I did some unexpected alchemy that produced similar results last Fall after the Klingspor Extravaganza in Hickory. I had mounted my Worksharp 3000 on a piece of unfinished White Oak plywood and had demonstrated the WS both Friday and Saturday on a number of chisels and plane irons, which generated quite a bit of metallic grinding debris around the unit. The ultrafine metal particles look like gray soot, but the surface of the plywood looked relatively clean after I wiped it all off. The achemy occurred when I took the tool back to my truck in the rain -- it got damp (I wiped off the WS, but just stowed the unit on my truck for the trip home). When I unloaded the truck the following morning, the entire surface of the plywood was permanently stained a gray-black. Let me see -- there was a Stanley #3, a #5 and 607 Bedrock plane irons, a few Buck Brothers chisels and grits up to 4000 -- Hmmmmm.