Post #626401 by KokoKele on Sat, Feb 25, 2012 8:42 AM

Oh, my, I think I might have been a bit dense about answering your last question. I'll bet you wanted to know how much plaster to actually mix up for each pouring for a mold your size. Right?

I always just guessed.

But if you're a genius, which I'm not, you can always use actual calculations!

After you get your answer, below, be sure to follow the instructions that come with the plaster exactly! The shop I buy mine from has an alternate instruction sheet that they give you, which they developed after years of trial and error, and it works great. If your shop gives advice like this, follow it!

Now, fun with math:

If your original piece is 3 inches by 8 inches, your mold box needs to be adjusted so that there is plenty of plaster around it after pouring. So you're filling a mold box that should be around 6 inches tall by, say, 12 inches long by six inches wide. Six times 12 times 6 is 432. That's the VOLUME of the mold box if those are your measurements. Now subtract the volume of the mug. That's a bit tougher. The formula for volume is pi times radius squared times height. Pi is roughly 3.14, and the radius is half the distance across the center of the mug, or 1.5 inches. So your formula for the mug volume is 3.14 times (1.5 times 1.5) times 8. The answer is 54 cubic inches. So 432 minus 54 equals 378 cubic inches.

BUT you only want to make enough plaster to fill up half the mold box, right? That means you only need enough to fill 189 cubic inches.

To get the number of quarts you need, divide your cubic inches by 58. 189 divided by 58 equals about 3.3 quarts. Since plaster takes up about twenty percent of the volume, I'd say you need about three quarts of water and about nine pounds of plaster. That should make more than you need.

So I guess you're making about a gallon of wet plaster. Sounds about right.

I'll bet some of the more seasoned mold makers out there have other methods for estimating their plaster needs.

Hope this was helpful!