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In previous tips on making tiles I talked about warping. Here are some of those tips related to warping, plus some additional tips.
Dry all pieces slowly and evenly. Make sure drafts aren’t causing uneven drying.
Make sure clay wetness is very uniform. Wedge extremely well, or warm the clay up in a pug mill.
If rolling a slab, roll it in multiple directions.
Keep pieces away from the edge of the kiln. You may even want to build “moats” around the piece to ensure even drying.
Use the right clay body. This might require experimentation, but some bodies just warp a lot more. Porcelain is especially difficult to throw without warping. Grog usually makes clay less likely to warp. Extra talc in the clay can also help.
Fire your rimmed piece upside down on a clay slab. You make the slab out of the same clay as your thrown piece, using a slab roller or rolling pin. It a piece slightly larger than the rim of your pot. Once the pot and the slab are leather hard, place the pot upside down on the slab and let them dry together. Bisque them together.
Dust the shelves with alumina hydrate so the pieces don’t drag while they are shrinking. Some people even make beds of alumina and sit the pieces in them. (Alumina will not stick to clay.) You can reuse the alumina if you brush it into a storage container. I have also used ordinary sand when firing large sculptures. Think of the alumina as little rollers for the clay to slide on.
Some people mix the alumina hydrate with wax and brush it onto the bottom of pieces. This avoids the messiness, but hopefully accomplishes the same thing. (Bonus: Alumina hydrate in your wax is also what you use to make sure lids don’t stick during firing.)
Make sure your piece is thick enough. The larger the piece, the thicker it should be to avoid warping.
Make sure the thickness is even throughout the piece. You may have thinner spots where you trimmed, or near rims. Thicker rims help a lot.
Sometimes under firing your clay (if the piece is not functional) will lessen warping.
Make sure you don’t stretch the clay out of shape while it is wet. It may remember that shape. Keep tiles flat at all times. For example, instead of peel them off your canvas, put a piece of wood over them, flip it, and remove the canvas from the clay. If you must carry slabs, carry them on a stiff board. When throwing, keep your rims nice and round at all times.
If you have to move a slab onto a slump or hump mold, roll it up lightly to prevent stretching. Unroll directly onto the mold.
For large platters, trim multiple foot rings. One should be in or close to the center to keep the plate from dipping there.
Round the bottom of the foot rim - makes it harder for it to get hung up on shelf imperfections.
Place platters upside down on foam for trimming to keep the middle supported and weight off the rims.
Cover rims with plastic or wax resist so they don’t dry out first.
Thoroughly compress the clay while throwing. Collaring, pushing, raising and lowering the clay while centering, compressing the bottoms. All these things work to compress the clay evenly.
Join slabs after they are leather hard (not while they are still very wet).
Don’t span pieces across shelves during firing.
Make sure you are firing on a flat, unwarped kiln shelf.
If you have all half shelves, you probably want to buy some whole shelves for this purpose. Even if you have whole shelves, you might want some spare smaller shelves to place on top of your shelves and provide a very straight surface. You can flip these shelves because there is no risk of kiln wash flaking onto the shelves below.
Dry tiles under compression if possible, such as between layers of drywall. Or make sure they get air from all sides (including the bottom) by using a wire rack.
Be sure to check out the previous tips mentioned above for more ideas on warping!