I was using a graphite crucible, heated to 1070C, using very minimal borax (crystals were very pure) and using a graphite mold pre-heated with a MAPP torch. The ingots were poured under flame to remove and oxygen that could oxidize the surface. When I poured the ingots I could see something floating on top of the molten surface and when it solidified the “crust” shown in the photos remained.
It looks like flux to me. It has a cloudy, glassy appearance and a slightly yellowish tint when viewed at the right angle.
These pictures were taken this morning AFTER soaking the ingots in a 10% sulfuric solution over night, and the “crust” is still there. I haven’t yet tried to use the 30% solution over heat as I've read on the forum yet, but will probably try that tonight.
Any ideas?? Thanks All.