefly before it disappears beneath the surface of the water. (This is called "slaking".)
Measuring Specific Gravity, This can be done in two ways: With a hydrometer. This is a glass tube closed at both ends and weighted with lead at one end so that when floated, a scale up the side can be read at the water surface. The higher the specific gravity, the higher the viscometer floats. However, one problem with this instrument is that thick slips tend to impede free floating and give an incorrect reading. In fact, my opinion is that in most situations a hydrometer is practically useless for this reason!
With a scale. Here, you weigh a known volume and divide the weight by that volume. For example, if 10 cc's weighs 17 grams, then 17/10=1.7. A simple technique is to counterbalance a 60 cc veterinarian's syringe, then draw up a specific volume of slip, weigh it, and divide the weight by the number of cc's. Don't get sloppy on this measurement. Do it several times to make sure you get the same reading every time. If necessary, measure a larger volume, get a more accurate graduated cylinder, or a better scale. You must have a reliable way to measure specific gravity.
Some slip suppliers quote specific gravity in ounces per pint (i.e. 29 oz/pt), however, this measure is not intuitive in my opinion.
It is very useful to have molds that you can put slabs of clay over (hump molds) or into (slump molds) for making platters and dishes. The benefit of working with a hump mold is you have access to the back (bottom) of the clay, for adding feet or decorating the back side. The advantage of working with a slump mold is you can work on the top surface while it is still wet. But you have to remove the item before it is completely dry to attach any feet or work on the back side
You can make a small press mold into a stamp for signing your work, or to impress into your piece and make designs. This works best if you want a raised design on your piece (since it is easier to carve into plaster than it is to carve away and leave a raised profile behind.) If you want a recessed design, read the next section. Stamps and sprigs are especially nice when used around a border, or across multiple pieces to achieve a repetitive design element.
Carving plaster sprigs and stamps.
Pour plaster into plastic containers such as margarine tubs and cottage cheese containers, about 1-2 inches thick. Normally after the plaster has set (about a half hour) it will release by itself, even without any mold release on the plastic. If you wait too long and the plaster sticks, you can always cut the plastic off. Now you can carve designs into the plaster, and use it for press molds.
It is important that you use enough straps or bands to hold the pieces of your mold together to prevent leakage.
Always pour slip in such a way that the initial contact of slip to mold is in a place that isn't critical, such as the bottom. At this point the silicates build up and may cause hard spots in your bisque that will no take decorations readily.
One method of avoiding a hard or hot spot when pouring, is to insert a tongue depressor or other flat object in the mold and pour your slip against it.
When pouring small molds, you may sometimes experience difficulty in getting the slip to drain from small pour holes. Try using an atomizer with a piece of small flexible tubing attached. Insert tube into the pour hole, turn mold over, and pump air into the mold. The slip will then run out freely.
You can make your own shape out of clay by pressing balls of clay together in roughly the shape you want. For example, you could rough out a rectangular casserole dish. A wooden paddle is useful for getting your clay close to the right shape. Then cut a template out of cardboard which is the profile of the cross section you desire. For a rectangular shape, you will need two pieces of cardboard, one for each direction. Pull this cardboard across the soft clay pieces to finalize the shape. One this shape is made, burnish the edges so they are very smooth, then build a moat around it (as above) and fill with plaster.
Pieces which explode in the kiln are generally the result of improper venting or improper draining of greenware.
Indented vertical lines are usually caused on the greenware by ‘drizzling' slip down the sides of a vase of pitcher. This can be repaired, most of the time by painting thickened slips into them and smoothing with a damp sponge.
Recognizing Clay Recipe Problems
Even if you achieve an optimum slurry, it will not necessarily cast well if the clay recipe itself is not right. Recipes that contain a lot of fine clay minerals (i.e. ball clay, bentonite) will cast slowly because the clay is not very permeable to the passage of water and they will release slowly because the clay is stickier and will resist release from the mold (although ceramic slip with its 50% typical ball clay is an exception because the high talc in the recipe helps vent the water). They will produce ware that will shrink more and crack more. Recipes that have inadequate clay or clays of very low plasticity will shrink too little and not release from the mold. They will produce fragile ware that fractures when being removed from the mold or during handling.
Heavy seams in greenware can be the result of not banding your molds tightly enough.
If pieces collapse when removed from the mold, try putting a foam rubber piece inside of the greenware and nesting them in shredded paper to dry.
Cracks around the pour hole of a greenware piece are commonly caused by tearing, rather than cutting, the spare off and may only appear after firing.
Pour the plaster over the slab (or other object). When the plaster has set, turn the whole thing over, peel the clay out of the plaster, and let the plaster continue to dry. When pouring these types of molds, it is best to pour enough so that your objects are covered with a couple inches of plaster, and have at least 2” of plaster around the outside. It is a good idea to mark your box 2" above your master and pour to your marks.
Glossary of Plaster Casting Materials
No. 1 Pottery Plaster - Formulated to provide stronger, longer-lasting ceramic slip molds. Standard of the industry.
Duramold Pottery - Compared to No. Pottery Plaster: 10% longer mold life, 28% higher wet compressive strength, 66% stronger dry compressive strength; makes more durable working molds, resulting in lower costs per unit cast.
Puritan Pottery - Slightly denser, longer-wearing mold material. Recommended for jiggering applications.
Ceramical - Low consistency, smooth-wearing mold material for use in ram pressing clay ware. Characteristics include low absorption, high strength, ability to purge easily, resistance to abrasion and wear.
Hydrostone ) - Often substituted for Ceramical in ram pressing applications. For more information see below.
The first step in doing cast ceramics is the most important and that is the cleaning of greenware.
It is true that greenware is fragile, but it is nothing to be afraid of when cleaning.
When holding a piece of greenware, do it firmly enough to hold it securely, but not hare enough to crack it.
The primary function of cleaning a piece of greenware is to remove the seam line caused by molds in the casting process.
It is useful to have hump molds elevated off the work surface so your clay can extend past the edge. This also makes it easier to trim the bottom if you want the clay even with the plaster surface. You can do this by adding a foot to the hump mold (making a mushroom mold).
You don't have to mess with plaster to make molds. You can make your own from bisque. Fire the bisque at a high enough temperature to give it strength, but low enough that it is still porous so the clay dries and does not stick. To make a bowl mold, throw a solid piece on the wheel, smoothing the edges well with ribs, etc. When it is leather hard hollow out the inside until your edges are about 1 inch thick. Fire.
The first stop in doing cast ceramics is the most important and that is the cleaning of greenware.
It is true that greenware is fragile, but it is nothing to be afraid of when cleaning.
When holding a piece of greenware, do it firmly enough to hold it securely, but not hare enough to crack it.
The primary function of cleaning a piece of greenware is to remove the seam line caused by molds in the casting process.
Use rubbing or denatured alcohol in a spray bottle (some people dilute, some don't). When the plaster mixing is almost complete, spray a couple bursts into the plaster. This breaks down the surface tension of bubbles on top and they disappear. After pouring you can repeat this when there are bubbles that have risen to the top of the mold. (This is especially useful if you will be using the top surface of the mold you are pouring.)
The best tool to use in the cleaning of greenware is one designed for that purpose with a sharp pointed blade.
You will find, when cleaning greenware, that wetting seam lines with a damp sponge will make the process easier.
To dampen greenware, you can use a sponge or any old plunger-type spray bottle. Remember to dampen not drown.
Fill a 20 ltr drum 3/4 full of turnings or smashed dry rejects (10 cent piece size).
Top with water and stand overnight.
Mix to a jam consistency with an electric drill fitted with a mixer/paint stirrer.
Add defloccculant 3/4 cup of bulk solution to a max. of 1 cup (250 mls) and mix with drill. The mixture will become quite thin. Allow to stand overnight.
Sieve and pour into well on plaster slab.
Bulk Solution Dispex:
9 cups water
1 cup dispex
One problem with various methods of mold release is they can clog the surface of the plaster, making it less porous, so clay sticks to it more. So mold release should be cleaned off as much as possible. A wet sponge after using your mold is a great way to maintain perfect molds.
Specialty Gypsum Products
Hydrocal FGR - A unique high strength cement with glass fiber used for fabricating glass- reinforced architectural details that are lightweight, fire resistant and thin cast.
Duracal Cement - Designed for concrete patching of highways, bridges, loading docks, etc. Can be drive on 60 minutes after set. Could also be used for outdoor statuary.
Hydroperm - Permeable metal casting product formulated with Hydrocal Gypsum Cement. Suitable for nonferrous castings because of smooth mold surface, carvability and controllable permeability.
Slipcasting: The paths to making a ceramic object are many: wheel throwing, hand building, coil building, press molding and last but certainly not least slipcasting. Although we may not think about it, slipcasting is the most widespread use of a ceramic technique, due to industrial manufacture, which tends to use this technique, which is most suitable to the manufacturing of multiples.
Once you have made a mold (see Making a Simple Plaster Mold), you can make hundreds of casts of the same object with relative ease. Molds do deteriorate with use however, and start to degenerate after a couple of hundred uses (more or less, depending on quality).
A mold is like any other too in that it will wear out after prolonged use.
Molds are expensive and should be cared for properly like any other investment
If you have poured a mold a number of times and it shows wear it is possible to scribe some of the detail back by carefully carving with a sharp tool.
It can take many months for a large plaster mold to completely dry out, so it will continue to get lighter as that happens. But it is usable immediately. If using the same piece of plaster over and over, it will eventually become too wet to release the clay. Let it dry and it will again work fine. If you can't wait for the plaster to dry, you can sprinkle with talc.
Slip-casting. In this case you buy or make plaster moulds, into which you pour liquid clay (slip.) The slip coats the inside of the mould, so when it hardens and you pull the mould away, a cast piece remains ready to decorate and fire. It would seem like if you poured slip into a mould, you would get a solid chunk of clay. But the slip only sticks to the inside surface of the mold at a certain thickness. You pour out the rest of the slip. When the piece dries it shrinks and separates from the plaster leaving you with a hollow piece.
You can make a sling out of canvas and use that for your mold. For example, take a piece of canvas and put it over the top of a large round garbage can. Where the canvas overhangs the can, wrap tightly with string. Place your slab on the canvas. You can get different curvatures depending on how tightly you pull the canvas. This same technique can be used with buckets for smaller slabs.
A method of creating ceramics other than throwing a pot on a potters wheel or handbuilding is using ceramics molds. This is an especially good method if you are planning to create a set of pieces that should match one another. Using molds can be tricky so you should do research and get some good tips on using ceramics molds.
No matter what the problem with the slip is, your first question should always be: “What is the specific gravity”? Until you know this and know it reliably, you cannot fix the problem. Assuming the specific gravity is 1.78-1.8 you should be able to continue an analysis.