Book 27, Question 118
How does plaster harden?
Plaster of Paris is a dry powder made from gypsum.
When mixed with water, tiny needle-shaped gypsum crystals grow through the mixture and lock together like a tangled mat, making it hard. The mixture warms up as the crystals form. People use plaster for casts, wall repairs, and crafts. Mixing and handling should be done with a grown-up, and plaster must never go down a drain.