Hot process soap making is the technique of taking the soap to trace - then applying continual heat for 1 ½ to 2 hours until the soap has fully saponified - then optionally thinning the soap with a bit of alcohol and pouring into molds.
The advantage is that neutralized hot process soap is gentle on fragrances, dyes, and does not discolor herbs or flowers like cold process soap making will. Fragrance oils - which sometimes seize cold process soap by bringing it to trace too quickly - will generally work in hot process soap making.
Hot process soap making also enables the soap maker to pour the soap into plastic molds without having to rush before the soap gets to a thick trace and is unmanageable. After pouring, it's unnecessary to insulate the soap molds as you would with a cold process batch, because saponification is fully completed while on the heat.
How to Make Hot Process Soap
- Prepare a large pot (like a large canning pot) of boiling water to create a double boiler effect when your soap making pot is placed inside. You may find it helpful to place a cookie cutter in the bottom of the pot so your second pot can sit on top if it.
- Make your soap as usual. Bring soap to trace. Ensure that the double boiler's water is at a slow boil. (Do not add specialty oils or fragrance into the soap at trace. If you wait and add them after the lye is neutralized, the specialty oils will have more effect, and the fragrances will be truer.)
- At trace, place the soap pot into your pot of boiling water, like a double boiler. Cover to retain heat and moisture. Cook your soap covered for 1 ½ to 2 hours, keeping the water in the bottom pot at a slow boil - remember to check your water periodically and add more if necessary. Stir occasionally to ensure mixing and heat distribution. The soap should begin to have a slight translucent look - this is a sign that the soap is neutralizing.
- After approximately 1 ½ to 2 hours, the soap will be neutral. This is the time to add any specialty oils or fragrance. If the soap seems too thick to work with, thin it with a bit of Isopropyl alcohol or Vodka. The soap will thicken quickly as it cools.
- Ladle your soap into molds, tapping the molds lightly to remove any air pockets.
- If some of your soap does harden before it is poured into molds, it can be re-melted using a double boiler.