Sunday, January 9, 2011

Making A Plaster Mold Using A Hard Boiled Egg

I have recommended that anyone who would like to learn how to make a two-part plaster mold, yet has some trepidation about learning how to do it on their in-progress doll parts, should practice making an egg mold. An egg mold uses a hard boiled egg as the model. The hard boiled egg is cheap, small, and it demands that you get the parting line absolutely right, or it will get locked in the mold. So here is a quick, short tutorial on how to make a two-part plaster egg mold.

Hard boil the egg, first.
Put the egg on a small donut of clay so it will not roll around.
This clay cushion should not extend out past the edge of the egg.
If it does, trim the excess off with a knife.

Try to get the egg as level as you can,
I'm imagining that the egg is on its side, with the big and small ends on the same plane, parallel to the work table.
That is, the egg does not have the small end up, nor the big end up.

To make the parting line, you will need a L square angle and a mechanical pencil lead.
Tape the mechanical pencil lead to the edge of the L square angle.
Now, carefully move the angle up to the egg, so the mechanical pencil lead touches the egg.
Move either the egg around the mechanical pencil lead, or the L square angle around the egg.
A line will be drawn at the point that is the parting line.
This same technique can be used with many doll parts.

Now that you have drawn the parting line, build-up the clay to the parting line.
Try to make the clay build-up perpendicular to the egg, and make it flat and smooth.
For a small part, like an egg, make the clay build-up 1.25 inches wide from the edge of the egg.
Take your time and do a good job.
Plaster mold making is a craft skill.

You can make the spare (the pouring funnel) from clay or carve it out later.
I prefer to do a combination of putting in a clay spare form, then carving it later.

You can use a knife to square up the outside of the clay build-up.
Once the outside edges of the clay build-up are square cut, place the coddles
around the clay build-up, and secure them with thick rubber bands or clamps.
Roll out some thin coils of clay and seal the inside corners of the coddles.

Apply parting agent to the egg and clay build-up.
A good parting agent is a 1 part liquid soap to 1 part water solution.
Weigh the plaster and water to be used.
The ratio for a plaster mold for slip casting is 2 parts water to 3 parts plaster by weight.
For example: 2/4 lb water to 3/4 lb dry plaster.
Always add plaster to the water.
Let the plaster slack (absorb the water - about 1 minute, or so).
Mix the plaster, squishing out the lumps, and not introducing air into the mix.
Mix until creamy (about 1 to 3 minutes).
Pour the plaster mix into the coddles until it is about 1 inch over the egg.
I like to pat the top of the poured plaster with my fingers to help release air bubbles.

Let the mold half setup for one hour.
Do not disturb it during this time.
The plaster will have an exothermic reaction during setup.

Once setup, remove the coddles, flip the mold over, and remove the clay build-up.
Leave the clay for the spare, if you did that.
You'll make the other half of the spare on this second half of the mold.
Carve the registration keys with a spoon. Always make at least three keys.
The registration keys do not need to be very deep. Do not undercut the keys.
Apply parting agent to the plaster surfaces and the egg, and the clay spare.
Put the coddles around the mold half and secure them with thick rubber bands or clamps.
Roll out some thin coils of clay and seal the corners of the coddles.
Mix and pour the second half of the mold.
Let it setup for one hour.

Once setup, the plaster mold halves may be prized open.
I often use a knife blade to do this. I insert the knife blade along the parting line
and tap it gently with a small hammer until the mold starts to crack open.
I move the knife all around the parting line.

Once the mold is open, remove the clay spare, and the egg.
Use a knife to bevel the outside edges of the mold.
Carve the negative place where the spare is to refine it.

If you are going to use the mold for casting wax, it must be saturated with water.
If you are going to use the mold for casting ceramic or composition slip, it must be dry.
Put the mold halves together, and put a band around them for drying.
Believe it or not, plaster mold halves can warp!

I hope this quick little tutorial helps!
Remember that plaster mold making is a craft skill.
Like all craft skills, you may not get it right the very first time.
Do not get discouraged.
Egg molds (or different-sized ball molds for ball joints) are small and cheap.
With every attempt, you will get better at making plaster molds.

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