There are two main reasons you need to rebatch your soap: To fix a batch of soap that you've made a mistake on and it is testing too alkaline, or to use ingredients (dried herbs, exfoliants) that don't react well in an alkaline enviroment or tend to turn brown. I usually don't rebatch unless I absolutely have to.
The first step in rebatching is to get the soap into as small of pieces as possible. If the soap has been curing for a couple of days, you’ll be able to use a cheese grater to grate the soap. If the soap is fresh out of the mold (too soft to even grate,) just cut it into small chunks.
You can use just plain water to help the soap melt. However, many people like to use milk instead of water. I like goats milk as it seems to help the soap melt into a smoother consistency.
A good starting place with a week-old batch of soap would be about 2 or 3 ounces of liquid per pound of grated soap. Start with 2…if it just doesn’t seem “wet” enough, add another.
The only problem with adding too much liquid is that the more liquid you add in the rebatching, the more that has to cure out of the soap before it’s good to use.
Take your grated soap and put it into a crock pot or a glass oven dish that you can tightly cover.
Add the liquid and stir it up gently.
If you’re rebatching to fix (the problem) that you left out an oil in the original batch, go ahead and add that oil at this time too. If the mistake you made was not adding enough lye water solution, you can add that now too with two cautions:
Don’t let the fact that this is a rebatch make you less careful with the lye – it’s just as dangerous.
If you’re adding lye water to correct a mistake (again, knowing exactly what mistake you made,) go easy on water you’ll add for the purpose of rebatching. Depending on how much lye solution you’re adding, it may be enough to wet the soap and get it to melt. If it’s not, just add the water a bit at a time.
Set your crock pot on “low”. Set the timer for an hour.
After letting the soap heat for an hour or so, open the lid and stir gently. At this point, you may just be able to mash it around a bit. You’ll see it’s probably starting to just begin to liquefy (to the consistency of thick applesauce,) and that the edges of the mass of soap are starting to get a bit translucent.
Gently stir it up and put it in for another hour or so.
After another hour, the soap will have liquefied more and will be more homogeneously translucent.
What you want is for it to be completely liquefied (or at least completely softened) and translucent. It will look like a big pot of soap going through a hot gel stage. Stir it up again, mashing out any big lumps, and let it heat some more.
In my crock pot, on low setting, this will take about 4 hours total. How much time it takes will depend on how warm your crockpot is and how much liquid you’ve put in.
When it’s to a consistency that you think it’s “pourable,” it’s time to add any additives (other than the lye solution) that you want to add. Stir it up well to make sure that the additives are well incorporated into the soap.
If you're rebatching unscented soap, add a little less fragrance than you normally would. 1/2 ounce per pound of soap is a good place to start.
Scoop or glop the soap into the lined mold you want to use, pushing the soap down with a spoon or rubber spatula, and tapping the mold on the counter to help the soap settle into the mold.
Let the soap set up for 24 hours or so. After that, you should be able to pop it out of the mold or slice it. Let it cure until it’s hardened completely. How much extra cure time you’ll need to add will depend on how much extra liquid you added.
As you can see it has a more rough, rustic look to it. You can get it to appear a little bit smoother with some practice.