Understanding Hot Process
The primary reason for making hot-process soap is that you can use it right away! Unlike cold-process soap, you don't have to wait for hot-process soap to cure. The “hot” part of the processing is cooking the soap over low heat until it neutralizes. This is a timesaver over cold-process soapmaking because in cold process you have to wait for the soap to neutralize on its own.
Is It Neutral Yet?
You can usually use hot-process soap as soon as it cools. Have phenolphthalein or litmus strips on hand to check the soap's pH. If you use litmus paper, follow the directions on the package. Know that litmus paper is not the best method for testing soap. The results are not always accurate, and it is difficult to read, but if you use phenolphthalein, the pH test is very simple: Put a few drops on the soap. If it turns pink, the soap is still alkaline. If the soap is neutral, the drops will stay clear. Test the soap when it is still in the slow cooker. If it is alkaline, cook it longer.
Molding and Unmolding
You can use individual bar molds for hot-process soap. Since the soap at time of “pour” is very thick, you need to be sure to cram the soap into the details of intricate molds. You will have better luck with individual molds if you use oils that produce harder bars, just as in cold process. Don't forget beeswax when making a recipe for individual molds!
You unmold and cut it the same way as you do in cold process. You will usually have to trim the top surface, since it is nearly impossible to get the top smooth, due to the clumpy nature of hot-process soap at time of pour. The longer you wait for the water to evaporate from the bars, the harder the soap will be.
Color and Scent Additives
Color and scent materials are added after “the cook.” You can use less essential oil this way, since it is not exposed to the batter when it is still caustic. Stirring in the additives can be difficult at this stage because the soap is very thick and gloppy, but it can be done without an enormous amount of energy.
You'll stir in fragrance and other things that need complete dispersion first. Then, if you're going to swirl or marble, you'll need to separate out the amounts of soap you're going to color. You'll tint each part individually, then mix them together. The effect will be different from the fluid swirls of cold process, but the effects are beautiful.
Make sure you read the information about the colorant and suggested uses. Responsible suppliers will provide this information. You can also read the archives of the soapmaking message boards for experiences of other soapmakers. Record your findings, keeping detailed records of how each colorant behaved in each technique.
If you're going to color the batch all the same color, you can do it at the beginning; if you're using colorants that are not affected by lye, such as ultramarines, you can add them at an earlier stage. With any lye soap batch, you can add ultramarines to the lye solution to ensure complete blending of color.
Remember when using any kind of colorant that the best way to know how to get exactly the color you want is experience. In hot-process soap-making, you can add colors that didn't perform well with cold process and usually get a better result. Know that there is no way to know exactly how it will look until you try.
There is a difference in the texture of hot- and cold-processed soap. Cold process tends to have a finer, firmer texture, while hot process can be more coarse and a little spongy. However, with practice, you can make hot-process soap with nearly the same exquisite texture as cold process.
The texture problems of hot-process soap comes from evaporation or lack of it. The amount of water used at the start is the same as in the cold-process recipe. If too much water evaporates during the cook, the soap can be grainy. If not enough evaporates, or if you use too much at the start, the soap can be spongy.
As long as the soap is neutral, graininess and sponginess are merely aesthetic issues. Another common result of too much water is bars that warp as they age. A good use for soap that is unattractive but otherwise completely fine is to use it for chunks in cold process or grated for hand-milling projects.
Cooking to Neutrality
There are many ways to cook the soap to neutrality. The easiest way is to cook the caustic batter in a slow cooker. The slow cooker hot-process method is gaining popularity over the common double-boiler method. The major drawback for many soapmakers is that there are more size limits on slow cookers than on stock pots. Some soapers take the stainless steel pot of traced soap and put it in the oven to do the cook.
Before you experiment with any kind of caustic-based soapmaking, you must have experience with more conventional methods. There are so many variables that can cause accidents, great and small, that you must know exactly what you're doing. Always respect the potential danger of hot, caustic raw soap.
There are soapmaking message boards dedicated to advanced techniques in hot-process soapmaking. You can find soapers who place their molds in the sun, on hot driveways, and in plastic bags inside the double boiler. Once you've gained experience with the basic methods, find a group of soapers who've come up with further specialized techniques and experiment.