Rebatching or handmilling soap is the process of melting down a premade cold process soap along with nutritive additives/fragrance etc and allowing it to set up again. This process allows you to add delicate additives such as herbs, essential oils and other ingredients that would otherwise be destroyed by the alkalinity of fresh cold process soap. It also allows you save batches that have seized or been ruined in some other way (If you've ever poured your soap into molds and then realized you forgot to add the fragrance or colorant, you know what I mean!).
Rebatching is best done when soap is fresh (5-7 days old). Fresh soap hasn't lost the water yet and will be soft and easy to work with. If your soap is older, you can still rebatch it but because much of the moisture has been lost, you will need to add a liquid to replace the lost moisture. If you are starting with a vegetable based soap, it will melt better if you use milk (cows milk, goats milk, buttermilk etc). For tallow based soaps, distilled water will work just fine.What you will need:
Use the grater to shred or grate your soap as finely as possible. Place in into the double boiler or crock pot if you are using one of these. If you are going to use an electric oven, place the soap into an oven proof pan that has a tight lid (or use foil). Now add the appropriate amount of liquid. If you are rebatching a tallow soap, use distilled water and if you are rebatching a vegetable based soap use ice cold milk. Remember that if your soap is very fresh, you may not need any but if you want, you can pour in a thin layer of liquid just to keep the bottom from scorching. Stir the soap until it is completely coated with liquid. The water in the double boiler should be touching the bottom of the top pot and should be maintained at a steadily slow boil. The crock pot should be turned to the lowest setting and the oven should be turned to 145F or 150F (if you know your oven tends to act hotter than it should according to baking recipes use the lower temp and visa versa).
Give the soap a good stir and then cover the pot and allow it to heat up. If you are using a double boiler, you will need to return to it very frequently about every 10 minutes. Remember to scrape down the sides every time you return to your pot. (If you are using a crock pot or the oven, you can check it every half hour or so.) After 2-3 checks, if the soap is still solid and it looks like it needs more liquid, add some and stir. This stage takes about an hour or more. Once the soap starts to smooth out, stop adding liquid or you'll wind up with soup. Remember you only need enough liquid to keep the soap moist like fresh soap. Keep checking to make sure the bottom is not burning. If the soap is beginning to scorch in the double boiler, add some cold water to the bottom pot of the double boiler to reduce the temperature. Eventually (somewhere between 1-2 hours), the soap will reach a stage where it is as smooth as it will get and the texture doesn't change anymore. You can then add your additives and stir well. Let it cook a little while longer, again stirring every so often and checking the bottom. Sometimes the soap will become a complete liquid, sometimes it wont really liquefy. Either way, as long as the consistency remains the same for a long time, it is ready to remove from the heat. Let it cool a little bit.
Once the soap has cooled down somewhat, you can either pour or push (depending on how liquid the soap is) it into a mold/molds or you can whip it with an egg beater to make floating soaps. (Spoon the whipped soap into molds.) Cover the molds and place them in a warm place for 48 hours until the soap hardens. Un mold, cut if necessary and allow it to cure for 3 weeks.