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, G2926BB supergloss and G3914A Alberta Slip black. Going on a hunch I mixed up a bucket of the G3914A (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, substituting as much CaO as possible for SrO. Firing schedule is PLC6DS. On hindsight, the gassing of the strontium could be a big plus for the development of the effect. On testing my other blacks, they did not work, that means the gassing of the G3914A (from the LOI of the Alberta Slip) must be even more important.