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When I was in the ironworker's union I have heard a fellow described as,"Couldn't pour pi$$ out of a boot if the instructions were on the heel." :D

Here is a post by Howard that might help. He is a stone collector and has gotten many in that condition.

Howard and others have successfully used oven cleaner. If you do a search in the archives you may find a post by Kees who had an Escher that got oil on it and there is a lot of feedback there. :gl: