Cleaning sterling grains

The sterling grains I had made (by re-melting and pouring into water about 500g of silver from a previous failed casting venture), gradually looked very dirty on the surface. They were steeped in sodium carbonate solution sat on aluminium foil for some days, which made them noticeably worse. Subsequent steeping in sulphuric acid made no change; nor permanganate, peroxide or bisulphite! I melted one of the larger lumps with some borax to get an interesting set of colours; white as in clean sterling on the part in contact with the crucible bottom, black on most of the top as in oxidised sterling, and a fluorescent yellow / green (which had been very golden in colour when red hot) on part of the top, possibly caused by the borax flux. The lump of silver was dumped in sulphuric acid for a couple of hours. Incidentally, these grains had been satisfactorily used to cast student work in early spring.
Subsequently I placed all the remaining tarnished grains in a ceramic shell open mould and set it in the enamel kiln at around 800°C and left it to reach dull red heat. On removal, as hoped, the surface of all the grains had changed from a kind of creamy mushroom grey to dark grey. They were cooled and dumped into dilute sulphuric acid, which satisfactorily converted the surface into the kind of clean white I associate with sterling silver.