Crazy Blue

A vivid cone 5–6 oxidation blue built on a high-zinc, boron-fluxed glossy base. Roughly 3% copper carbonate reads turquoise-to-blue against the zinc-rich host, and a small 0.2% cobalt carbonate addition deepens it to a clean, saturated blue. A bright, glassy surface that wants to flow, so it is happiest on vertical forms with room above the foot.

Notes

Summary:

• Vivid copper-and-cobalt blue on a glossy, high-zinc cone 5–6 oxidation base.
• Very high ZnO flux (0.39) with low Al₂O₃ (0.24) — bright and fluid, runs if applied thick.
• Watch for crazing: test glaze fit on your clay body before functional ware.
• Best treated as decorative — copper glazes can leach, so leach-test before any food-contact use.
• Oxidation only — in reduction the copper shifts toward red and zinc misbehaves.

Details — based on the calculated Seger UMF (flux unity):

Li₂O 0.17 · Na₂O 0.10 · CaO 0.33 · MgO ~0.00 · ZnO 0.39 | Al₂O₃ 0.24 · B₂O₃ 0.19 · SiO₂ 2.22

The defining feature is the very high ZnO (0.39) flux paired with low Al₂O₃ (0.24). R₂O:RO is ≈ 0.27:0.73 — heavily weighted to the RO side because zinc dominates the alkaline-earth fluxes. The melt is driven by three complementary fluxes: B₂O₃ (0.19) from Frit 3134, Li₂O (0.17) from spodumene, and the large ZnO load, all of which lower the melting point so it matures cleanly in the cone 5–6 range. At SiO₂:Al₂O₃ ≈ 9.1 with low alumina the glaze sits firmly in the glossy zone and runs fluid — expect it to move if applied thickly, so leave clearance above the foot. The low alumina and high silica-to-alumina ratio also raise the risk of crazing, so test glaze fit on your clay body before using on functional ware; combined with the copper content, treat it as decorative unless leach testing clears it for food use. A ZnO load this high (0.39) can also encourage zinc-silicate crystallization or cloudiness on slow cools, so cooling rate will affect clarity. Colour comes from the additions: in this zinc-rich host the copper carbonate is pushed away from green toward turquoise/blue, and the 0.2% cobalt carbonate adds a clean blue accent — together the vivid "crazy blue". This is an oxidation recipe: in reduction the copper would shift toward red and zinc behaves unpredictably.