Yup - although if it is genuinely pure potash (potassium carbonate), you might burn your face off with alkali burns... pure potassium alum (or potash alum or alum of potash) is the stuff.
My dad turned up with some alum from the Indian barbers! £2.00 for a block. Seems to be quite nice. Not as convenient as a razo rock but it will do for now.