Steve Slape sent me these notes (from a Dan Ketteridge workshop) in a thread I started about uses of wood ash:

“Seawater can also be used to add minerals but you must precipitate out sodium
chloride. This can be done using lye, which is pot ash and rainwater. You want to raise
the pH of the water to 10.4 by leaving it overnight. After one night there will be clear
water on the top of the barrel, which is the sodium chloride and the milky water
underneath will be the other elements.”

I tried googling for the ratios salt water to ash without much success…

Hope that helps…