The basic chemistry of all plasters is the same, calcium sulfate. The difference is the particle size. The coarser the particle size, the greater the spaces between the individual grains. Granted, it is hard to visually see or feel the difference, but it is significant. The coarser the plaster, the more water absorbent it is and the weaker it is, since the gaps between the particles can hold more water, but are not as tightly knitted. There are many different plasters, many with only subtle differences which don't concern most potters.

ROVIN CERAMICS maintains in inventory the following plasters, which cover virtually all the potential uses of the clay artist.

#1 Pottery Plaster -- Most water absorbent, but the weakest. Used for slip cast molds, wedging tables and other applications where water absorbency is the main criteria. Used extensively by greenware mold makers. Also the plaster of choice for glass molds, in conjunction with ground silica, etc.

#1 Moulding Plaster -- Harder and denser and less water absorbent than #1 pottery plaster. Used as a sculpture plaster, as well as wedging tables, mostly because it is stronger. Has been used for slip cast molds where the mold has many sharp edges which must be incorporated in the final cast shape. Casting time is much longer and number of cycles before drying is decreased.

Hydrocal -- not stocking - The hardest and least water absorbent. Cast as sculptural pieces such as lamps, ceiling molding, etc. Also used as tile press molds.

For Best Results
Always store plaster in a warm, dry, clean place. Do not use plaster that is lumpy. As a result of improper storage; it has absorbed moisture from the air and will have lowered its strength. Always use room temperature, potable water.

Sift or spread plaster evenly into the water (never add water to plaster) allowing no lumps to form. Soak the unmixed plaster for 2 to 4 minutes and mix until slurry attains the consistency of thick cream.



The setting time is affected by water temperature, speed and duration of mixing. The warmer the water, the faster the set. The longer the mixing procedure and greater the agitation, the faster the set. The greater the plaster to water ratio, the faster the set. Older plaster sets slower. Keep all mixing equipment clean.