Getting a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of both chemistry and logic. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB supergloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I decided that I wanted one with high surface tension (in the melted form). I created the G3912A by calculating in my account at insight-live.com, substituting as much CaO as possible for SrO. My firing schedule, PLC6DS, produced the tile on the far right. Jackpot! I tried that at cone 5, 6, 7 and 8 and it worked at all of them. Surprisingly, my other tests done later were failures. Why? I thought that the gassing of the strontium carbonate (as it decomposes and releases CO2) was a factor in the original success, but apparently the bubbles generated from the Alberta Slip, in the G3914A black, must be the key (the other two bases have a low LOI).
Creating a Cone 6 Oil-Spot Overglaze Effect
In this video I use my Insight-live account to do the chemistry to convert an existing MgO-matt base glaze into a tin-opacified, high-surface-tension melt for double-layering over a gloss black. I also use it to record my physical testing.
A mottled glaze created by double-layering glazes of different melt fluidity and color