Here is how to make a multi-piece plaster mold without any arithmetic.
(This is off the top of my head, so please double check these steps.)
First of all you must figure out the parting lines.
Parting lines define the edges of each piece of the mold.
You make one piece at a time.
Put the part to be molded on a molding board.
Put some clay balls under the part so it will not move, and so the parting line is level,
or is as level as you can get it.
Build up a clay bed to the parting line.
The clay bed should extend at least 4cm (1.5 inches) all around the part.
The clay bed should be perpendicular to the parting line.
You can make a spare (the pouring hole) at this point, or wait and carve it later.
If you make the spare at this time, it can be made from clay.
For a two-piece mold, I usually make half of it in clay at this time.
Make sure the spare is large enough so that slip can be easily poured into it.
Make sure the area where the spare attaches to the part is big enough so excess slip
can be poured out of the mold when the slip reaches the proper thickness.
Put coddles around the clay build-up and seal all the edges with small coils of clay.
Apply parting agent with a soft brush or sponge, to the part ,the clay build-up, and the coddles.
Estimate the amount of plaster you will need.
Pour water into a clean mixing bowl. The amount of water equals your estimate.
Rapidly sift fresh plaster into the water until a dry lake bed surface appears.
It will be cracked like a dry lake bed. Always add plaster to water.
Wait until there is no dry plaster. This is called slacking.
Stir the plaster-water mix until it is creamy.
Pour the plaster mix into the coddles, covering the part.
Allow the plaster to set up for one hour.
Turn the part over.
Remove the clay build-up.
Carve registration keys into the first half of the plaster.
Add the second half of the spare, if you are making it now.
Put the coddles around the first half of the mold.
Seal the seams of the coddles with coils of clay.
Apply parting agent to the part, all the exposed plaster, the clay spare, and the coddles.
Mix and pour plaster the same as above.
Let sit for an hour.
Remove the coddles.
Carefully open the mold halves.
Remove the clay spare.
Bevel the edges of the mold with a knife.
If you did not create a spare before, carve it into the plaster now.
Put the mold back together and set it aside in a ventilated area to dry.
When it is dry, you may pour slip into it.
Depending on the humidity where you are, it may take several days to a week?
I hope this helps?
Plaster is a traditional sculpture and doll mold making material.
Like any other traditional material, it requires practice to make good molds.
I was taught plaster mold making by a teacher in University, many years ago.
Once I got good at it, he had me teach other students in his class.
The example I used to teach 2-piece plaster mold making was a hard boiled egg.
If you don't get the parting line exactly right, the egg will be locked in the mold.
It is a great plaster mold making exercise, doesn't cost much, and helps you learn.
Once the skill of plaster mold making is learned, it will help you make any kind of mold.
To date, the most complicated plaster mold I've ever made had 12 pieces, for a torso.
Each piece was made, one at a time. Each piece was poured into an area defined by a
clay build-up that was built-up to a parting line on the part being molded.
Registration keys were made in each piece, so it fit exactly to the piece next to it.
Parting agent was always applied, especially where plaster touched plaster.
You must remember that plaster will stick to plaster!
Use a parting agent if you don't want fresh plaster to stick to a piece already poured.
Each mold is different, so there isn't a specific one way to make plaster molds.
You must always figure out each piece of the mold, each time you make a mold.
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