Rebatching Soap: How to Save Failed Recipes
A Bad Recipe Isn't Always Lost if You Know How to Rebatch Soap
Reading Time: 4 minutes
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.
For rebatching 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!
Melanie Teegarden is a longtime professional soapmaker. She markets her products on Facebook and her Althaea Soaps website.
We have a very heavy lye soap. My friend accidentally put in too much lye. We are going to follow your instructions and add oil until it is the correct PH. When we dry it, how long will it take to cure?
Hi Lynn, since this rebatch is technically a hot process, it should be usable as soon as it hardens, if it’s done correctly. Hot process soap takes much less time to cure since much of the water dissipates as the soap boils. So you could wait that standard six weeks, but would only need a week or so.
Thank you for your detailed instructions on fixing oily soap! I made a lovely HP soap yesterday, but didn’t have quite enough lye, especially after adding the scent and color. It was a small batch, so I think I’m going to make a second batch, but with no superfat this time, and once it’s almost done cooking, I’ll add back in the crumbled, oily soap. Does this sound like a good plan?
Hi Elena, that does sound like a plan, as long as the oily soap is grated into pieces small enough to mix. Since it’s 0% superfat, the worst-case scenario would be that the soap might be a little drying if the other soap isn’t oily enough…but from how you describe it, you should be fine.
I doubled my pumpkin spice soap recipe. I doubled all the ingredients but the lye. I was supposed to put 14 oz of lye in but I only put 7 oz in. If I rebatch and add the 7oz of lye, will that fix my soap?.
Hi Marissa,
I made my first batch of homemade soap and it is extremely oily. A heavy layer of oil is pooling out of every mold.. I’m thinking about using a glass bowl over a pot of boiling water to melt all my oil soaps and then adding in a little more lye. How does that sound? Maybe I didn’t mix everything enough. I really don’t know what I did wrong bc I’ve never made it before. But last time I mixed everything by hand, so before I try reaching it to fix my oil soaps I was going to get a mixer to help make sure it’s stirred enough. I made them 5 days ago and they’re still very oily
Good day!
We made a soap just last day and it turns out to be so itchy, maybe because we put too much lye and very little oil. We haven’t measured the ingredients very well. I think that our soap is lye heavy. We are planning to reverse it using the method that you have suggested above but we don’t have olive oil. Can we use other kinds of oil instead of olive oil?
I hope you respond to this question because I badly need your help.
Thank you so much and have a blessed day!
Hi! Thank you for sharing. I am wondering if I will be able to use the rematch method with my soap. This is my second batch in a row that has done this, but when I unmold and flip the bars over I have these lighter patches that look like lye. There’s also some strange spots that have appeared to come up from the top as well. Is it still safe to do the rematch method if I have undissolved pieces? My measurements were spot on; however, I did see some pieces of undissolved lye left as I poured the lye water (actually goat’s milk) into the oils. I’m wondering if my lye is bad too? Thanks for any input.
Hi, My second batch of cold process soap failed. I did not stir the lye in well enough with the water. After I made the soap and poured it into the molds I found a layer of lye sludge on the bottom of the bucket. On inspection of the soap cooling in the molds- I see undissolved lye crystals and super oily layer on the tops. I’m guessing what happened is the lye did not dissolve fully and the crystals remained in the soap. Some of the lye remained on the bottom of the bucket and did not mix in leaving my soap extra oily.
This seems like a two part problem- the lye did not dissolve and there is not enough lye hence the extra oil.
Is it possible to rebatch this and start with adding a little water to help the crystals dissolve? And then add some extra lye solution to help the rest of the oils reach saponification?
Thanks so much for any advice! I am sad to waste so much materials…
I have made lye/tallow soap. It turned out fine. Now I want to remelt and add fragrances, color and additives. I have tried twice but am unable to melt it to a smooth, thin enough constancy to pour into special molds for a smooth soap with all the mold features. These were to be Christmas presents for family and I don’t know if I will be able to make them. Please help.
Hi dear, thanks for all the explanations. My carrot soap is not foaming well. I used a combination of PKO and carrots oil. Can I melt it and add foam booster, what kind of foam booster is advisable to use. Thank you
Hi, During the process of trying to make an antiseptic bar soap I guess I took a wrong route to my production and it got watery. Is it redeemable? Thanks