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Charles Latex is derived from rubber tree sap, is fairly inexpensive to buy and very labor intensive to use. And it shrinks considerably.
The rubber you are looking for is called RTV silicone (for room temperature vulcanizing). It is important to know if your sculpture is made of Plastilene or true clay (wet earth). Some plastilene's contain sulfur and sulfur can inhibit the cure of certain silicones. (tin) Wet clay contains moisture- which can inhibit the cure of other types of silicones. (platinum ) You may also need to get a bag of glass microspheres, called cabosil or aerosil. This is used to further thicken the rubber to a paste-like consistency.
Your best bet is from Silicones Incorporated, a product called G.I. 1000. You will need to buy two additives, first a 'thixotropic agent' to mix with the silicone. This agent makes the silicone thicken so that it can be laid up by hand in thicker coats without running off. The second is 'ultrafast catalyst' which helps the action of the thixotropic and causes the rubber to cure rapidly enough to apply several coats in a day.
Your sculpture needs to be sealed with either mold soap or a spray acrylic (lightly) so that the silicone won't sink into the surface.
It helps if you have a vacuum pump and chamber that you can use to elimanate air bubbles from the mixed rubber prior to applying your first coats. Be aware that high temperature and humidity accelerate the curing of rubber- you do not want to use so much ultrafast that the material siezes up before you can get it smoothly applied
Your first (face) coat needs to be finely applied and not very thick, take time to eliminate airbubbles. You must wait for this coat to fully cure before proceeding. The second (face) coat can be thickened as thick as the agent will allow. You must wait for this coat to fully cure before proceeding.
After the first 2 face coats- it is time for the fill coats. You should do all the filling at once and it will take a lot of rubber. Measure out several buckets worth of rubber, so that each will require the same amount of catalyst. Use the full amount of thixotropic agent but go lite on the ultafast for these coats. You want these coats to take a longer time to cure. Starting with one bucket- add the catalyst and mix, adding aerosil all the time until the rubber is a stiff paste. Smear this on, building up the average thickness and filling in undercuts. Move immediately to the next bucket and do the same- do not wait for it to cure- you want to do all the filling before the first fill coat cures.
When you have all the undercuts eliminated, and the high spots uniformly covered with at least 1/4 inch- mix up one last bucket with no thixotropic and some ultrafast. Do not add aerosil, you want this 'gloss' coat to be thin. Using a brush, wet it with silicone and brush over the fill coats, smoothing the fill coats down . The idea is to leave as smooth and uncomplicated a surface as possible on the outside of the rubber. Let it cure overnight- longer if it gets below 55 degrees F. at night
You will not need to reinforce the silicone with cloth, it has a tear strength of 100 pounds per square inch. Also, I find that my hands work better than any brush in applying the silicone- wear disposable gloves.
You do not need to put a separation agent on the exterior of the rubber- virtually nothing will stick to cured silicone. You can make a casing (mother mold) out of fiberglass reinforced polyester resin or fiberglass reinforced epoxy resin but you can also make a casing faster, cheaper and better out of Sisal reinforced plaster. Sisal is the raw hemp fiber from which manila rope is made. Look for it at construction supply houses. For a large mold- build a wooden frame that you can tie the plaster into for added strength, the plaster casing itself should only be about a half inch thick on average.
I strongly recommend hiring an experienced moldmaker to assist you and w
Before you buy.
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