Casting slip is a fluid clay mixture that pours like heavy cream into plaster molds, building up a solid clay wall as the mold absorbs water. The key to making it is deflocculation: using a small amount of a chemical additive to make clay particles repel each other, so you get a pourable slurry with far less water than you’d need otherwise. A well-made casting slip contains only about 30-32% water by weight, compared to 60% or more if you just stirred clay into water without a deflocculant.
What Makes Casting Slip Different From Regular Slip
Regular slip is just clay mixed with water until it’s fluid. It works fine for joining leather-hard pieces together, but it holds too much water to cast with. If you poured regular slip into a plaster mold, the excessive water would cause slow drying, warping, and cracking. The cast walls would be weak and uneven.
Casting slip solves this by using a deflocculant, a chemical that disperses clay particles so they float freely instead of clumping. This means you need dramatically less water to reach a pourable consistency. Less water in the slip means faster casting, stronger walls, and fewer defects. The body recipe itself also differs slightly from a throwing or handbuilding clay. Casting bodies typically use more kaolin and less ball clay, with little or no bentonite, since too much of these sticky, plastic clays can cause problems with deflocculation and mold release.
Ingredients You Need
You need three things: dry clay body, water, and a deflocculant.
For the clay body, the hobby casting industry has long standardized on a simple recipe of roughly 50% talc and 50% ball clay, with minor additions depending on the firing temperature you want. You can also buy pre-formulated dry casting slip from ceramic suppliers, which takes the guesswork out of the clay recipe itself. If you already have a favorite clay body for throwing, you can often adapt it for casting by reducing the ball clay and bentonite content (aim for about 1% bentonite instead of the 3-4% used in plastic bodies).
For deflocculants, you have two main options:
- Sodium silicate is the traditional choice, inexpensive and widely available. You need very little, ideally as close to 0.2% of the dry clay weight as possible. The downside is that sodium silicate can gradually clog the pores of your plaster molds over time, shortening their life. Many experienced casters minimize this by using the absolute smallest effective amount and mixing for a long time.
- Darvan 7 is a synthetic alternative that extends mold life significantly compared to sodium silicate. It requires a slightly higher dose, around 0.3 to 0.5% of dry body weight, but produces slips with a wide, forgiving casting range. It works well with most whiteware and stoneware bodies.
A third option, Darvan 811, is used primarily for vitreous and semi-vitreous bodies. It offers longer casting range, higher solids content, and better viscosity stability than the traditional sodium silicate and soda ash combination. It also eliminates the hard spots and scum that sodium silicate can cause.
Step-by-Step Mixing Process
Start by weighing everything. For a 10-gallon batch using sodium silicate as the deflocculant, a proven starting point from Aardvark Clay is 100 pounds of dry slip, 5.5 gallons of water, and 220 grams of pure sodium silicate (not a diluted solution).
Begin by adding your water to a clean bucket or mixing container. Add about two-thirds of your deflocculant to the water and stir it in. Then slowly sift in the dry clay, mixing continuously with a drill-mounted mixer or a dedicated slip mixer. Add clay gradually, not all at once, to avoid lumps. Once all the dry material is incorporated, let the mixer run for a while. Sodium silicate in particular benefits from long, continuous mixing times to reach full deflocculation with minimal chemical.
After the initial mix, check the specific gravity and viscosity (more on that below). Then add the remaining deflocculant in small increments, testing between additions, until you hit your target fluidity. This two-stage approach is standard practice: get the density right first with the minimum deflocculant needed, then fine-tune the viscosity with small additional doses.
Let the finished slip rest for at least 24 hours before casting. This allows air bubbles to rise out and the chemistry to stabilize. Sieve the slip through a mesh screen (80 to 100 mesh) before use to catch any lumps or debris.
Testing Specific Gravity
Specific gravity tells you the ratio of solids to water in your slip. It’s the single most important quality measurement. Most clay-based casting slips work best at a specific gravity of around 1.75 to 1.80. Beginners should target 1.75, which gives a more forgiving margin of error.
To measure it, you can use a hydrometer designed for ceramic slips, or simply weigh a known volume of slip and divide by the weight of the same volume of water. For example, if 100 milliliters of water weighs 100 grams and 100 milliliters of your slip weighs 175 grams, your specific gravity is 1.75.
If your specific gravity is too low, the slip has too much water and will cast slowly with weak walls. If it’s too high, the slip may be too thick to pour cleanly into detailed molds. Getting this number right before you start casting saves a lot of frustration.
How to Adjust Problem Slip
The most common mistake is adding too much deflocculant. There’s a sweet spot: too little and the slip is thick and sluggish, too much and it actually reverses, becoming thick and gel-like again. This over-deflocculated state is difficult to fix. If you suspect you’ve gone past the optimum, you can try adding a tiny amount of vinegar or Epsom salt (a flocculant) to pull the chemistry back, but prevention is far easier than correction. Always add deflocculant in small increments.
If your slip is casting too thin and pieces aren’t releasing from the mold properly, try adding about 0.5% bentonite. This makes the slip slightly stickier, slowing down both casting time and release time, which often solves release problems. When draining excess slip from a mold, hold it nearly horizontal rather than flipping it upside down quickly. This prevents suction from pulling the still-soft cast away from the mold wall.
Pinholes in finished pieces usually come from air trapped in the slip. Screen and stir your slip gently before each use, and let freshly mixed batches rest overnight. Cracks during drying often point to uneven wall thickness or drafts. Cast in a consistent environment and rotate molds occasionally for even absorption.
Casting With Your Slip
Pour the slip steadily into your plaster mold, filling it completely. The plaster immediately starts drawing water from the slip, building a layer of firm clay against the mold wall. For most pieces, a wall thickness of 5 to 10 millimeters works well, which typically takes 10 to 15 minutes depending on the mold’s absorbency and the slip’s specific gravity. Leaving the slip in longer produces thicker walls.
Once you’ve reached the wall thickness you want, carefully pour the excess slip back into your container. Tilt the mold slowly and let it drain completely, then leave it inverted at an angle so the last bit of liquid runs out. The cast piece will continue to firm up and shrink slightly, eventually pulling away from the mold walls on its own. At that point you can gently remove it, clean up seam lines, and let it dry evenly before firing.
Between pours, stir your slip gently to keep it homogeneous. Casting slip is reusable: just pour the excess back in. Over time, the slip will thicken as it loses water to the molds. You can add small amounts of water and deflocculant to bring it back to the correct specific gravity, but always re-test rather than guessing.