Before caustic soda came in tidy beads, soap-makers made their own alkali by leaching wood ash with water. The result — an impure, potassium-rich lye — is how soap was made for centuries. It is also the least predictable ingredient you will ever work with, so read to the end.
What you are actually making
Hardwood ash contains potassium carbonate, which water dissolves into a caustic solution (historically called "lye water" or "ley"). Because it is mostly potassium, ash lye makes soft, paste-like soap rather than a hard bar — unless you later convert or salt it out.
The traditional method
- Pack a barrel or hopper with clean hardwood ashes over a bed of straw and gravel.
- Pour soft water — rainwater is ideal — slowly through the top and collect the dark liquid that drains out.
- Re-pour that liquid through fresh ash until it runs strong.
How they tested strength — and how you should
The old test was to float a fresh egg or a potato: if it bobbed up with a coin-sized patch showing above the surface, the lye was "strong enough." That is a folk heuristic, not a measurement.
Ash lye has no reliable, repeatable concentration. You cannot calculate it precisely — you must make a small test batch every single time.
Working safely
Leached lye is still caustic: goggles, gloves, ventilation, and "lye into water" apply exactly as they do for modern NaOH. The romance of the old way does not lower the risk.
Curious how the numbers settle once you switch to measured NaOH or KOH? Compare in the calculator.