In the Studio: Mixing Casting Slip: Tips and Tricks David Scott Smith

Appears in the Nov/Dec 2020 issue of Pottery Making Illustrated.

A liquid clay body, also known as casting slip, cannot be made by simply adding an excessive amount of water to dry or wet clay. A dry mudflat is an excellent example of why adding additional water will not work: the particles spread further apart as extra water is added, and when the clay dries, it cracks and falls into pieces. Chemicals called deflocculants help liquify clay bodies without adding excess water. A deflocculant works like two magnets turned so the same charged poles face each other, creating a situation where the clay particles now repel one another. Added to a clay body in very small amounts, a deflocculant can liquify a clay body with approximately 40% water added by weight, which is not much more than the 20–30% of water that is usually added to mix clay for handbuilding or wheel throwing

Mixing casting slip can be tricky. I have learned a few tricks over the years that may help you mix your own casting slip successfully.

Mixing Casting Slip

• Recalculate your recipe for a manageable batch size. When starting with a 100-pound total recipe, divide all parts of your clay body recipe by 4. A quarter of a 100-pound recipe (25 pounds total) usually mixes to about 2½ gallons, which is an easy amount to work with and use immediately.
• Weigh the dry ingredients and place them into a clean bucket.
• In another bucket, weigh 40% distilled water, or 10 pounds for a 25-pound batch of slip. Some recipes call for slightly more or less water, but it’s safe to start with 40%. It’s very important to use distilled water; the minerals in tap water can cause the deflocculant to not work correctly.
• Mix a small amount of sodium silicate, no more than a couple of ounces, in a clean container with water. Use 1 part sodium silicate to 4 parts distilled water (1). Add a very small splash (no more than an ounce) of this solution to your bucket of water. Not all clay bodies adhere to the 0.2% formula for deflocculating slips—sometimes adding 0.2% is too much. Sodium silicate is an extremely powerful deflocculant, which is why I dilute it with distilled water; there is more margin for error.
• While mixing (I use a paint mixer and a drill), slowly add the dry materials to the water (2). Only add more material when the previous scoop is completely mixed. The mixing blade should be running continuously throughout this process.

• The mix will begin to thicken and even appear somewhat chunky (similar to lumpy oatmeal). At this point, add a very tiny splash of the sodium silicate/distilled water solution. As the solution mixes into the slip, there should be an immediate change, and the slip will become very fluid again.
• As you continue to add dry material, each time the slip thickens to a lumpy, sluggish consistency, mix a very tiny amount (up to a teaspoon) of the deflocculant solution into the slip.
• After you have mixed all of the dry materials into the bucket, use a spatula and clean the sides. Spend a few minutes mixing the slip on high speed (3). Sometimes I add another tiny splash of deflocculant solution (up to a teaspoon) if the slip seems thick or doesn’t pour smoothly. It’s a good indication that the slip is mixed correctly when the whole bucket of slip rolls and mixes easily, and when you dip your hand into the slip it pours smoothly back into the bucket. I don’t do any tests, other than dipping my hand to see if it pours easily. I mix some slips on the thin side (translucent porcelain), and some a bit thicker (anything functional).
• Run the slip through a 60–80-mesh sieve before using it.

Mason stains and oxides can be used to color or marbleize casting slip. Use a small amount of distilled water to pre-mix the stains before adding them to slip (4). Pre-diluting the stain will help it mix smoothly, without clumping. Run the colored slip through a 60–80-mesh sieve before using it. You can marbleize slip by pouring colored slips together. Be careful to not over mix or the colors will all blend together.

Troubleshooting

Why does the slip turn out like pudding?

• It’s very easy to accidentally mix too much deflocculant into the slip, and when this happens the slip becomes thick like pudding. The only way to fix this is to make more slip. The bad batch of over-deflocculated slip can be added in very small increments to new batches that have not yet had deflocculant added to them.
• It’s very important that you use fresh deflocculant. Often ceramics suppliers will purchase materials in bulk, and portion them into smaller containers for customers. Sometimes these items sit in the stores for months or years. If you buy a pint of sodium silicate from a supplier, and every batch of slip you make turns into sludge, most likely you have old deflocculant. This is one reason why I prefer sodium silicate over Darvan 7, it seems to have a longer shelf life. Try ordering deflocculant from another supplier and see if that fixes the problem.

Tips for Pouring/Casting Plaster Molds

• Plaster absorbs water from the slip and allows the clay walls to solidify, so the amount of time the slip sits in a mold determines the thickness of the clay walls (6). For example, if I’m casting translucent porcelain , I only allow slip to sit in the mold for 15–30 seconds, which yields a piece with very thin walls. If I’m casting a coffee cup, I want the finished piece to be fairly thick, so I might leave slip in the mold for 15–20 minutes.
• As plaster wicks water from the slip, the slip level will drop in the mold; it’s important to continue to top off the level of slip until you’re ready to empty it (7). Designing your molds to have a reservoir for excess slip can help.
• Check the thickness of your cast by cutting a small divot in the clay that has built up on the reservoir wall. Once the walls are thick enough, invert the mold over a bucket and drain the excess slip (8).
• Only remove casts when they are firm enough to handle without deforming (9)