I spray it with WD-40 first and scrub with a wire brush until all the loose stuff is gone then the soak goes much faster. It only soaked for a couple of days but I pull it out 3 or 4 times a day and scrub with a fingernail brush, wash with Lava soap then return to the soak. That combination seems to work pretty well. You wouldn't believe it was the same stone as before. It was black on the top half.