Rebatching soap is an excellent way to prevent waste and turn your valuable oils and fats into a useful product, even if mistakes have left the soap imperfect or unsafe to use. If your soap turns out lye-heavy (with a pH at 10 or above), you can add oils or fats in small amounts until the pH reaches a safe and mild number 8. If your soap is soft and oily, melting it back down and adding small amounts of lye solution can save it.
Rebatching, also known as hand-milling soap, is the process of shredding down and processing soap with heat until a molten, homogenous state is reached. The soap is then poured into the mold, cooled, unmolded, and sliced. After an appropriate cure time, this process renders a hard, long-lasting natural soap. It is similar to the process of working with melt-and-pour soap – shred, melt, make additions, and mold.
For some, rebatching soap (or hand-milling) is their preferred soap-making technique. It is easy to make one large, basic batch of 0% superfatted soap, which can then be shredded and used in separate batches to create laundry, dish, and skin soaps. The main difference between utility soap and body soap comes down to superfatting – adding extra oil to a recipe beyond what is needed to fully react with the lye.
To rebatch soap, you will need the following: olive oil or lye water solution (depending on the problem you are fixing), a slow cooker with a low setting, a spoon – not aluminum – for mixing, any botanicals, extracts, fragrances, or colors you may want to add, and a mold. If your soap is oily and requires lye solution, mix the solution according to the original recipe. (Leftover lye solution can be poured into a drain, just as you would use drain cleaner.) Make sure you have pH testing strips, available at any pharmacy. Remember, when using lye for soap, to use all necessary safety precautions including gloves and eye protection. A ventilator mask is also a good idea to prevent inhaling fresh lye fumes, but if you don’t have one, an open window and a fan provide enough ventilation to keep things safe.
Lye-heavy soap occurs when there is not enough oil in a recipe to react with all the available lye. This leaves free lye in the finished soap and makes it caustic and unsafe for use, even for laundry or cleaning purposes. You can tell if a soap is lye-heavy if, after a few days of curing time, it still registers a pH of 10. Lye-heavy soaps also tend to become very hard and crumbly very quickly in the mold, but this is not always the case. If in doubt, always check the pH to make sure it is safe. pH testing strips can be found at any pharmacy and at many online retailers.
To correct a lye heavy batch, shred the soap as finely as possible, using gloves to protect your hands, and add to a slow cooker set on low. Add 1 tablespoon of distilled water and cover. Allow the soap to cook, stirring occasionally, until it has melted into a homogenous solution. Add olive oil, 1 ounce at a time, to the solution and stir well. Cook for an additional 15 minutes, then check the pH. Continue this process until the soap tests with a pH of 8. If the soap foams up while mixing, spray it with a small amount of alcohol to prevent bubbles from forming air pockets in the soap. Only use a small amount of alcohol – too much can reduce the lather. Once the soap tests at a pH of 8, remove the lid and turn off the slow cooker. Allow to cool for 10 to 15 minutes, add your botanicals, fragrances or colors, or the best essential oils for soap making, then pour into molds and cool.
To correct an oily batch of soap, proceed in the same way as above, shredding the soap (or mashing it, if too soft) and adding to the slow cooker on low. If the soap has separated into an oily layer on top of solid soap, be sure to add both the solids and the liquids to the slow cooker. Instead of adding plain distilled water, add 1 ounce of lye solution (mix according to your standard recipe ratio of distilled water to lye) and allow to cook until fully melted. Test the pH. If it is below 8, add another 1 ounce of lye solution and wait 15 minutes. Test again. Continue in this way until the soap tests at a pH of 8. Turn off slow cooker, cool briefly, make any additions you wish to make, and mold.
Once cool, rebatched soap is safe to use immediately. However, a 6-week cure is still recommended to drive off moisture and make a harder, longer lasting bar of soap.
Have you tried rebatching soap to fix a failed recipe? How did it go? Let us know in the comments!