Question:

How do I cast slip?

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.

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.

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.

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.

Flabbiness

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

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.

Thinning

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.

Recipe: cone 06-04 Ceramic Slip

Recipes
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
Talc 50.0
BallClay 50.0
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.

Bad Draining

Bad Draining
Description: Slip failing to drain from narrow sections
Cause: Fluidity too low or thixotropy too high
Remedy: Increase the water addition or increase deflocculent addition.

Recognizing Over Deflocculation

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.

Bookend/lamp weight

If you need to weight down a bookend or lamp, you may find it easier to use a combination of sand and plaster of paris. Plaster alone may crack the piece and sand can ooze out.

Wreathing

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

Pinholing

Pinholing
Description: Small holes just beneath the surface on the mould side of the article
Cause: Fluidity too low
Remedy: Increase water addition.

Cracking

Cracking
Description: Small cracks where handles join the body of the article
Cause: Thixotropy too low
Remedy: Decrease deflocculent addition.

Recipe: Terracotta Casting Body

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.

Pouring

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.

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.

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.

Brittleness

Brittleness
Description: Difficult to fettle or cut
Cause: Thixotropy too low
Remedy: Decrease the deflocculent addition.

Double casting

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

Banding

Keep the table where you band your molds and the bands themselves as clean as possible to prevent bits of dried slip and trash from falling into the molds as you band them.
If you band your molds with the pour gate down, it will help to keep the inside cavity clean.

Slow Casting

Slow Casting
Description: Casting time too long
Cause: Fluidity too high or thixotropy too low
Remedy: Decrease water addition or decrease deflocculent addition.

Handling plates

If you are working on greenware plates and have trouble handling them without breakage, try carrying them around in paper plates.

Specific gravity

Trouble Shooting
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.