Slip is a liquid mixture of clay and water, sometimes with small amounts of additional minerals and chemical additives. In its simplest form, it’s just clay particles suspended in water to create a creamy, pourable consistency. But the exact recipe varies depending on whether you’re using slip to cast objects in molds, decorate the surface of pottery, or join pieces of clay together.
The Basic Ingredients
Every slip starts with two things: clay and water. The type of clay and the proportion of water change based on what you’re trying to do, but those two ingredients are always the foundation. A simple decorating slip can be nothing more than a specific clay body mixed with enough water to make it brushable or dippable.
Casting slip, which is designed to be poured into plaster molds, gets more complex. A good casting slip contains no more than 32% water by weight. That’s significantly less water than you’d expect for something that needs to flow like heavy cream, and it’s only possible because of the third key ingredient: a deflocculant.
What Deflocculants Do
Deflocculants are the ingredient that separates a basic clay-and-water mixture from a true casting slip. They’re electrolytes that cause clay particles to become electrically charged so they repel each other instead of clumping together. This repulsion keeps the particles evenly dispersed, making the slip fluid without needing a lot of extra water.
The two most common deflocculants are sodium silicate and a synthetic dispersant sold under the brand name Darvan. For a porcelain casting slip, a typical proportion is roughly 0.35 parts Darvan per 100 parts dry clay powder. Stoneware bodies that contain iron oxide can require up to double that amount. Sodium silicate works similarly and is often used alongside or instead of Darvan. A 10-gallon batch of casting slip might use around 220 grams of pure sodium silicate.
Why does keeping the water content low matter? Less water means faster casting times in the mold, less shrinkage as the piece dries, and fewer cracks in the finished work. The deflocculant does the job that extra water would otherwise do.
Other Additives in Casting Slip
Beyond clay, water, and deflocculant, casting slips often include a few more ingredients tailored to the firing temperature and final product. A low-fire casting slip (designed for firing at lower temperatures) typically contains:
- Ball clay: A very plastic, fine-grained clay that suspends the other ingredients and gives the cast piece enough strength to hold together before it’s fired.
- Talc: A non-plastic mineral filler that fires white and helps control how much the piece expands and contracts with heat. This matters for getting glazes to fit properly on the surface.
- Barium carbonate: Added in very small amounts (around 15 grams per 10-gallon batch) to neutralize soluble salts in the clay. Without it, those salts can migrate to the surface during drying, leaving a white, scummy residue called efflorescence.
Higher-fire casting slips for stoneware or porcelain swap out some of these materials for feldspar, silica, and different clay blends, but the principle is the same: clay provides plasticity and structure, fillers and fluxes control how the piece behaves during firing, and deflocculants keep everything flowing.
Decorating Slip vs. Casting Slip
Not all slip is meant to be poured into molds. Decorating slips are applied to the surface of pottery to add color, texture, or a smooth finish. Their recipes are simpler but follow different rules.
A decorating slip is generally composed of 50 to 90% clay, with the remaining percentage split between colorants (like metal oxides for color) and a small amount of flux to help the slip bond to the surface during firing. The high clay content means these slips shrink and move similarly to the clay body underneath, which is critical for preventing the slip from cracking or peeling off.
Engobes sit between slips and glazes. They contain only 25 to 50% clay and include a higher percentage of fluxes, which are minerals that lower the melting point and help the layer fuse to the surface. Engobes are also more likely to contain calcined (pre-fired) clay, which has already gone through its shrinkage, making the engobe more stable when applied to pieces at various stages of dryness.
Why Shrinkage Rates Matter
One of the trickiest parts of slip composition is matching its shrinkage to the clay body it’s applied to. When slip shrinks significantly more or less than the underlying clay during drying and firing, the result is cracking, peeling, or a defect called shivering where the slip layer flakes off.
As a general rule, a difference of about 1.5 percentage points in shrinkage rate (for example, 11.5% slip over a 10% clay body) is close enough to work for thin applications. The greater the difference, the more likely you’ll see problems. If your slip does crack over a particular clay body, reducing the water content and adding a small amount of deflocculant can help. The deflocculant keeps the slip fluid enough to apply while lowering overall shrinkage.
How to Check if Your Slip Is Right
Potters measure slip consistency using specific gravity, which is simply the weight of the slip divided by the weight of an equal volume of water. For casting slip, the target range is 1.75 to 1.80. You can test this at home by weighing a known volume of slip and dividing by the weight of the same volume of water.
Slip also has a useful physical property called thixotropy. This means it becomes more fluid when you stir or agitate it, then thickens again when left to sit. The clay particles form a loose network structure at rest, and stirring breaks that network apart temporarily. This is why a jar of slip that seems too thick will often loosen up with a good stir, and why freshly poured slip in a mold will gradually firm up as it sits undisturbed against the plaster walls.
If your slip is too thick even after stirring, the fix isn’t usually to add more water. Adding tiny increments of deflocculant is more effective and won’t compromise the final piece. If it’s too thin, you can let it sit uncovered so some water evaporates, or add small amounts of dry clay and remix thoroughly.