Two matte mechanisms: One crazes the other does not

These two glazes look the same, they are both cone 6 satin mattes. On the same porcelain. But the matteness "mechanism" of the one on the left, VC71, is a low Si:Al ratio melted by zinc and sodium. The mechanism of the one on the right, G2934, is high MgO melted by enough boron to also have plenty of SiO₂ and Al₂O₃. The "baggage" of the mechanism on the left is high thermal expansion and crazing (drastically reducing strength and providing a space for a germ zoo). If your ware develops this your customers will bring it back for replacement. No change in firing will fix this, the body and glaze are not expansion compatible. Period.

Related Pictures

This is crazing. This is functional ware. Is this good?

AI generated with the prompt: A crazed glaze on a stoneware pottery mug.

This glaze is "stretched" on the clay so it cracks. When the lines are close together like this it is more serious. If the effect is intended, it is called "crackle" (but no one should intend this on functional ware). Potters, hobbyists and artists invariably bump into this issue whether using commercial glazes or making their own.

"Art language" solutions don't work, at least some technical words are needed to understand it. Crazing is a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. The amount of change is very small, but ceramics are brittle and glazes are rigidly attached. If they are stretched on the ware cracks will occur to relieve the stress (usually during cooling in the firing but sometimes much later). All glaze manufacturers advise against crazing on functional ware.

Why does VC71 not cutlery mark when it is barely melting?

Left: VC71 cone 6 silky matte glaze. Right: An adjustment that adds boron melter and SiO₂/Al₂O₃ (which preserves their ratio). The dramatic improvement in melting was unexpected. Even though B has the same Si:Al ratio, it is completely glossy. Why? A (left) is simply not melting completely, that is why it is silky matte (not a true matte). Yet it feels like a good silky matte and is resistant to cutlery marking. Why? Touch alone can be misleading. Cutlery marking usually happens with matte glazes or heavily opacified whites, this is neither, it is an under-fired glossy glaze, fired just high enough not to mark.

Is the V.C. 71 pottery glaze a true matte?

The top glaze is V.C. 71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. V.C. 71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).

Links

Glossary	Co-efficient of Thermal Expansion The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking.
Glossary	Calculated Thermal Expansion The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
Troubles	Glaze Crazing Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.

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