PLASTER MOLDS FOR CLAY ARTISTS
Potters’ molds are generally made of plaster. Some are as simple as a sheet of hardened plaster while others are considerably more complex depending on the number of pieces it takes to fully surround the model.
Hardened plaster is rigid and will become stuck in areas where “undercuts” exist. The artist mold maker must design the mold such that each piece can be removed easily from the form. The more undercuts there are, the greater the number of pieces required to make the mold useful for clay.
Plaster is made from refined gypsum where the chemically bonded water has been removed in a slow cooking process. When the powder made from this process (i.e. the plaster) is mixed with water in the appropriate proportions and stirred for the appropriate amount of time, the plaster recombines the water that was removed in the heating process to form a solid over the few minutes of its set time.
Plaster is the preferred material because the hardened material absorbs water from the clay. This absorption by the plaster leaves the clay firmed where the two meet, giving form to the desired shape.
Mold making requires discipline and skill to do well and represents a significant investment of time by the artist. While commonly thought of as a tool of mass production, in the hands of an artist mold making can be a means of producing unique, one-of-a-kind art objects. Indeed, sometimes it is the only way of achieving the desired form.
Casting Slip is a liquid suspension of clay where the ions are charged negatively. This charge allows the clay to flow freely with a lower water content than would otherwise be the case. Casting slip does not settle and makes for an even cast from the slip mold.
The Model is a form that the mold is made from. The artist can make the model out of clay, plaster or some other appropriate material or it can be a found object. The mold is built around the model in as many stages as are defined by the number of undercuts.
Types of molds
There are many different kinds of molds used by the clay artist such as drape and slump molds, sprig molds, press molds and slip molds. The artist is free to mix and match processes, attaching molded pieces to parts made by different processes such as slab building and wheel throwing.
Sprig Molds are usually small detailed molds where wads of clay are pressed into them to make copies. The resulting sprigs are applied with slip on top of the surface of a still damp clay form.
Press Molds are a type of mold where clay is literally pressed into the working face of the mold to create the desired form. These can be single piece molds or multi piece molds where the parts must be joined with slip.
Drape Molds and Slump Molds are single piece molds where a sheet of clay is patted down on the surface of the plaster to create the form.
Those artists who primarily use molds for their pieces are Rachael Kroeker and Kevin Stafford.