Left: The base recipe of GA6-C (80% Alberta Slip, 20% Frit 3134, 4% rutile). That recipe is stable; it does not run. Right: A 2% addition of copper carbonate transforms it into a runny, melt fluid glaze. It also produces a pleasant green even on this red-burning body (a good green is not easy in oxidation ceramics). Using a catch glaze (e.g. a frit-reduced version), this could be made safe on vertical surfaces. However, there are red flags here. Having only 2% copper would not destabilize a typical glaze (making it leachable). But phase separation is occurring here, likely because the base contains significant rutile. That means the phases are copper-bearing and possibly copper-concentrating. And, as noted, the copper has a greater-than-expected impact on the melt. Third, commercial variegated copper glazes can even leach in acids (as happened with the mug inset after a night with lemon juice). While the GA6-C recipe does have a 0.3:0.7 R2O:RO ratio, other indicators make it a course of wisdom to consider the GLLE test before use on functional surfaces.

This picture has its own page with more detail, click here to see it.
A closeup of a glossy Cone 6 glaze having 4% added copper carbonate. The bottom section has leached in lemon juice after 24 hours. This photo has been adjusted to spread the color gamut to highlight the difference. The leached section is now matte.

This picture has its own page with more detail, click here to see it.
Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice overnight. The white areas indicate leaching has occurred! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such normally tend to leach metals, supposedly the manufacturers were able to tune the chemistry enough to pass tests. But the overlaps interact, like drug interactions they are new chemistries. Cobalt is clearly leaching. What else? We do not know, these recipes are secret. It is better to make your own transparent or white liner glaze (either as a dipping glaze or brushing glaze). Better to know the recipe to have assurance of adherence to basic recipe limits.
Are commercial glazes really guaranteed food safe? When manufacturers claim adherence to standards like ASTM D-4236 what are they saying? The small amounts of liquid glazes are safe for the artist to brush on. They are not making claims about leaching on finish ware.
| Oxides | CuO - Cupric Oxide |
| Materials |
Copper Oxide Black
The purest source of CuO copper oxide pigment used in ceramic glazes. |
| Glossary |
Flux
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk