Start by preparing a gelatin base.
Remember we will be foaming it, so you do not need to make the same volume of gelatin as your prosthetic...assume roughly half
Now that your gelatin base has cooled, you should see that the material itself is quite solid and heavy, and though pliable it's never really in a workable state at any point of the heating/cooling phases.
In order to make prosthetics and effects that are light enough to stay on your skin, we will 'foam' the gelatin, using the same basic process as in a science-fair volcano.
Ideally for this, we want to introduce as little liquid as possible, as the more water there is in the gelatin, the more it will collapse and degrade. But, as powdered acidics are less likely to be found in your average house, vinegar can be perfectly effective for small quantities of foamed gelatin. If preparing larger quantities I would recommend acquiring powdered citric acid or tartaric acid.
Still, using vinegar in my tests, You can prepared prosthetics a couple of weeks in advance and they survived admirably in the fridge.
So take your base, and heat it in the microwave for 20 seconds; if this is not enough time to melt the gelatin, do small extra bursts of 10 seconds rather than a long heating. Do not let it get too hot or it will break down and become useless, as well as turning into a superheated liquid of doom.
Add the color (acrylic paint) if you did not already at the base stage, and the bicarbonate of soda (Baking soda) and stir it in thoroughly. You will get a small amount of reaction usually at this stage so you want to act quickly (as the gelatin is cooling as well). So, add the vinegar (less than a teaspoon is fine for small quantities of latex, but you can adjust depending on how much foam you want. The reaction will begin immediately, so quickly stir to mix the ingredients evenly.
You can now spoon the foam into pre-made moulds, or use tools to create prosthetics by hand, though you have to work quickly with it as it cools rapidly.
Foamed gelatin does not keep well, as reheating it will 'de-foam' it to some extent, and the more foaming ingredients you add the less stable the foam becomes. Because of this it is best to only do as much as you need as and when you need it.
