Crafts Plaster of Paris is gypsum rock which has been heated to drive off the chemically-combined water and then pulverized into a powder. This powder, when mixed with water, will set into a hard, rocklike substance, much like the original gypsum rock; this is one characteristic which makes plaster a suitable material for ceramic molds. There are many types of plaster - each one intended for a definite purpose; pottery plaster is the best type to use for molds. Pottery plaster, when mixed with water and poured over an object, will faithfully reproduce the shape and texture of that object.

It is not just a matter of mixing some plaster and water and then pouring it over the form that is to be reproduced; the ratio of plaster to water is vital to the quality of the finished mold. If too much water is used, the mold will be soft and weak; while the mold will be too hard and not absorbent enough if too much plaster is used. Best results are ensured if the plaster and water are carefully measured, using one quart of water to each 2-3/4 pounds of plaster.

Using the correct amounts of plaster and water, the plaster is sprinkled onto the water and allowed to slake (or soak), for a few moments, then carefully stirred so that no air is introduced into the mixture. When the plaster begins to thicken, it is poured smoothly, without splashing over the model or form to be reproduced in the mold.




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May 19, 2012