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How are lye or Sodium Hydroxide made, or even Potassium Hydroxide. It's one of the key elements in soap manufacturing, but a natural resource supply would be preferable after the shift.

Sodium Hydroxide/Caustic soda/NaOH
derived from electrolysing table salt.
Potassium Hydroxide/Caustic potash/KOH
derived from leaching hardwood ashes.
(The white ash does indeed leach stronger potash.)

Leaching is simply the process of straining water through the ash to obtain a strong caustic liquid. Rainwater is preferable and filtering of any remaining ash will be required to clear the solution. (Note Lye is, as mentioned, very caustic and damaging to both eyes and unprotected skin. Provisions should be made to provide some sort of protection while using it. Contact with should be treated immediately by rinsing the effected area with cold, clean water thoroughly before treatment.)

Most of the available recipes mention sodium hydroxide as the source of lye, as this is more readily available commercially. There seem to be very few using the potassium hydroxide so some experimentation will be required to allow for the conversion of your own mixes. Although sodium hydroxide can be produced if you have some kind of power source and the necessary materials to make electrodes. (Note potassium hydroxide can also be used as an electrolyte for batteries.)

Offered by Brian.

If the ash is black, obviously it is more charcoal than ash, which is not desirable for lye making.

Offered by Steve.