How to Cast a Mould
The piece of work to be cast should be finished to the best of your ability. In this case, we want to reduce our 18 inch Peggy of the Cove doll to a 12 inch so we can mass produce her. The first image on the right is the finished doll. Below is the sculpt head ready for casting.
Porcelain shrinks when fired. We may have to go three or four reductions to accomplish the final results. That's a lot of moulds.
Spray the entire head with a release agent. You can buy pottery release agents for casting plaster moulds, but Pam or any non-stick agent works just as well.
CONTAINER FOR MOULD
Select a container that is large enough to allow for a thick mould. If the mould is too thin, it may break.
Pottery Plaster is not expensive and can be purchased at your local pottery supplies dealer. Make sure you keep it sealed and dry after opening. Exposure to air and dampness will destroy it's properties and it will not set up properly. You'll end up throwing it out.
Instructions for the proper water to plaster are available from your supplier. I mix equal parts of water and plaster and add one or the other if necessary. The warmer the water and temperature, the faster the plaster sets. You have approximately three to five minutes before the plaster is too hard to set the sculpt. Avoid breathing the plaster. Using a mask is advised.
SETTING THE HEAD
Fill a container about half full of plaster mixture. Pull away any bubbles with a paint stick or brush, that rise to the top. A decompression chamber is used by the professionals but they cost in the $500 range. Bubbles are going to be your enemy and are hard to eliminate. When you pour the mixture into the mould container, you'll see how free of bubbles the mix is at the bottom.
When it begins to thicken, gently set the sculpt half way into the plaster. For certain areas you may have to add a little plaster or scrape away. Make sure curves will not prevent the sculpt from releasing or break delicate parts like the ears.
This sculpt is made of Smooth Cast Resin and very strong. Normally the sculpt is made with Sculpy which is quite brittle in thin delicate sections. Often an ear or tiny fingers will break when removing them from he mould.
If you look closely, you'll see it was necessary to scrape a little plaster away from the side of each ear.
Before the plaster hardens solid, take an object to form a hole so both parts of the mould will fit and line up when completed. The end of this metal rasp works fantastic. Spin it slowly as it bores to the required dept. Usually I make three or four holes for accurate positioning.
While the plaster is setting, I prepare the second patch of plaster.
Here's the critical point. When the first half of the mould is hard enough (usually within 10 minutes) spray the entire surface with a good coating of release. Two coats are better. If you forget to do this... your baby will be encased in plaster. It will take a hammer and chisel to crack open your sculpt. Most likely it will be in pieces too.
If necessary and most often is, use a soft paint brush to evenly smooth the release over the face. That's the area that may obtain too much release. The plaster absorbs it.
Now you're ready for the second half. If you pour it instantly after mixing, the chances of eliminating bubbles forming on the face of your mould are much greater. Remember, bubbles rise and that's what you want them to do... rise to the top on the second half.
Notice the X near the bar code on the container, between the two strips of overflow, that's where the bottom of the head is located.
REMOVING THE CONTAINER
After the second pour is hard enough, cut a slit in the top of the container and peal it off. By removing the sculpt before the plaster is rock solid, the sculpt has less of a chance of breaking.
Remember where the X marked the spot just in case you can't see the bottom of the neck. Cut away any plaster and gently separate both sections. If they don't release, use a chisel or putty knife to slip between the two sections. If the plaster has blended into the lower section, scrape around the middle and the lines will be distinctive. Then wedge apart.
REMOVING TOP SECTON
After the top section is removed, gently wiggle the sculpt. If it refuses to budge, it may be necessary to scrape away an area that is preventing the sculpt from releasing. Try not to go overboard. You want your mould to be as perfect as possible.
Now your mould is finished. Let it dry for days before using. Of course it depends on the climate. If you live in a desert area it will dry much quicker that places like the rain forest.
Lots of success in your adventure. If at first you don't succeed... you know.
Notice: One of the posts broke off when removing the top section. In our case this isn't critical.