Gelatin is more translucent and moves better than some materials, has a very realistic texture, and doesn’t take much makeup to get good cover. However, gelatin breaks down with heat and perspiration; that is, it melts and dissolves, respectively. There are several effective workarounds for sweat-related problems, but heat, such as working under hot stage lights or on location near the equator, is another matter, and there’s not much you can do about that except use something other than gelatin.
Gelatin is considered to be hypoallergenic, that is, allergy-free for use on most people. That’s a good thing. Gelatin is also considered hygroscopic, which means that it has a tendency to absorb moisture from the atmosphere. This is both good and not so good: Good in that it allows gelatin to be soluble—to liquefy and dissolve. With the addition of water, the gelatin particles swell and expand, actually absorbing up to 10 times their weight in water (which, in turn, can make gelatin appliances somewhat heavy); not so good in that gelatin appliances can swell in proportion to humidity changes in the air and shrink over time through evaporation.
One way to help minimize this change due to humidity is to substitute glycerin for almost all the water used in the gelatin appliance formula. This is very good for creating a variety of wounds and injuries—cuts, burns, bullet holes, swelling, and the like. This gelatin can be colored with flocking material, food coloring,
or powdered cake makeup and stored in small squeeze bottles to be heated until the gelatin liquefies. This is similar to a number of gelatin effects kits that are available commercially. Just be very careful not to overheat the gelatin; because it is organic material, it can be severely damaged or ruined if heated too much (or too often), and you don’t want to burn your actor by applying gelatin that is too warm. Always test the temperature before application!
As I mentioned, glycerin is also hygroscopic, so some formulas replace some of the glycerin with Sorbitol, which is derived from corn syrup and is less affected by changes in humidity than glycerin. Sorbitol also increases the structural integrity of gelatin formulas, making them more tear-resistant, which is critical for prosthetic work. (I’ve seen formulas that added Elmer’s® white glue for the same purpose with good success.) Zinc oxide can also be used in small amounts to add strength and greater tolerance to temperature changes (remember, gelatin tends to melt when heat is applied). You will have to experiment when using zinc oxide because it will affect the translucency of your finished gelatin appliance. Zinc oxide powder is the preferred form, but sometimes it can be tough to find; zinc oxide ointment will work, though. Regardless of the formula you wind up using and calling your own, keep your finished appliances in air-tight plastic bags, away from the light and in a cool place. They will last much longer.
For prosthetic appliance work, the gelatin you use will need to be light and soft—foamy. So, the recipe I’ll give you is for a foaming gelatin. The resulting gelatin will not be as light and soft as foam latex, but it will be substantially lighter and spongier than a solid gelatin appliance and definitely lighter than silicone gel.