Gelatin or Gelatine is an animal protein capable of retaining many times it's weight and volume in water or certain other liquids.
Gelatine is an excellent medium for Prosthetic makeup due to it's flesh-like consistency, elastisity, high tear threshold and ability to capture fine detail when properly prepared.
An added bonus which makes it especially optimal for beginners is the ability to recycle gelatine, so long as it isn't heated beyond a certain threshold. Flawed prosthetics may be remelted and recast repeatedly.
By using low-water components to prepare gelatine, such as glycerine and sorbitol, the life of a gelatine prosthetic can be increased many fold. due to the component's hygroscopic properties, water does not leech or evaporate from the substance, reducing shrinkage.Gelatine prosthetics may be thinned or dissolved with Witch hazel. Warm witch hazel works best,but whether Witch hazel dissolves or corrodes is debated . Many artists leave a bit of the overflow on the blending edge so that during application they can pull the blending edge out as it is adhered and then use witch hazel to cut away the edge. This works well,but being able to brush away a thick edge as is done with acetone and cap plastic is not in my experience.
For adhesives, silicone adhesives lead prosaid use for application. Gelatine is not porous to sweat or air, so any undried prosaid will not dry and sweat can cause little pockets of moisture and resolvented prosaid. Even the hydraulic force of sweat can lift a piece. Also gelatine is not quite as mobile as skin and can eventually pull away in mobile areas in the face. For stunt players or desert movies gelatine is not the best choice. Like silicone it can be heavier than foamed products.
That said, the sheer advantage of having media that matches the translucency of skin and the ease of production makes it very popular. Pieces can be molded to match the actors skin tone by adding facial powders,pancake makeup,flocking,or othe dry pigments.With the right application, pieces can be applied that are indetectable. Molds can even be run on location since very little in setup is required.Easily 20 or 30 runs of a mold in a day are possible. Old age makeups, look alike makeups and other realistic makeups are good choices for gelatine or silicon.To match skin tone , add warm colors, or add breakup; gel makeups (like gel blush or tan gels) or alcohol activated colors seem to blend in to the piece better while most normal makeups and PAXs tend to look like they are on the surface. Again translucency vs opaque coloration.