Rebatching is the usual way to turn soap scraps into useful bars of soap or to correct a failed batch of soap. Rebatching involves preparing soap scraps by grating them into small pieces if needed, mixing the soap with a small amount of water-based liquid, heating the mixture until the soap largely dissolves, and then putting the soap into a mold to harden.
Another way of reclaiming soap scraps is the time-honored method of "salting-out" a soap. Salting-out is similar to rebatching, except a large excess of water is used when dissolving the soap. The water is removed from the dissolved soap by adding sodium chloride (table salt) to the mixture. The salt reduces the solubility of the soap so the soap then separates from the salty liquid and forms a floating layer. The soap layer is removed from the pot and formed into bars or soap powder.
This tutorial and videos show how this salting-out process works.
What can salting-out do that rebatching cannot?
Salting-out is more time consuming than rebatching, but unlike rebatching, it can remove a lot of additives from the soap, including a large part of any color and fragrance. The amount removed will vary on the type of additives and the depth of color.
Most of the glycerin naturally present in hand-crafted soap will be removed by salting-out.
Unwanted superfat can be reduced by mixing and simmering the soap in a weak lye solution rather than plain water. This "boiling" step with lye is done as a separate operation before the soap is salted-out.
What is salting-out not able to do?
What salting-out will not do is change the essential nature of the soap. If the soap scraps are from bath-soap recipes, then you will end up with bath soap after the soap is salted-out.
Salting-out will not work on potassium-based soap, such as liquid soap made with potassium hydroxide, KOH. Salting-out will also not work on highly soluble sodium-based soaps, such as 100% coconut oil soap. These types of soap are highly soluble in water. Adding salt does not reduce the solubililty enough to make the soap solidify and separate.
Salting-out is adapted from the "boiled" soap making method
The following process of boiling and salting-out has been adapted from the time honored "boiled" soap making. Bear in mind that this process, like much of soap making, is an art, not an exact science. Please use my instructions as guidelines, not hard and fast rules.
The pictures in the written tutorial below are from my first salted-out batch. These videos show how my second batch turned out --
Part 1: Prepare and melt the scrap. https://youtu.be/DedPOZIOnfI
Part 2: Salting-out the melted soap. https://youtu.be/ZHAtoLDF2PA
I have salted-out over eight batches of soap with varying results. The end result has always been usable soap, but the appearance of the salted-out soap has varied depending on the apparance of the scraps I started with.
Grate the soap into fine shreds with a salad shooter, food processor, or hand grater if needed.
Weigh the shredded soap. You don't need to be super precise when weighing ingredients -- fairly close is good enough.
Measure water roughly equal to twice the weight of soap. This is a starting amount. You may need less or more water. I recommend using distilled, deionized, or reverse osmosis water or even rain water. Hard-water minerals in tap or spring water can affect the quality of the finished soap.
Water weight = 2 X (Scrap weight)
Measure salt (sodium chloride, NaCl) equal to about 25% of the water weight. Again, this is a starting amount. You may need less or more salt. I recommend the cheapest salt (sodium chloride) you can buy -- iodized or non-iodized table salt, kosher salt, or canning salt are all fine. Even water softener salt will work in a pinch, but the coarse crystals will take longer to dissolve than fine grained salts. Don't waste your fancy expensive salts on this project. Absolutely do not use epsom salt (magnesium sulfate).
Salt weight = (Water weight) X 25 / 100
Optional -- Calculate the weight of NaOH if you want to remove some or all of the superfat in the soap.
Estimate the average superfat. If most of your soap batches are 8% superfat, then use that number. If you think about half are 3% and half are 7%, then use an average of 5%.
NaOH weight, grams = (Average superfat %) X (Soap weight, grams) X 1 / 1000
This weight will remove about half of the estimated superfat. To remove ALL of the superfat, double this weight of NaOH. You can use a few grams more or less lye than these estimates. Even if you add more lye than is needed to saponify all the superfat, the boiling and salting-out processes will remove any excess lye. The finished soap will not be lye heavy.
Example: I have 1500 grams of soap at 6% superfat. About how much NaOH should I use?
For LESS superfat: NaOH weight = 6 X 1500 grams X 1 / 1000 = 9 grams
For NO superfat: NaOH weight = Previous answer X 2 = 9 grams X 2 = 18 grams
So I could use anywhere from 9 to 18 grams of NaOH, more or less, to reduce or eliminate the superfat in the soap.
Weigh out the NaOH you want to use. Add this lye to about one fourth of the water you measured out earlier. Mix until the lye is fully dissolved. You will use this weak lye solution right away -- see "Liquefy the soap".
CAUTION: Use safety gear and safe practices for working with lye. Even a weak solution of lye can cause dangerous burns and permanent injury if splashed on the body!
Liquefy the soap
Put about one fourth of the water (or the optional weak lye solution) into a container that can be heated on the stove. I recommend a heavy bottomed stock pot or Dutch oven to reduce the risk of scorching.
CAUTION: Never put NaOH in an aluminum container. Even if you do not use any NaOH, I still recommmend using a stainless steel or porcelain-enameled pot -- not aluminum and not cast iron -- when working with soap.
Heat the water to a simmer and gradually add handfuls of shredded soap. Stir until most of the shreds are melted before adding another handful.
The mixture should not boil. It should gently simmer at 180 to 200 deg F (80 to 95 C). The photo below shows the digital thermometer I used to make sure my soap did not get too hot.
If the soap does start to boil, it will foam and rise in the pot. If this happens, immediately remove the pot from the heat and stir constantly until the soap settles. Lower your heat setting, put the soap pot back on the heat, and resume cooking.
If the soap starts to stick to the bottom of the pot, add more of your reserved water as needed. If you need to add even more water, add it! The amount you originally measured is only a guideline.
CAUTION: There is some hazard to cooking soap on the stove like this, so please stay nearby and stay alert. If you don't correct problems fast, the soap can easily scorch or boil over, make a huge mess, and even cause a fire.
Stir with a whisk, spatula, or stick blender to break down stubborn lumps and keep the soap well mixed and evenly heated. Allow the soap mixture to gently simmer until all lumps are gone and the mixture is as smooth as possible. It should look like smooth gravy before you go to the next step:
If you didn't use any NaOH, skip over the next section "Simmer longer to saponify" and go directly to "Salt-out the soap".
Simmer longer to saponify
If you added NaOH to the soap mixture, allow the mixture to gently simmer and saponify for another 15 to 30 minutes. Stir freqently to encourage saponification and discourage scorching.
Reduce the heat if the soap shows any signs of rising in the pot. In the old fashioned kettle method of soap making, this is called the "boiling" step, but I don't recommend actually boiling the soap mixture. It can easily boil over and make a huge mess! Just a gentle simmer is all you need.
At the end of the cook time, carefully check the soap for "zap" (see "Zap test" below). If you used a smaller amount of NaOH, a "no zap" result means the saponification is done and you can go to the salting-out step. If you used a larger amount of lye to remove all of the superfat, the soap may remain mildly zappy even after a thorough cook.
Zap test: Remove a small dab of soap from the pot and allow it to cool. Do NOT lick the soap directly. Instead, dampen your fingertip with water (or spit), rub it on the cooled soap to get a thin smear of soap on your fingertip, and then lightly touch your fingertip to your tongue.
A "zap" means there is excess lye in the soap. The sensation feels like an instant static shock or as if you touched your tongue to a battery.
A "no zap" means there is no sensation of a static shock. "No zap" soap might leave a pungent, sour, metallic, or salty taste in your mouth, but an odd taste is not a zap!
Regardless of the results, rinse your mouth with cool water until any lingering zap or unusual taste is gone.
If you think the soap needs more cook time, simmer it for another 15 to 30 minutes. Retest for zap if you like; ideally the zap should be milder than before or may even be completely gone.
Regardless of the results of this second zap test, go onto the next step of salting-out. Do not cook any further. Any excess NaOH will be removed by salting-out, so don't be too concerned if the soap still has some zap.
Salt-out the soap
Continue to simmer the soap. Toss about one fourth of the salt you measured out earlier into the soap mixture. Gently stir with a spatula or spoon until the salt dissolves. Set your whisk or stick blender aside -- these tools will mix the soap too much at this point. The goal now is to let the soap separate from the water.
Repeat with another one fourth of the salt and gently mix until dissolved. As the salt content increases, the soap will begin to look curdled. Eventually the soap will form a separate layer floating on top of a dark watery layer. The mess might look a little like this after a time:
Continue to add the salt, stir gently, and give the soap time to separate from the water. The soap layer will eventually change in texture to something that looks like oatmeal or chunky applesauce. The curds will no longer coat the spatula; they will slide off leaving the spatula wet but clean. When the soap has "grained" like this, stop adding salt.
Stop stirring and turn off the heat, but leave the pot on the warm burner so the contents of the pot cool slowly. The goal at this point is to let the fluid soap continue to separate from the water.
Allow the soap mixture to quietly cool until the soap layer solidifies. This might take an hour or two, so take a much deserved break! If you want to stop at this point, just cover the pot and let it sit in a quiet place until you can come back to the project in a few hours or the next day.
Separate the solid soap from the water layer. One way to do this is to lift the soap out of the soap pot with a slotted spoon, ladle, or even your fingers (if the soap is cool enough). Put the curds into a cheesecloth-lined colander to drain. This method is slower, but no heavy lifting is required.
Alternatively, you can pour everything from the pot into the colander in one go. This method is faster, but you have to lift a soap pot filled with warm soap curds and salty water.
Regardless of how the soap ends up in the colander, give it plenty of time to drain. Discard the salty water.
Optional -- Salt-out a second time
Measure out a second portion of salt. If you know the amount of salt you used for the first salting-out, measure that amount again. Otherwise, measure the same amount of salt that you originally weighed out.
Measure out a second portion of water. You will need anywhere from half to the full amount of water you originally used. If you think your soap has a lot of water from the first salting-out, use less water. If it seems dry, use more.
Put the water and soap back into the pot and bring the mixture to a gentle simmer. Add more water as needed to keep the soap from sticking to the bottom of the pot.
Once the soap and water are fully mixed and simmering, salt-out the soap again. Now that you know what to look for, the process will go easier and smoother.
Optional -- Salt-out a third time
If you decide the soap needs a third salting-out to reduce any lingering scent or lighten the color, repeat the salting-out process. Remember every salting-out reduces the amount of soap you will harvest, so don't overdo. I would say most soap needs only one or two salting-outs.
Rinse the soap curds
After the last salting-out, lightly spray the drained soap curds with cold water to rinse off any remaining salty water. Do not over rinse to avoid washing too much of your hard-won soap down the drain.
Dry the soap
Transfer the soap to large flat container, break up larger lumps with your fingers, and spread the soap out to dry further. Use a fan, if you like, to help the soap dry faster. When the soap is damp dry, the curds can be used to make soap bars or soap powder.
Alternative 1 -- Make soap bars
Put the damp crumbled soap back into the soap pot on low heat. There may be enough water still in the soap for it to melt properly, but add a small amount of water as needed to keep the soap from sticking or burning.
Optional: Add sodium lactate solution at about 3% of the original weight of soap scraps. This will make the soap more fluid and pourable.
When the soap is fully melted with no lumps, add fragrance, superfat, color, or any other additives you desire. Pour or spoon the soap into molds and allow the soap to cool and harden. Cut it into bars when it is firm enough to handle.
The soap bars will need time to dry. If your experience is like mine, you will also see the soap lather much better after several weeks, so be patient -- give the soap plenty of time to cure.
The picture below is what my first salted-out batch looked like after being molded into bars. The soap scraps were fairly light colored, so the salted-out soap was a pale ivory and I was happy with the result. The batches I have done since then have used darker colored soap scraps (including some soaps with a large doses of activated carbon), so the salted-out soap is also darker. While this soap is less appealing to look at, it is still useful.
Alternative 2 -- Make soap powder
Continue to crumble the soap with your fingers into smaller and smaller pieces. Allow the crumbles to dry until they no longer feel damp or cool to the touch. Ideally, the crumbles should be pliable and waxy, like firm cheese, for best results when powdering soap.
When the shreds are at the right moisture content, use the blade attachment in a food processor to break up the shreds into a fine powder. Use this powder at the sink for hand and dish washing or household cleaning.
If the superfat is low, you can also use the powder in your favorite laundry soap blend. My current dry laundry soap recipe is 50% by weight of soap powder and 50% by weight of washing soda.
Further reading
Bakmthiscl. My procedure for salting out. Soap Making Forum. 7 March 2013. https://www.soapmakingforum.com/threads/my-procedure-for-salting-out.32717/
Boyago. Salting out for laundry soap. Soap Making Forum. Post #13 dated 22 February 2015. https://www.soapmakingforum.com/threads/salting-out-for-laurndry-soap.52103/#post-495735
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