Steam is excellent for removing ash. A handheld steamer its great for removing ash or making soaps shiny. Without the steamer you could use a teakettle and hold the soap in the stream with tongs. Pressing plastic wrap on the surface of the soap untill its unmolded will prevent ash. Its sodium carbonate and happens when free sodium from the lye reacts with carbon dioxide in the air. It is also more likely to happen in soap that is a little lye heavy. Are you using 'food grade' lye?
There are a few things you can do to prevent it: don't unmold the soap untill its tongue neutral, press plastic wrap against surface to minimize its contact with air, beeswax or soy wax is said to prevent it too (1-2% of oils).
It is best to avoid unmolding and cutting soap thats starting to form ash, before its tongue neutral because it will then get it on all sides exposed to air.
