Just to preface what I am about to post...I don't know who posted this silicone mold recipe originally, nor do I remember where I got it from on the internet (I saved it to my computer a few years ago). If anyone knows, please let me know and I will be more than happy to give them credit. The information here has been valuable to me and I hope it helps someone else out.
"I was just lurking around your site when I came across some references to making molds from silicone sealant (caulk). There are a few things one should know about this process that will make it a whole lot easier.
First off, use only 100% silicone, preferably clear. DAP makes a pure silicone sealant as does GE. GE markets both Silicone I and Silicone II formulas but either will work. Siliconized latex caulk or any other silicone blends will disappoint you every time. Even the colorants used to get black, white, and almond silicones alter the characteristics of the product. The colored silicones will work, but trust me, start with clear.
Second, silicone caulk does not cure well in thicknesses beyond 1/4 inch. The skin that forms eventually becomes thick enough to encapsulate the uncured silicone which can remain soft for days or weeks. In very thick layers, the inner silicone may not cure at all. Some people have tried to market "catalysts" for silicone caulk. For tin or platinum cure two-part silicones, you do have to have a separate catalyst, but the catalyst for silicone caulk is water. Pure water does not disperse well in silicone but there are two vehicles that provide the required moisture and can easily be incorporated into the caulk, acrylic paint and glycerine.
It doesn't take a lot of either to do the job and don't try to use them as a thinner for the silicone. Too much will result in a product that sets up very fast, but is very weak. The glycerine has the added bonus of reducing the adhesion of the silicone, making it easier to remove from the object being copied.
I use the hobby acrylics that come in small flip-cap bottles. About three drops per ounce of silicone is sufficient. I add the glycerine by dipping a tongue depressor size mixing stick into the glycerine (available at any pharmacy, I've found K-Mart to be the cheapest). Dip the stick into the glycerine about a half inch for each ounce of silicone. For large batches a half teaspoon of acrylic paint and a teaspoon of glycerine will cause a uniform cure in a full caulking tube's worth of silicone.
The silicone will not seem to take the glycerine right away, but keep mixing and soon it will all come together. The paint makes mixing easy, just stir until the color is uniform. When mixing, be careful not to whip air into the mixture. Bubbles become trapped and result in holes or spongy spots in the surface of the mold.
This formula should give you about 15 minutes working time with a cure in 1-3 hours depending on heat, humidity, and variables in different brands of products. To be safe, allow 4-6 hours (or better yet, overnight) before demolding important projects).
This mold making material will come off of most hard, non-porous surfaces without the use of a mold release. Plaster or clay can be sealed with flat acrylic spray available from art supply or craft stores. A tablespoon of Ivory dish soap can be mixed with a pint of water and bushed over non-porous or sealed surfaces and allowed to dry for an extra measure of security. You can also use the soap mixture on organic surfaces such as leaves. Don't use petroleum jelly or cooking spray. They work, but what a pain in the . . . , well, they make spreading the silicone on the model almost impossible. Don't use silicone based release sprays. They work great for every other molding material I know of, but they become part of a silicone mold and lose their effectiveness.
Carefully paint a detail coat on your model with a disposable brush, making sure to work it into all the details. Silicone can even handle minor undercuts due to its elasticity. Once the surface is well coated, add a layer of silicone 1/2 to 3/4 of an inch thick. You can make a neat surface on the outside of your silicone mold by smoothing it with your fingers wet in the same Ivory and water solution. Allow the silicone to fully cure. Molds of small objects need no additional support. Larger molds should be surrounded by a wall of wood or plastic at least one inch taller than the highest point of the silicone and leaving about an inch of free space all around. Coat the back of the cured silicone, the surface on which it is sitting, and the inside of the retaining wall with petroleum jelly (it's OK at this point) or your soap mixture and pour a plaster mother mold over your silicone mold.
Once the plaster has cured, remove the retaining wall and invert the entire assembly of model, silicone mold, and mother. Gently remove the model and then carve a small notch at some point where the silicone meets the plaster. This is just a reference point which will allow to realign the silicone mold with the mother.
Clean the silicone mold with acetone or Ivory and water. Store it in the mother mold, dry and out of sunlight. It should last for years and give you many castings. Any mold release can be used with it, but typically cement and plaster can be cast in it without any additional release.
To cast in this type of mold, place the mold in its mother prior to filling it. After the cast has cured, lift the cast and silicone mold out of the mother mold together, then gently pop the cast out of the silicone mold.
I hope this wasn't too lengthy and confusing, but it may help you avoid the trial and error I went through.
Silicone caulk is not as strong or elastic as two-part mold making silicones, but it is not as expensive either, $3 per pint compared to $18-40. If you must thin it, use xylene, with appropriate gloves and ventilation or respirator. Thin sparingly, as this further reduces the strength of the end product and results in a softer mold that is more likely to deform under the weight of plaster or cement.
It will handle modest undercuts due to its flexibility. If the undercuts are more severe, the silicone mold can be slit with a razor blade or xacto knife, along one side, in a line that will facilitate the removal of the model and subsequent castings. The slit will be held closed with tape, an elastic band, or by the mother mold, during casting. A fine line will be left on the cast, which can be cleaned up after demolding. Very complex designs can require molds with two, three, or more parts and require more expertise and experience than I can pass on here.
Several of the silicone manufacturers have good tutorials on their sites. A link to one of them is posted below. Dan Perez Studios also has a tutorial on their site which explains the making of a silicone mold for a complex plastic model.
The best book I have ever found on mold making is The Prop Builder's Molding and Casting Handbook by Thurston James. It covers silicone as well as a couple of dozen other molding and casting materials aimed at film and theater.
If the silicone is curing unevenly or too slowly, you can try adding some heat and humidity. Don't place water in direct contact with the mold, but you can try something along the line of placing it in a small bathroom with a hot shower running or place it on a rack above a pan of hot water in a cardboard box. Both the heat and the humidity will help curing. Don't get it above about 100 degrees F.
If you need a fast plaster shell for it, you can buy a roll or two of plaster gauze from a hobby or craft store. This is pretty much the same product as doctors used to use for making casts for broken bones. Two or three layers should be sufficient to make a quick mother mold over the cured silicone. An alternative would be to mix plaster of paris to a pancake batter consistency with very warm water. Fold a piece of cheese cloth to three layers thick and dip it into the bowl of plaster. Squeeze out the excess plaster and then lay the triple layer of plaster impregnated cheesecloth over the cured silicone just as you would have used the plaster bandage. Keep this warm and dry and it should cure well enough to be used in about an hour.
If you are making your casting in plaster, remember that mixing with warm water makes plaster of paris set faster as does drying it in a warm place. Don't spot heat drying plaster with something like a blow drier though, as it will promote cracking."
