As artists, some of us are unsure about whether or not slip casting is too close to mass production for our tastes. Will my pieces still be unique? Will they still look handmade? Will other artists think I’m cheating? Once you learn more about the process though, you may begin to feel that slip casting is simply one more tool you can use as an artist. I started slip casting so I could produce pieces that could be sold at reasonable prices. The result is that I’m able to spend a lot of time designing and handbuilding complicated original pieces, and reproduce them multiple times.
Roll out an 1⁄8 inch-thick slab. You can add pattern to this however you like. I roll my slabs onto a pre-made, textured plaster slab (1). Let the clay stiffen up a little, then cut it out in whatever shape you want (2). Alternatively, you can throw a base form for your mold. Or if you have found a shape that you really like, you can handbuild your mold to match that shape. For these bowls, a plastic lid had exactly the profile I was looking for, so I could drape a slab into the bowl, making sure it was even on all sides.
After draping your slab in your support structure, place it on whatever surface you’re going to cast on. Starting from the foot, slowly build up coils all the way around the outside to create a -inch border (3). Plaster won’t stick to plastic or glass, so these are great surfaces to cast from. If you need to use a porous surface, like a Masonite bat, you can protect it by brushing on several coats of liquid soap, such as Murphy’s Oil Soap, diluted slightly with water. Smooth the surfaces out with a rib, and make sure the top is flat. Vinyl floor trim or metal flashing, both available in most homestores, are perfect for making round molds—plaster won’t stick to it, and it’s easy to work with. Wrap the trim around your form (3), pushing it firmly into the clay footprint. Line the bottom of the outside with a clay coil, making sure to press firmly all the way around so it seals. Run a coil vertically up the trim seam as well, either inside where the two edges meet, or on the outside if the inner and outer seams are close to each other. Use a binder clip at the top to hold it together tightly. For larger forms, tie a length of rope or twine around the outside of the floor trim for extra support and resistance against the force and weight of the liquid plaster.
1 Roll an 1⁄8 inch-thick slab and add patterns to it or roll the slab on a pre-made textured slab.
2 After the clay stiffens up a little, use a fettling knife to cut out whatever shape you want.
3 Place your slab on a support structure that plaster won’t stick to, build up smoothed clay, and wrap vinyl trim around the form.
4 Pour plaster over the form. It helps to hold a plastic ruler at the edge of the bucket while pouring to get a cleaner cast.
Pouring the Top Mold
Small molds don’t need a lot of plaster. I use 70% water to 100% plaster, by weight, so my molds are a little more resilient and don’t degrade as fast. To fill the volume of a 5-inch-diameter mold, you will need 755 grams of plaster to 529 grams of water. Sift the plaster into the water slowly, letting the little clumps absorb and sink before adding more. Make sure to wear a dust mask. Let the plaster sit for a minute or so, and then tap the bucket on the table a few times to dislodge air bubbles. Now, stir the plaster for a minute or so, being careful not to stir too much air into it. I do this with my hand, so I can break up any clumps. The consistency will start to change and become a little thicker, like heavy cream, which is when it’s ready to pour. Bang the bucket on the floor several times to dislodge any air bubbles. These little bubbles can ruin your texture if they end up on the surface of the mold.
When pouring plaster, hold a plastic ruler at the edge of the bucket, and pour through it (4). This keeps the bubbles mainly at the top of the plaster, so you get a cleaner cast. Start in a corner, pouring slowly, and let the plaster gradually fill the entire area until it is about inch above the foot of your bowl. Gently tap the table near the mold repeatedly to force out any remaining air bubbles. The poured plaster needs to set for about 45 minutes. It’s ready to be released from the cottle when it is cool to the touch. Caution: Any excess plaster must not go down your drain; even small amounts can mess up your plumbing. Rinse your bucket and tools in a bucket of water designated for plaster clean up, then dump the water out in an inconspicuous part of your backyard, compost it, or let it settle, drain off the top water and put the plaster in the trash.
Remove the vinyl trim and the coils. This clay should only be used for making plaster molds now, as clay that has plaster in it can explode in your kiln or crack after firing. If the plastic bowl stuck to the plaster, pry it away gently with a knife. Leave the clay slab in the mold. Scrape the outer edges of your form with a fettling knife to get rid of any sharp edges. Add a small slab to the edge of the mold. This will create a void in the mold rim called a lock release, where a tool can be inserted to pry the mold sections apart. Using a round trimming tool, create at least three registration keys around the rim (5). These should be about the size of half a marble. Wipe your surfaces off with a sponge to get rid of most of the plaster bits.
5 After the plaster sets up, remove the support structure, add a clay lock release, and carve 3 registration keys into the rim.
6 Place a weighted plastic bottle on top of the form when pouring the second part of the mold to create a foot for the bowl form.
7 Separate both parts of the mold and clean up all outside edges. Pay special attention to the foot area and the lock release system.
8 After the mold dries and is ready to cast, pour casting slip in the mold slowly so it has a chance to fill all the areas.
Pouring the Second Mold Section
Now we are ready to create the top half of the mold. I find old plastic vitamin bottles work well for creating the foot and pouring hole. Fill the bottle with water, so it doesn’t float away or crush when you cast the next part (see 6). Center your bottle on top of the mold. Brush on two coats of Murphy’s Oil Soap on all exposed plaster. Wrap the vinyl trim around the outside of the mold as before, and seal all edges with a coil. Mix another batch of plaster, and pour it inside the mold (6). Let the plaster set up.
Release and Clean
Peel off the vinyl trim. Rotate the plastic bottle in a circular manner, and you should be able to pull it out. If it sticks, you can remove it in the next step instead. Clean up the outside edges: you may have to scrape into the plaster to find where your lock release is (7). Use a wooden tool to pry the two mold halves apart. They should separate easily as soon as you push something into the lock release and lift (see 10). Clean and rinse all outside edges as before. Set your mold aside, either on a wire rack, or something elevated, so that air can reach all sides as it dries.
Slip Casting Using the Mold
For those new to slip casting, I recommend working with commercial slips that have already been adjusted. Casting slip should always be mixed well before each use. Try to avoid stirring too much air in. Before casting into a dry mold, run the mold briefly under running water, or spray it with a water bottle. If the mold is too dry, it will suck moisture out of the slip too fast. If you are making multiple casts in a day, you don’t need to wet the mold after the first cast as the plaster will retain some of the moisture from the piece you just cast.
9 Once the bowl is cast to the desired thickness, usually after about 15–20 minutes, pour the remaining slip back in the bucket.
10 Wait until the cast bowl is ready to come out of the mold, then push a wooden stick into the lock release to open the mold.
11 Use a plastic or rubber rib to help remove the cast if it’s stuck in the mold.
Slip-cast porcelain plate, Cobalt Blue and Turquoise Clear Glaze G19, fired to cone 6.
Pour your slip into the mold slowly so that it has time to fill all the areas of the mold (8). Fill it at least 3⁄8 inch above the foot. As the slip drys, the level in the middle will go down, but the area that touches the side walls will dry and naturally form a foot. You can control how thick, and how high the foot is by adding more or less slip. Let the slip sit in the mold until it’s dry, at least half an hour, but it can also be longer. The clay will no longer look liquid, and will be pulled away from the edge of the mold slightly. Small bowls can be cast solid (see 8). For larger pieces, you will need to fill the slip level much higher (see 9). Let the slip sit in the mold for 15–20 minutes, and then pick the mold back up and shake it gently to get the slip moving again. Now pour the excess back into your slip bucket (9). This forms a foot on the inside of the mold, but allow the base of the bowl to be a normal thickness. Gently push a needle tool in at the center of the bowl, and check your depth. If it is too thin, pour a little slip back into the mold.
Releasing the Plate
When the cast form is ready to release from the mold, put a wooden tool in the lock release and separate the sides (10). If the bowl doesn’t easily release, use a plastic rib to help loosen the bowl (11). Push gently from the other side once you have done this, and the bowl should come right out. If your bowl doesn’t come out on the right side, try dusting the textured side of the mold with talc before you cast it again. If it won’t hold it’s shape, you peeled it too early. If the form warps, let it sit longer next time and wait for it to dry until it releases on its own, or use compressed air.
Smooth any rough edges or mold seams on the bowl Let your small bowls firm up before covering them loosely with plastic overnight. Flip them upside down the next day, and let them dry without any plastic. When they’re completely dry, bisque fire them. To avoid damage in the firing, don’t stack the pieces in the kiln. Each piece should sit on its own foot on the kiln shelf.
Pieces that have two casting surfaces like these usually have some small amount of air between the two sides. I have found that dipping the pieces in glaze can cause uneven expansion and it is possible for it to crack or split open. I prefer to brush the glaze on instead. I fire all of my porcelain very slowly, in both bisque and glaze to help minimize any cracking or warping, and let any trapped air or moisture slowly escape.
Paul Barchilon is an artist and teacher in Boulder, Colorado. He is fascinated with the mysteries of the circle, and was seduced by fire and earth’s magic at a young age. Visit www.barchilonceramics.com.
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