Procedure

Procedure:
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.

Opening the mold

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.

Hump/Slump molds/Masters

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.

Recognizing Under Deflocculation

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.

Mold Release/Agents

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.

Bisque Molds/slump and hump

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.

Recipe: Porcelain Clay Slip

Cone 6 Porcelain Clay Slip
Flint 10.0
Ball clay 20.0
Kaolin 20.0
Nepheline Syenite 48.0
Talc 2.0
Water 36.0% of dry amt
Soda Ash 0.05-.1% of dry amt
Sodium Silicate 0.05-.2% of dry amt
or
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.

Making a mold with a master

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.

Bisque Molds

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.

Mixing airless plaster

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.

Pottery Plaster

Buy plaster. When casting molds, Pottery Plaster #1 is best because the particle sizes are small and will capture detail the best. But if you can´t get this, normal Plaster of Paris will work.
Put on a dust mask when handling dry mixtures of this type.

Air bubbles

Air bubbles (pinholes) in greenware can come from air in the slip from stirring too briskly or pouring too rapidly.
Air bubbles on rims of pieces can be reduced by rocking the mold while pouring.

Mold lines

If you have a mold line that is difficult to remove from a piece of greenware, dampen the piece slightly. It will come off easier and lessen the chance of breakage

One-piece molds.

The simplest mold to cast is the one-piece variety. There is no need to band it. It is easily drained and, in most instances, the casting comes free without difficulty.

Water-plaster ratio

There are actual suggested ratios of plaster to water for various plaster mixes, and some people use those. But most people I know use some rule of thumb similar to the one here, 2/3 water to 1/3 plaster to achieve the correct consistency.

Filling a crack

If you are filling a crack in greenware with slip, be sure to work the slip into the cracks as deeply as possible to form a solid bond. You may find using a stiff brush will help, using a back and forth movement of the bristles.

Fill handles

You should always fill the handles of cups with pouring. If they do not fill by letting excess slip settling into them, paint them full with a brush.

Mixing plaster

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.

Modifying Molds

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.

Dis-assembly

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.

Trimming and carving

There will be a point where the plaster is hard, but still wet enough to easily carve. This is a good time to put the item on your wheel and trim it, or trim off the rough edges by hand, or start carving your designs.

3-D Casting and Tooling

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.

Drying your mold

Let air circulate around your plaster mold as it dries. You can put it on top of your kiln or over a radiator to speed drying, but keep elevated so it dries evenly.

The perfect surface

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.)

Recycling

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.

Parabolic surfaced molds/wheel

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.

Pouring large molds

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).

Seams

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.

Use the excess

You can make various sizes of earrings by pouring slip into the various sized keys in your mold.

Preventing undercuts

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.

How to

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.

Draining small holes.

A simple way to drain those molds with small pour holes is to insert a straw and blow in it as you drain the mold. The straw acts as a vent allowing air in to displace the slip.

A Block

Did you know that a block is the reverse impression of a finished ceramic mold, usually made of hydrocal or rubber? Plaster impressions of this block are taken that become your casting mold.

Repair

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.

Specific Gravity

Specific Gravity
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.

Mold Release: Description

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.

How to

How to Slipcast Ceramics
Slipcasting is a widespread ceramic technique, suitable to the manufacturing of multiples. A plaster mold is required.
Here´s How:
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.

Tips:
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!

Inspect

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.

Opposite Pour Holes

If a mold has two pour holes on opposite sides, lumps may form on the bottom while the other side is being poured. It might work well to pour the top of a piece first since cone-shaped tops usually don't make lumps.

Stability of piece

Be sure to test how well your piece of greenware sets on its base. By cleaning the bottom effectively, your finished piece will not wobble

Cutting the spare.

Always cut the spare or pour ring from your mold rather than tearing it off. Tearing can cause undue stress and may result in a crack.

Sieve alternative

Next time you strain slip, use a piece of women's hose rather than a sieve, as it will produce a much smoother paste.

Description

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).

Smelly slip

A small amount of bleach added to slip, glaze or recycled clay mixtures eliminate mold which may occur and cause smelling. The bleach may be caustic on your hands so you may want to wear gloves when handling. Though this mould introduces plasticity to your clay.

Wood Molds

Wood
Wooden bowls can often be used directly without any coating. Wood is porous so clay doesn't stick. Keep an eye out for wooden bowls at garage sales.

Making Stamps/Positive

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.)

Larger molds

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.

Sand Seams

After the bulk of seam lines have been removed with a tool, the next step is to sand the lines with a sander sponge.

Storage

It is important not to dry your molds too fast. Store them on slated shelves so that the natural air can circulate around them.

Mold Release/Reverse molds

You can pour plaster into another plaster mold to make the reverse of it. You need mold release in this case, or the plaster will stick.

Quick drying #2

Never put a wet mold on top of a kiln to dry. The uneven heat may cause the mold to warp.
If you must force-dry a mold do it in front of a forced-air electric heater. If it is too close to hold your hand in that spot, it is too close for the mold. Turn mold frequently.

Making your clay master/wheel

A round hump mold can be made out of clay on the potters wheel. Just center a lump of clay, use a rib or trimming tool if desired to alter and smooth the shape. Then plaster can be poured around this to create a slump mold.

Hump/Slump molds/ description

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

Things to watch for

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

Slip

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.

Slipcasting

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).

Final step

The final step in cleaning is to sponge your piece lightly with a damp sponge. This will guarantee smooth greenware and remove excess dust.

Spilled 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

Sticking

If you have a new mold that sticks together and, no matter what you do, will not open, try saturating the seam area all around with water by pouring it right onto the mold. Then go ahead and cast it in your normal manner and it should open easily.

Separation in mold

If a plate or bowl separates into two layers, either you opened the mold too soon or you drained it too soon.

Preperation.

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.

Handles.

When pouring cups, be sure to lie the mold with the handle down after draining so any excess slip will fill the handle and avoid leaving a bacteria trap.

Recipe: # 10 Stoneware casting slip

Clay 25 kg
Water 11.4 litres
N42 Sodium Silicate 130 grams
N40 Dispex
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.

Flabbiness

Flabbiness
Description: Soft casts difficult to handle without distortion
Cause: Thixotropy too high
Remedy: Increase deflocculent addition.

Adding plaster(end)

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).

Making your clay master

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.

Greenware breakage

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.

Faults in greenware

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.

Mixing Time

Mixing Time
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.

Keep insect free

If you live in an area where bees, bugs, wasps etc might build a nest in your stored molds, be sure to cover the pour holes.

Clay Recipe Problems

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.

Ill-fitting lids

To avoid a poor fit on pieces that have lids, be sure to place lids on bottoms and allow them to dry together. Even if you should accidently mis-shape the piece, the lid will still fit.

Casting-spot

Casting-spot
Description: Discoloured patch appearing on the mould side of the article after firing
Cause: Thixotropy too low
Remedy: Decrease deflocculent addition.

Two piece

Two piece molds come in two types. Some have the seam line on the finished casting running vertically and some horizontally.

Slip Casting Bodies

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

Double casting

Double casting is a two (or more) colour casting technique using multiple colours of slip.

Slip casting: description

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.

Save Debris:Mender

While you are cleaning a number of pieces, be sure to save the greenware bits that you cleaned off. When you finish, catch all the scrapings in a bisque cup and fire. This residue can be used as a fine grog that can be used to fill in cracks in bisque.

Scribing

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.

Wreathing

Wreathing
Description: Small uneven ridges on the slip side of the article
Cause: Thixotropy too low
Remedy: Decrease deflocculent addition.

Mushroom mold/description

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).

Alternative

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 importance

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.

Close holes

A piece of terry cloth, when rubbed on a piece, will help close holes faster than a sponge on greenware.

Making Deflocculated Slip from Turnings

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