[fanfan]

Boil the potash down until its pH is around 10 (basic - lye)

How would you tell? What would you look for?

[Daniel II]

Today you could do it with pH strips. If I recall, basic yields shades of blue to purple and acidic shades of red to yellow.

Our ancestors probably had a more formulaic approach.

You might be able to tell in that if there is not enough lye, you'll still have a greasy emulsion floating on the top. Too much lye and all the grease will react and you'd be left with water (and lye) on top that you could just pour off.

Any chemists out there?