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Happy holidays, and warm wishes to all for a wonderful New Year! A couple of years ago I posted about engobes, but I couldn't retrieve the thread. Different question this time. Using locally found clays, firing at 1100°C, I spray engobe over stencils before bisque firing, then add glaze. The engobe is my own laboriously developed recipe (kaolin 15, calcined kaolin 15, Silica 20, Zirconium carbonate 10, Feldspar 20, Boron frit 5, talc 10, Ferro frit 3110 10, nepheline syenite 10, Borax 5). Don't ask me how this recipe got so complicated, I have forgotten. I tested a dozen different engobes, and all disappeared into the glaze. This one is the best and doesn't usually fall off either. The usual glaze is my glossy clear workhorse (Ferro Frit 3124 60, Nepheline Sy. 15, wollastonite 12, kaolin 10, silica 12 and lithium carbonate 1). This all works well, although the engobe does get faded by the glaze but with no incidents, but any addition of copper oxide (2%) or cobalt carbonate (0.3 - 0.5%) - or for that matter any other glaze with those oxides in - results in disaster: crawling, pinholes, and some odd holes that look as if something blew up under the engobe, folding out the engobe+glaze. Nothing is wrong where the clay was covered by the stencil. Iron seems safe, and much more cobalt (1%) also seems OK. Besides, in my last firing I tried an alkaline copper glaze (2% CuCO3), and it washed out the engobe completely into dark blue, while staying green on the stencilled shapes. It all ran like crazy too. Photos attached, of a plate with the plain combo and of various faults. Can anyone think of an explanation? I'm baffled. Why should the oxides have this effect? Thanks! Finished plate that was particularly lucky: Bisqued ware These are messily stencilled over the glaze, but it shows how the engobe gets lost in the clear glaze. This shows a blow-out, a Robin Hopper engobe and clear glaze with a wee bit of cobalt. (2019) A lot of activity happens on the edge of the engobe, but not only. The engobe is mottled when I spray it on still-damp clay. The runny copper one, same set of cups, same stencil, same clay and engobe.