He this is interesting I have made soap in the past but never with leftovers from a stb
I would say give it a try its very easy
the basic recipe I used for making olive oil soap is
500 gram olive oil
64 gram lye
178 gram water
you can add hard fats as well like palm fats
just calculate the amount of lye you used and convert it to the right ratio
then heat the fat to 50C
and the lye mixture to 50C
Then mix the lye in the fat and stir for one hour or use a stick blender then throw in a mold and let it go hard for a day or two.
After that dry the soap for a couple of weeks
hope this helps and let me know how the results are
btw check some online soap forums for lye water and fat ratio calculators
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