|Monthly Tech-Tip |
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.
The top glaze is VC71, 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. VC71, 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).
Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.