an article from 1972 that says this about making wood ash lye at home:
To make lye in the kitchen, boil the ashes from a hardwood fire (soft woods are too resinous to mix with fat) in a little soft water, rain water is best, for about half an hour. Allow the ashes to settle to the bottom of the pan and then skim the liquid lye off the top. You can do this daily and when you've got enough of the weak solution, start the soap making process by boiling the liquid down until it'll float an egg. Now put that meat fat, left-over cooking lard and vegetable oil into a kettle not over half full, and heat the whole mess until all the liquid has been rendered out of the solid scraps. While it's still hot, add this clean grease to the bubbling lye and continue to boil the mixture, stirring all the while, until it reaches the consistency of thick cornmeal mush.The article goes on to say that added salt makes the soap set into bars more easily, and that non-hardwood ashes are too resinous to produce soap. That last statement could use a good skeptical questioning, since pine tar soap is renowned for its anti-inflammatory properties against ailments such as psoriasis.