If you or someone you know is into old (actually very old) recipes, odds are you’ve seen this listed as an ingredient here and there. Pearlash is refined potassium carbonate, an alkaline salt found in wood ashes that also goes by the name potash. Potash was used for a lot of things back in the 1700s and 1800s, especially glassmaking. These days we mostly know it as a fertilizer, but once upon a time it was used to leaven things like corn cakes since it makes bubbles when it gets wet. Given that potash was made from wood ash, its effect on the flavor of corn cakes was as you might expect, but hey, at least the texture was lighter.
The problem was partly solved by a chemist named Antonio Campanella who, in 1745, invented a way to refine potassium carbonate by heating it and burning away more of the ashen residues. That got rid of enough of the burnt wood flavor to make it an acceptable leavener many American home bakers. The first printed recipes containing pearlash appeared in 1796. If you’re in possession of a recipe dating to that time, just substitute baking soda but use about half again as much.