This piece was fired on a textured surface of bio-soluble fibre paper, kilnwash and plaster, on a ceramic fibreboard shelf. I used various blue frit capped with 6mm Tekta. I suspect the plaster/kilnwash texturing was still damp, as the darkening has happened in the area of deepest texture, and the color is unaffected near the edges, where moisture could escape. I don't usually dry the texture before firing, but am in a temporary workshop and think there is less circulation of air and warmth than in my previous workshop. When I wetted the plaster mix to clean it off the fired panel, it gave off a sufurous, rotten egg smell, and there is a stain on the fibreboard shelf too.
Has anyone else come across this dark staining, or have any alternative thoughts as to what might have caused it? I also wonder whether the shelf will be safe to use again?