Before creating a new tile design, it is important to consider your mold. If what you are after is a very uniform geometric or solid line you may consider starting with a plaster blank.
What you will need:
plexiglass 12"x 12"
4 coddle boards
Murphies Oil Soap
As you determine your design, remember clay shrinks and you will want to scale up your design accordingly. (for example, my clay shrinks at about 12% so a 4 inch tile I will need to start with a 4 1/2 inch mold.)
now that you have determined the size of your mold choose a smooth, level surface on which to begin. Use a solution of 2:1 water and Murphies Oil Soap to clean plexiglass and seal coddle boards.
Lay your plexiglass onto your work surface and begin to assemble your coddle board frame.
Coddle boards are a valuable set of tools and worth constructing. Made simply from a 1"x 4"x 18" piece of wood, sanded and sealed with polyurethane, and an angle bracket. Coddle boards are adjustable and re-useable.
Clamp your coddles together and seal around the outside edges and inside corners with a coil of clay. You are ready to mix your Plaster.
USG Pottery Plaster #1 is measured by weight in a 10:7 plaster to water mixture. Always add plaster to room temperature water and mix at high speed with a mechanical mixer for maximum strength, absorption, and hardness. Pour plaster slurry into the prepared frame to a depth of 1 1/2 inches.
Use a paint brush to gently lift air bubbles from the plexiglass surface.
Remove coddles when plaster has hardened. The smooth surface of the plexiglass has created a equally smooth plaster mold. You are ready to carve.