Making a ceramic mold requires diligence. Project tools are a square board, ruler, water-based clay, Purelube, talc, four C-clamps, four wood boards cut longer than mold size, a knife, rubber mallet, screw driver, wedge, drill with mixer attachment, gallon bucket, paint brush, orange shellac and denatured alcohol. Your casting object can be clay, glass, ceramic or wood.
Place the cast object on a movable board and indicate the dissection, or seam line, with a marker. Do a "clay up," molding a clay pillow from the seam line to an inch beyond the widest appendage. Smooth and square the clay and edges. Thinly cover the object and clay with brushed coats of orange shellac; fully dry and sprinkle with talc. Brush on Purelube slickness enhancer; brush off excess.
The plaster mix needs to configure to the project size. Use the formula of 2 3/4 lbs. of No. 1 pottery plaster mixed into 1 quart of cool water, placed in a gallon bucket. Slack, or rest, added plaster one to two minutes. Mix with a drill mixer for under two minutes. C-clamp four overlapping boards to form a snug square around the clay. Pour plaster so it reaches 1 inch over the highest object point. Cure for 20 minutes and remove the casting boards. Lightly scrape the plaster top and slant the edges to prevent chips. Grasp the mold and clay; gently rotate to loosen it from the work board. Flip the mold and pry the clay from the plaster.
Clean clay particles from the mold. Use alcohol on shellac residue. Carve depressed seam keys kitty corner, top and bottom with a dime or quarter. Dust the surface and seams with talcum and Purelube the surface. Mix plaster and place C-clamps around the boards, filling to 1 inch above highest object point. Wait 30 minutes, remove the casting boards, scrape the top and slant edges as before.
Part the mold with a wedge and rubber mallet. Clean the interior, remove the casting object with a piece of wood under a screwdriver. Carve a pour hole in the base with a fettling knife. Your ceramic mold is ready.
Allow the new ceramic mold to completely dry out before use. Join the mold halves with rubber straps. Stir slip with a dowel to avoid bubbles (mixture of clay or Bentonite and water with liquefying deflocculate Calgon or Daravan 7). Mix to the thickness of condensed milk. Homemade slip recipes are at the websites Pottery the Art (http://kengeorgepottery.tripod.com/id36.htm) and Judy's Corner for Crafters in The Business (http://judysbookshop.com/blog/2009/07/06/how-to-make-your-own-ceramic-clay-slip).
Purchased slip can be used for white casting, pastelines, porcelain or terracotta. Pour slip at the mold base to prevent silicate build up. Pour against a tongue depressor and fill slowly, avoiding air bubbles. Gently tap the mold sides with a rubber mallet to release trapped air. Your greenware cures in an hour or more, depending on size. Clean greenware with a damp sponge and seam with a fettling knife. Greenware is bisque fired to 1,800 kiln degrees to remove residual moisture from the clay, hardening the surface. Your molded ceramic craft is ready for glazing and a final firing.