Crafts The term "ceramic" can be applied to a vast number of products - from fine china to sewer tiles and bricks - and a multitude of processes are employed for their production; since this series of articles deals only with hobby ceramics, we will be concerned with the production of ware cast in plaster molds, and its preparation and decoration.

Long ago, someone discovered that a lump of clay placed on a piece of plaster became dry and hard where it came in contact with the plaster, and reproduced any scratches, dents or other markings that happened to be on the surface of the plaster. This led to the development of molds into which slabs or pellets of clay were pressed by hand, enabling a potter to reproduce any number of identical shapes. A mixture of clay and water (called "slip") could also be poured into these early molds and the plaster would absorb the water from it, leaving a shell of clay against the plaster. The longer the slip remained in the mold, the thicker became the shell of clay, until the plaster was saturated and then the process would cease. If the potter poured off the remaining slip, a casting duplicating the mold shape would result. The only problem was that greenware obtained in this manner was difficult to remove from the mold, was sticky when wet and extremely brittle when dry, and had a high shrinkage rate. Another draw-back was that, since the slip was composed of about equal parts of clay and water, the mold rapidly became saturated with water, and could not be used again until it was allowed to dry.




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Home > Crafts > Ceramics > Molds And Slips

November 20, 2008