Ceramic slip is clay mixed with water to a liquid consistency, and the method you use depends on what you need it for. A simple joining slip takes minutes with scrap clay and water. A casting slip for pouring into molds requires precise ratios of water, clay, and a chemical additive called a deflocculant. Here’s how to make both types.
Joining Slip vs. Casting Slip
These two types of slip serve completely different purposes and require different approaches. Joining slip (sometimes called “slip and score”) is the glue you use to attach two pieces of clay together, like adding a handle to a mug. It’s simply clay dissolved in water to a thick, creamy consistency. You can make it in a few minutes from whatever clay body you’re working with.
Casting slip is engineered to be poured into plaster molds. It needs to be fluid enough to pour easily but dense enough to build up a solid wall inside the mold quickly. A good casting slip contains no more than 32% water by weight, which is far less water than you’d expect for something that pours like heavy cream. That low water content is achieved through chemistry, not just mixing.
How to Make Joining Slip
Start with scraps of the same clay body you’re joining. Dry the scraps completely, then break them into small pieces and drop them into a container of water. Let the clay absorb water for several hours or overnight until it softens into a slurry. Stir until it reaches the consistency of thick yogurt. If it’s too thin, let it sit uncovered so some water evaporates. If it’s too thick, add small amounts of water and stir.
For a stronger bond on difficult joins, add a splash of vinegar to your slip. Vinegar acts as a flocculant, encouraging clay particles to clump together and making the slip stickier. Some potters also tear small pieces of toilet paper or paper towel into their slip. The paper fibers create an internal structure that reinforces the bond between pieces. This “paper slip” is especially useful for attaching delicate elements or repairing cracks in greenware.
To use joining slip, score both surfaces you want to attach with a fork, needle tool, or serrated rib. Apply a generous layer of slip to both scored surfaces, press them firmly together, and smooth the seam. The combination of scoring (which creates mechanical grip) and slip (which fills gaps and fuses the clay) is what makes a lasting bond.
How to Make Casting Slip
Casting slip requires three ingredients: dry clay (or a prepared clay body), water, and a deflocculant. The deflocculant is the key ingredient that separates casting slip from regular slip. It’s a chemical that causes clay particles to repel each other, keeping them suspended in far less water than would otherwise be needed. Without it, you’d need so much water that your castings would shrink excessively and take forever to dry in the mold.
The two most common deflocculants are sodium silicate (liquid water glass) and a synthetic alternative sold under the brand name Darvan 7. Sodium silicate is the traditional choice, used at roughly 0.2% of the dry clay weight. Darvan 7 requires a slightly higher dose, typically 0.3 to 0.5% by weight, but it’s less likely to clog your plaster molds over time. Many experienced casters prefer to keep sodium silicate as low as possible and supplement with Darvan to get the best of both.
Step by Step
Weigh your water first, aiming for about 28 to 32% of the total batch weight. Pour the water into your mixing container and add the deflocculant, stirring to dissolve it completely. Then add the dry clay gradually while mixing. Adding the clay slowly prevents lumps from forming and lets you monitor the consistency as you go.
Mix thoroughly. For small batches, a jiffy-type mixer chucked into a power drill works well and is the most accessible option for home studios. For larger or ongoing production, a blunger (a dedicated mixing machine with a built-in agitator and spigot) is worth the investment. Blungers come in various sizes and keep the slip moving continuously, which prevents settling.
Once mixed, run the slip through a 40-mesh sieve to catch any lumps, unmixed clay, or debris. This step is easy to skip and hard to fix later. Lumps in your slip become lumps in your castings.
Checking Specific Gravity
Specific gravity tells you the ratio of clay to water in your slip, and it’s the most reliable way to know if your batch is right. Most potters and hobby casters target a specific gravity around 1.75, meaning the slip is 1.75 times heavier than the same volume of plain water. Industrial operations, particularly for large items like sinks and toilets, push that to 1.8 or higher.
To measure specific gravity, you can buy a hydrometer or use a simple DIY method: weigh an empty container, fill it with exactly 100 milliliters of slip, and weigh it again. Divide the weight of the slip (in grams) by 100. If your slip weighs 175 grams per 100 milliliters, your specific gravity is 1.75.
A slip at 1.79 specific gravity (about 28% water) poured into a dry plaster mold will build a 3 to 4 millimeter wall thickness in just a few minutes. If your specific gravity is too low, the slip has too much water: castings will be thin, slow to build, and prone to excessive shrinkage. If it’s too high, the slip won’t pour smoothly.
Adjusting a Slip That Isn’t Right
The most common problem with casting slip is getting the deflocculant amount wrong. Too little deflocculant and the slip will be thick and sluggish despite having enough water. Too much and it can actually reverse the effect, causing the slip to thicken or become gel-like. Add deflocculant in tiny increments, mixing thoroughly between each addition, until the slip flows freely.
If your stored slip settles into a hard layer at the bottom of the bucket, the problem is usually one of two things: not enough clay content to keep particles suspended, or unintentional chemical changes in the water. For slip that keeps settling, adding 1 to 2% bentonite (a highly absorbent clay) to the recipe gives the mixture more structure to stay in suspension. If the slip has become over-deflocculated from dissolved sodium in the water, you can reverse it with a flocculant. Dissolve Epsom salt (magnesium sulfate) in water to create a saturated solution, then add the clear liquid from the top of that solution to your slip in small amounts, mixing at high speed after each addition. Stop as soon as the slip noticeably thickens.
For a bucket that’s completely solid at the bottom, pour off the standing water into a separate container (save it), then work Epsom salt solution directly into the hard layer by hand or with a spatula. Once it loosens, gradually add the saved water back while mixing.
Protecting Yourself During Mixing
The most serious hazard when making slip from dry materials is crystalline silica dust. Clay, feldspar, and many other ceramic raw materials contain quartz, and inhaling fine silica particles over time can cause silicosis, a permanent and irreversible lung disease. OSHA sets the workplace exposure limit for respirable quartz at 0.1 milligrams per cubic meter of air over an eight-hour day. NIOSH recommends an even stricter limit of 0.05 milligrams.
For a home or small studio, practical protection means wearing a properly fitted N95 or P100 respirator whenever you handle dry clay, not just a paper dust mask. Work in a well-ventilated area, ideally with airflow that pulls dust away from your face. Wet-mop or sponge surfaces rather than sweeping, which just throws dust back into the air. Add dry materials to water (not water to dry materials) to minimize the amount of powder that becomes airborne. Once the clay is wet, silica dust is no longer a concern, so the critical window is the dry-mixing phase.