You know that you are wanting to make potassium hydroxide with the wood ash (used liquid soap), vice sodium hydroxide that cold process soap makers use producing a harder bar.

Good luck with this venture though, it is commendable(as well as interesting).

You'll be wanting fresh, well-burned ashes from hardwoods. The solutions of lye suck up carbon dioxide very rapidly and leave you with potassium carbonate instead of the hydroxide (lye). Plan to do the burning, leaching, boiling down, and actually making the soap all in a row, or have large, airtight containers to store it in between steps. These soaps tend to be very harsh on skin (since you're only guessing at the amount of lye that you have), but make good laundry soap.

I remember doing this as a kid. We built a v-shaped trough out of two boards which ran on a slant from a barrel with a spigot at the bottom into another, smaller basin below. We put the fireplace ash in the top & poured water over it, let it run down the trough into the other basin. I remember pouring the lower basin back into the top barrel many times-- I guess to increase the strength of the lye. I remember the soap we made from it was really really hard & brittle & harsh & it seemed to sit on the shelf curing forever (although I think it was really probably just a standard 6 weeks). I have absolutely no idea where my mother got the recipe from or what it actually was, except that we also rendered the fat from months of bacon grease. I do remember we fragranced the soap by wrapping it in old teeshirts soaked in perfume. I STILL remember what it smelled like 40 some years later!

simplemary,

I would guess that this soap would have been very deadly for the "potty mouths"!

And, probably hard on clothes.

I think that most pioneers would just measure with ladles to a certain ratio of the lye vice rendered fats.
It's interesting that someone would like to go through the old process, as I would.

Making items the "Old Fashion way" catches my interest in most things.