You could spray a little alcohol on the soda ash and leave for less than a minute, It has worked for me, I just blot up the excess liquid and let air dry, you may see the sprayed area has a satin look, but much nicer to me than soda ash. I have found this will take it off. It is strange I don't like spraying my finished soaps with alcohol, but wouldn't think twice about the soap that is just poured.