Tuesday, April 8, 2014

09 Plaster Production Molds Nº 20

Once the plaster build-up is done, it is time to move on to making the production mold. The oil-clay slab that I rolled out a few days ago will work as a support for the plaster build-up because the carving wax foot is higher than the plaster build-up is thick. I cut out a foot shape in the oil-clay slab and set the plaster build-up on top of it. Then I trimmed the excess oil-clay around the edges with a fettling knife. Click on any image to enlarge it.

Can I use the same coddles I used to make the plaster build-up, for the final production mold? Maybe. The sole of the foot sticks up about 0.25 inch. That, plus 1.50 inches is 1.75 inches. So yes, I will be able to use the same coddles, with a little extra space to spare.

This is one of the old diagrams I made for figuring out registration keys. The positive keys for mold pieces 1 & 3 must be on those pieces in order for those pieces to be removed sideways. At least, that is how I think it should be.

If I put some positive oil-clay keys where I want the negative keys to be on the sole piece of the mold, then the sole piece will have negative key holes. That would allow the top pieces to slide out sideways. The possible problem of doing it this way is that any of these oil-clay keys might move when I'm pouring the plaster for the sole piece. Alternatively, I can carve them after I pour the sole piece of the mold.

I will probably carve them after I've poured the mold piece.

Some Mold Arithmetic

Most of the time, I can estimate more or less how much plaster I need to mix by simply guessing how many cups of water will fill the coddles. When I feel I need to be more accurate, I use some mold arithmetic.

I will be the first to admit that I am not very good with numbers. That is one reason why I went to Art School instead of studying Math or Science. But one thing I have found out is that being able to do some simple arithmetic can save time, money, and materials over the long run. So this is what I figured out for the mold piece for the sole of the foot:

Using 1.0 pound of water and 1.5 pounds of plaster, I made the 4.5x7.0x1.0 plaster build-up with a little bit left over. To make a mold piece that is 4.5x7x1.5 inches, I will need half again as much water, so:

.75000 (one part)
2.25 (three parts)
.66666 (consistency of 67)

I hope that is right?

1.75/2.625 (pounds - 2:3 ratio)
.66666 (consistency of 67)

That gives me a little bit more, but keeps the same consistency of 67, or 2:3 (water to plaster, by weight).

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