Terra cotta slip has two purposes. First, it serves as a simple coating, like a glaze, for hand-built or wheel-made pieces of terra cotta pottery. A slip-coating is customarily made of the same clay as the piece to be covered; because of that, slip usually adheres more easily than a glaze of other chemical composition. Second, slip is uses to make plaster-casted dishes or other objects. Anyone interested in making pottery needs to know how to make a slip.
Making a Terra Cotta Decorative Slip
Combine clay, whether powdered or already made, with water and stir until you have produced a liquid the consistency of heavy cream, mayonnaise or thick gravy. Commercial clays and slip powders have proprietary recipes. Create your own proportions by combining two parts clay with one part water in a small cup and stirring thoroughly. Let the consistency be your guide.
Paint slip on your nearly dry unfired pot. The slip serves as both a coating for the raw clay and a glue, holding coils or other hand-built techniques together smoothly.
Explore coloring part or all of your slip as you become more expert. Potters' suppliers offer a range of coloring chemicals; what are called mason stains are judged most appropriate and stable for coloring slip. Colors can be brushed on or applied with a homemade pastry-bag. Make your decorating bag out of glazed paper, plastic or mylar, and stir colored slip as you fill it, to prevent air-pockets.
Making a Terra Cotta Casting Slip
Explore the use of defloccant agents, like sodium silicate or soda ash, when making a casting slip. Again, exact recipes are closely guarded. In general, casting slip formulation is described as half clay, half fillers and fluxes. The object of adding sodium silicate or another deflocculant is to cause clay particles to distribute more evenly in water and to use less water, hastening drying.
Stir slip very thoroughly. If you are making a large quantity, a mechanical clay-mixer will make the task easier. Scrape flakes of drying slip off your container and discard.
Make enough slip to allow for shrinkage in casting. A plaster mold works by soaking up the water in the slip and leaving the clay behind in solid form. As slip shrinks in the mold, add more, gently, to compensate for shrinkage, if desired.
About this Author
Janet Beal has written for Demand Studios since May 2008. She writes about gardening, the home, child development, and cultural issues. Her work has appeared on early childhood education and consumer education Web sites. She has a Bachelor of Arts in English from Harvard University and a Master of Science in early childhood education from the College of New Rochelle.