What is the right measurement for making plaster of paris that only add water?

Answer 1

You put an amount of water in a bucket that you think will be about the right amount. Then begin sprinkling plaster in the water, a little at a time, until a peak forms in the middle -like a mountain or cone. Then, and only then, begin to gently stir the plaster into the water. When it seems just a little watery-er than you would like, stop stirring, and start applying the plaster. It works FAST, so be ready to work immediately.

The measurements are inexact because plaster and water have a chemical reaction that changes according to temperature. It takes experience to be good at guessing the amounts needed.