Mixing Plaster

Mixing plaster can be done proportionally by formula following a water-plaster ratio of 1 to 1-3/8 by weight, or by eye.

By eye
Plaster is a material that cures: the gypsum reacts to the water by heating up to a paste before it hardens. Therefore, the colder the water, the longer the working time. When pouring plaster into a bucket of water, make sure that the plaster is evenly distributed around the bucket. If it is not, one may end up with more water than plaster. Maximum saturation is achieved when the surface clumps of plaster (created when the plaster hits the water) no longer submerge under the level of water. At this point, wait a few moments before mixing the plaster. When the mixture has reached a consistency of cream, the plaster is ready to use.

Wait a minimum of an hour for partially cured strength. For full strength, store the plaster form overnight in the driest setting possible. As there are different plaster formulas and strengths -- for example, the difference between Plaster of Paris and Hydrocal (US Plaster) is significant; cure time and cure strength may be differ from product to product.

Successive layers of plaster adhere to one another. This can be avoided by using waxes or oils.

If you mix many batches of plaster in the same container over and over, remove as many pieces of dried plaster as possible from the container before beginning to mix a new batch. Small pieces of plaster left in the container will cause the new batch to begin curing more quickly. Using partially-cured plaster upsets the chemical reaction and may cause the plaster to be weaker than if it had been used fresh.