Mixing Plaster

(excerpt)
How to mix Plaster:

1. Always weigh out the amount of plaster you need, then look up the water by weight percentage. For example USG # 1 Pottery for making molds uses 70% water to plaster. So 10 lb. of plaster requires 7 lb. of water. First weigh out the plaster you need, take that amount and multiply it by .7,(point 7), (7/10) this gives you how much water to use.

2. Generally speaking the larger the percentage of water the softer and more absorbent the plaster will be. The harder the plaster the less water used.

3. Hot water will speed up the set time and cold water will slow it down.

4. Place the water in a suitable sized container. Always add plaster to water. Pour the plaster into the water and let slake for one minute. Mix by hand or with a Jiffy Mixer, being careful not to suck air into the plaster. When the mixture is lump free it is ready to pour; don’t wait for it to start to thicken.
(end excerpt)

This is a note from a supplier. In all my packed boxes from the ceramics shop, I have the complete charts for every variety of plasters and cements. I promise, as I continue to unpack and arrange, when I run across these charts, I’ll be back with the information.

After studying the formulas, years ago, I devised a shortcut which I describe in the ebook. I’ve never had a problem using my little shortcut.

Just a couple notes:
Hot water truly does speed up the setting time. I prefer room temperature, as then my timed activities are right on.
I have, due to interruptions, left plaster sitting for up to an hour, without the chemical reaction starting, simply because I had not begun to mix.
Plaster is not panicky stuff. :)
Always, after shaking in the plaster, let it sit (untouched) for at least 5 minutes before mixing. This is called slaking. It gives ever particle an opportunity to come into contact with water.
Mix for 3 minutes steady (5 minutes for hydrastone). Let sit for 3 minutes. Mix again for 3 minutes and pour.

If using plaster or hydrastone to pour into rubber molds, tap the sides of the mold to release trapped air bubbles.