(For readers coming across this question at the bottom of a post on wild nutrition, if you’re interested in rat rendering and/or soap-making please check out my fat rendering post HERE, and at some point in the not too distant future I’ll link from it to a soap-making post. Otherwise, please scroll past this comment.)

Hi Brett, the short answer (describing the method I use; there are others) is you melt a clean rendered fat (see HOW TO RENDER FAT) in a stainless steel pot. In another non-reactive container you (carefully) combine water and caustic soda to make lye (our parents/grandparents made lye much less precisely using wood ash and water). You pour the lye into the fat to make soap batter, stir (yay for stick blenders) until it emulsifies and begins to thicken, pour into molds and allow it to cure (saponify) over a 4 to 6 week period. If your measurements were accurate, once cured there’s no caustic left – the saponification process has converted the lye and fat into soap.

I’ll write a proper how-to post on basic soap making sometime soon.

Hi Veronique, thanks for commenting 🙂

Here is a study that indicated that boiling vegetables was the best way to remove soluble oxalic acid and that it is removed into the water – so yes you would have to discard the cooking water. I’m not sure what role temperature plays in all of it.