Why Does Plaster Harden? / Mr. Denjiro’s Happy Energy!

DENJIRO: Do you know what this is? It looks like glass, but it’s actually gypsum. It’s a rare, transparent crystal of gypsum called “selenite.” It’s a group of thin crystals, so if you shine light at it… it lets the light through and shines. Today, we’re going to perform an experiment to learn about the properties of gypsum.
This here is powdered gypsum. First, let’s heat this up.
NARRATOR: If you heat up powdered gypsum, you get water vapor. Gypsum is a substance made of calcium sulfate and water. When you heat it up, the water decreases. The resulting substance is known as “calcined gypsum.” We have gypsum and calcined gypsum, or heated gypsum. They both look the same, but we’ll see a difference when we add water. A few minutes have passed. Let’s try touching them. Here’s the calcined gypsum with water.
BOY: It’s hard!
DENJIRO: The calcined gypsum, which had water burned off, hardens because it bonds with water and we get gypsum crystals again.
NARRATOR: Regular gypsum, which is already bonded with plenty of water, doesn’t harden when water is added.
DENJIRO: Let’s try making a plaster figure utilizing this property. I’m pouring a mixture of calcined gypsum and water into a mold, and now I’m going to wait an hour. We have a plaster figure of a rabbit. Gypsum is also used for orthopedic casts as well.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!”