Monday, June 15, 2015

09 Plaster Production Molds Nº 210

Before I mix the plaster, I like to do a wee bit of arithmetic to figure out the volume. The formula for volume is Volume = Length X Width X Height. The result is expressed in cubic inches or cubic centimeters, whatever. This first mold piece is 8 inches long, 6 inches wide, and 3 inches high, or 8L X 6W X 3H.

So first I multiply everything to find out the cubic inches.
8 X 6 X 3 = 144 cubic inches.

There are 14.44 cubic inches in ONE CUP.
So next, I divide 144 by 14.44 to find out how many cups there are.
The reason for this is because I will figure out how much water I need.
ONE CUP of water weights about 8 ounces.
144 / 14.44 = 9.97 cups.
I can round that 9.97 up to 10.
10 cups X 8 ounces = 80 oz.
The ratio of water to plaster is 67 to 100, or about 2 to 3.
80 / 2 = 40
40 X 3 = 120
So I will weigh 80oz of water, and add 120oz of plaster to the water.
80 / 120 = 0.66666... which is very close to the consistency I want: 67.

I always mix up a little bit more plaster than I actually need. It is much easier to toss the extra plaster in the lined trash bin than it is to mix up more plaster if I didn't mix enough. In this case, I will be mixing up about 7.5 pounds of plaster.

If I wanted to get even more precise, I could subtract the volume of the upper thigh that is sticking up above the oil clay build-up, from the volume of the mold piece. I would probably do that if I was using an extremely expensive mold material like silicone rubber.

In other news, I see that I am getting close to running out of plaster. So I will need to go to the local building supply store and buy another 100 pound bag of No.1 Industrial Moulding Plaster. The last 100 pound bag of plaster I bought cost about $44 USD. I wonder how much the price has gone up since then? I'll let you know.

UPDATE: The 100 pound bag of USG Moulding Plaster cost $43.75 plus $2.95 tax for a total of $46.70 USD. So each pound of plaster I use costs about 47 cents.

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