Plaster Of Paris: Of all casting materials the most used by sculptors is plaster of paris (calcined gypsum, or sulphate of lime, or Ca SCU . Y2 H2O). Except for its fragility it is an almost ideal casting substance. It is available everywhere cheaply. It is safe to handle, simple to use, and sets rapidly. Also it expands slightly on setting and thus tends to fill the mold completely to produce a clear cast. Usually when a work in clay has been modeled it is cast immediately into plaster, for the clay will not long maintain its shape under most conditions. Such a cast requires the making of a piece mold in plaster and then the use of the piece mold as a negative to cast a positive which will be a replica in plaster of the original clay. Specially prepared plasters are also used in making piece molds for the casting of nonferrous metals.
Undercoat plaster For deep or wide cracks and holes, first apply an undercoat plaster. On absorbent surfaces, such as brick or lightweight block, choose a browning coat. If you are plastering over concrete or stone, use a bonding coat.
Finishing plaster For the final layer, apply a lightweight finishing plaster, such as a pre-mixed gypsum plaster to which you need only add water. This is more convenient than plasters that have to be mixed with sand.
Gypsum plaster coving is normally available in 6J^ft (2m)lengths, either 4in (1 Ocm) or Sin (1 2.7cm) wide - although longer lengths can be specially ordered. Like Styrofoam coving, plaster coving is also mounted with a special adhesive, but good wa and ceiling plaster surfaces are essential. Cutting miters for the corners can be tricky, but plaster coving is always supplied with a template. It is wise to practice cutting miters on scraps of wood before tackling the coving itself.
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