This Instructable shows how to make a two-part plaster mold of a ceramic bowl (or just about anything, for that matter) that can be used for slipcasting. Slipcasting is a ceramic manufacturing process in which liquid clay (A.K.A. 'slip') is poured into a plaster mold. The plaster absorbs the slip's water, leaving clay behind. The extra slip is then poured out, and you're left with a clay shell. The beauty of slipcasting is that you can make as many replicas as you'd like, and the beauty of making your own mold is that you can customize any shape that you want!
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Step 1: Prepare the Bowl for the Plaster Pour
To prevent the bowl from floating away once the plaster is poured, fill the inside of the bowl with clay and secure it to the bottom of the bucket. Be sure to seal the crack with a small amount of extra clay. Then, brush diluted liquid soap (hand, dish, laundry, etc...) onto all of the surfaces inside the bucket. Wipe away excess soap with a paper towel. The soap acts as a release for the plaster once it's set.
Step 2: Mix and Pour the Plaster
Mix the appropriate volume of plaster and pour it evenly into the bucket, making sure to fill at least 2 inches higher than the bowl. Let the plaster set overnight or longer.
Step 3: Remove the Mold From the Bucket
Turn the bucket upside down and gently tap the mold on a table, rotating as you do so. Stop once the mold releases and falls out of the bucket. Clean up any extra clay on the rim of the bowl.
Step 4: Secure Plastic Bowl to the Original Bowl
To create a reservoir for the slip while slipcasting, you'll need to make a second mold. Using any object that has the same diameter as the bowl and a good draft angle, secure the object with clay to the first mold. You can also build this piece out of clay if necessary. Build a retainer wall out of clay (pictured in the next step) to contain the plaster pour. Dig at least three registration holes in the plaster mold for an easy fit once both molds are complete. To create the release, wet the surface of your first plaster mold, then soap all of the surfaces and wipe clean with a paper towel.
Step 5: Pour the Plaster for the Second Mold
Place a weight on top of the plastic bowl to prevent it from floating away in the plaster. Mix and pour plaster into the ring-shaped reservoir. Fill at least three inches high. Let this sit for an hour, then remove the clay and the plastic bowl.
Step 6: Final Mold Ready for Slipcasting
Once the plaster is bone dry (this could take up to 4 weeks without an oven), your two-piece mold is ready for slipcasting! Now, you can fill the mold with slip past the rim of the bowl to allow for the slip level drop, as moisture is drawn out of the slip. Once the clay is ready, remove the top mold, and use the rim of the bottom mold as a guide for cutting off the excess clay.