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Making Molds Without Plaster
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This tip isn't technically about using plaster. It’s about how to use molds without plaster. But what if you don’t want to use plaster, buBt you still want press molds, or slump and hump molds? Never fear, as there are other options!
You can make press molds out of clay and bisque them. These work quite nicely and clay doesn't stick to them. Carve the inverse of your desired design into leather hard clay. For example, if you want raised lettering, carve the letters out of the press mold and the inverse will be raised. Remember to make your design about 10% larger than desired, to account for shrinkage.
Bisque also makes great slump and hump molds. These can be thrown, or handbuilt, or molded from a found object.
You can use found objects (mixing bowls, platters) directly by coating with vegetable oil, PAM, WD-40, etc. or by covering with plastic wrap or newspaper. Usually, you will use these as slump molds, as the insides of your bowls and platters have the nice curvature you are looking for.
Wooden bowls can often be used directly without any coating. Wood is porous, so clay doesn't stick. Keep an eye out for wooden bowls at garage sales.
You can also make a sling out of canvas and use that for your mold. For example, take a piece of canvas and put it over the top of a large round garbage can. Where the canvas overhangs the can, wrap tightly with string. Place your slab on the canvas. You can get different curvatures depending on how tightly you pull the canvas. This same technique can be used with buckets for smaller slabs.
You can also cut holes in the canvas where you want the feet to go. That way you can attach the feet while the clay is still wet, and not have to worry about getting it off the sling at exactly the right time, when it is hard enough to hold the shape but still soft enough to add the feet! Cool huh?
A similar approach to the above is to put the canvas over a plywood box. Staple the canvas to the outside of the box. Again, the curvatures can be modified by how tightly you pull the canvas. You can staple just two ends, or all four ends for different effects. If you want to make this more versatile, make a version where you can vary the amount of curvature. Attach screws or nails to the outside of the box.
You can also use clothes baskets. They come in round or rectangular, and are inexpensive.
Another thing you might consider is making a form out of loosely crumpled and dampened newspapers. This allows a more loose look. Shape the damp, crumpled newspaper into the form you like, and cover it with plastic. The newspaper will dry and this hump mold will likely be useable for a while. If it starts to come apart, you can squirt the newspaper with water and re-shape.