Whether you are a hobbyist soap maker or a professional, eventually, something will go wrong. While soap making is chemistry, it is also art, and there is always that biggest cause of error involved – the human element. Knowing what went wrong, where, and if you can fix it, how, are some of the awesome skills that come from experience, and one of the first that most soap makers learn is the skill of how to rebatch soap.
My first mistake happened because I was up very late on a Saturday night (between 2 and 3am) and Hulu/Netflix wasn’t holding my interest, so I decided that it was the perfect time to make soap. I did use a tried and true recipe (my favorite Shea & Mango Butter Soap needed restocking), but I decided to resize the batch on the fly, so that I could have MORE SOAP. Not only did I not run the expanded recipe through my soap calculator (the percentages wouldn’t change, just the total amounts), but I didn’t write down my calculations either. I did them in my head (after a long day with no sleep – see above).
I also split the batch, to try a new technique, The Ghost Swirl. I did my calculations, made the soap, everything seemed to work. I eventually went to sleep, and came back the next day to check on my soap. I should have had beautiful, hard bars, ready to cut. What I actually had was a gloopy, soft, semi-solid, not-quite soap. Something had gone wrong.
I sat down to review everything I had done the night before, and realized that I had made a crucial mistake in my early morning math adventures and calculations. I master batch my lye water. I make it to a standard concentration (i.e. 1 oz. of lye water equals x amount of lye and y amount of water, always). I had gotten that concentration wrong when I made my calculations. This meant my soap did not have enough lye to complete the saponification process that had begun.
The solution, since I knew exactly how much lye I was missing (the difference between what my recipe called for, and the amount I had calculated), was to add the missing lye and rebatch / hot process the soap to finish the saponification process. This was only possible because I knew EXACTLY how much lye was missing. This is not a general solution for suspected lye-heavy and/or possibly over-superfatted soaps.
Why Rebatch Soap
Rebatching, in general is often used to fix a general scent, color or texture issue with an already complete soap. Perhaps, your fragrance faded, and left you with an unscented or barely scented soap that you were making for a particular reason, you could rebatch the soap to add additional fragrance (up to the skin safe limit). If your fragrance oil (or other ingredient) behaved badly, it can cause your soap to accelerate trace, rice, or volcano, leaving you with a badly textured and/or misshapen soap; rebatching the soap can help you to smooth out the texture, and make a lovely, usable bar.
The techniques of rebatching can also be used to add ingredients that would not be stable during the process of saponification. This is done by making a base bar of soap, an original recipe, or Castile, or whatever your preference is for the bar of soap being created. Because saponification is a balanced chemical process, in which lye + water + oils/fat interact to create soap, some ingredients that are more acidic or basic can alter or interrupt that chemical process. However, those ingredients may still be beneficial to the overall qualities wanted in a bar of soap. One example of this is the baking soda added to my Skin Soothing Soap. Because baking soda is very basic, it interferes with the necessary reaction to create soap. However, it can easily be added to the rebatch process.
In addition to correcting mistakes, making a soap more esthetically pleasing, or adding ingredients that may not react well with the saponification equation, soap can be rebatched to rescue scraps, trimmings, ends of loaves, or imperfect bars. These soap scraps and pieces can be combined, with themselves and/or with a fresh batch of hot processed or previously cold processed base soap to create a new loaf. These rustic loaves can an attractive and functional way to reduce waste.
How to Rebatch
In most cases, you would begin by preparing the soap that you want to rebatch. This can be done by shredding the old soap with a cheese grater, or food processor, or by cutting the soap up into small cubes and pieces. You want to increase the surface area of the soap and reduce the size of the particles so that it can be “melted” into a new smooth finish. In the case of the soap I am using for my example, I did not have to cut or grate the soap, as it was a semi-gelled consistency, easily separated with a spoon.
The next requirement is a heat source. This is for “melting” or cooking your new soap. This is a hot process soap technique, so any fresh soap added to your rebatch will be hot processed at the same time, reducing curing time for your soap. The increased temperature also means that your soap is more likely to gel, as mine did. There are various techniques for doing this in a double boiler on the stovetop, or in the oven, even, but most people these days use a crockpot, as I did in my example.
The last thing that you need is a bit of liquid to help with the cooking process. This can be any liquid that you might use in your soap making, though it is usually water. Other people might use milk or such, but remember that this is a higher temperature process and don’t scorch your ingredients. I used the lye water (lye dissolved in distilled water) that I needed to bring my recipe into the proper ratios. You do not want to add too much liquid, as you are essentially going to cook this off, like reducing a sauce, until your soap in a thick batter ready to go into your molds. The less liquid you use the sooner your soap will be ready to use after rebatching, as long as you have enough to complete the process as described below.
Add your shredded / cut up soap to your crockpot or double boiler. Add just enough liquid to wet down the ingredients (maybe 4-6 oz. or so). You can add more water later. Cover your crockpot, and set on low to begin to cook. Keep an eye on it, this isn’t dinner – the process will seem to be going slowly, and then will suddenly be in the correct state. Also, no one wants scorched soap, if your mixture seems too dry or chunky, add another 2 oz. of water. You are trying to reach the consistency of instant mashed potatoes. Stir occasionally, to incorporate the liquid, smooth your soap batter, and incorporate any added ingredients, but remember to try and keep the crockpot covered to maintain the temperature.
When your soap batter has reached the desired consistency, spoon it into your molds. Tap down your molds on the counter (fairly aggressively) to get out any air bubbles in the finished mixture. Those bubbles will result in holes in your soap. Smooth or decorate your top as you desire and let the soap sit for about 24 hours to harden and set up. Unmold and cut. You will have successfully rebatched your soap!
Further Reading: How to Rebatch Soap to Fix Mistakes