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Can someone help me work out determining the amount of mold plaster to mix up? I have shallow open-faced molds to build, each measuring 9x12x2 inches deep. I am using R&R 101 and know the proportions of mix to water - I'd like to know what finished volume of mixture to shoot for. In the past I just guesstimated how much to mix up and either ended up with too much left over or forced to quickly make up additional mix to complete the mold. If someone can walk me through the math on this one it will help save materials, and/or mad scrambles to top up the mold.

TIA... Faye

Faye, an easy way to do this is to fill the empty space with short grain dry rice. That is, the space that you want to occupy with the mixed investment.

Empty the dry rice into a measuring cup. The volume of the dry rice is the amount of water in your investment.

This method will give you a little too much investment. A little more is better than a little less.

This method also works with mixing plaster, concrete, etc

Peter

I hope this is right! My notes from Milon Townsend's class was this:

for a round flask (even with a wax positive in it) the formula is:
3.14 x radius (inches) x radius (inches) x height (inches) divided by 17 = weight of wet mix in pounds

if it is a square flask the formula is:
X x Y x Z (inches) divided by 17 = weight of wet mix in pounds

I can't locate my handout for the class, but this was written in my notebook.
You start with water and add the R&R to the water. You could weigh out the correct ratio first without mixing, I suppose just to see if it kinda looks as if it will work.

Let us know if it does work!

Thanks all. I lost my notes from Milon's class but a friend retrieved the info. Molds are made - alll looks good.

A useful quick reference table for proportions and volumes of a refractory plaster mix is on the last page of Ted Sawyer's Bullseye tipsheet on Box Casting.

Nikki

Since I make my molds in layers, and hand-build them, I have an entirely different method (although I use Peter's when I'm box casting). But when you start making molds, always grease up a margarine tub or takeout tray and keep it at your plaster station. If you mix too much plaster, pour off the excess into the tubs--slide it down the sides instead of splashing it on the bottom.

When they cure, you've got excellent little blanks for carving into and making small molds. I've used them for rings, earrings, small figures, or hollowed them out for vessels. Wear a respirator when you work on them.

Great idea Cynthia!

Morganica wrote:Since I make my molds in layers, and hand-build them, I have an entirely different method

Cynthia, do you allow each layer to fully set before applying the next layer?

Peter Angel wrote:

Morganica wrote:Since I make my molds in layers, and hand-build them, I have an entirely different method

Cynthia, do you allow each layer to fully set before applying the next layer?

Hey, Peter...this just popped up with an old date and I saw I didn't respond. The longer you wait between layers, the more likely the layers are to delaminate (i.e., pop apart at the most inconvenient time possible). So I usually wait until the plaster has stopped moving on the mold and is beginning to set up, but not until it's knock-on-it-and-it-rings set up. Generally, that's about the time it takes for me to clean out the plaster bucket, refill with water, dump in the new plaster and additives, let them slake and then power-mix them.

Just to be on the safe side, I like to "key" my molds in between pouring, i.e., add some texture and undercuts to the surface to ensure that the new layer locks into the old even if it does start to delam. I usually do that as I'm pressing in the layer, i.e., getting a tiny bit of water on my glove and going over the surface, pushing the new plaster into the old to make sure there aren't air cavities.