|Monthly Tech-Tip |
It is about the oxide chemistry, as shown calculated below the recipes in my account at insight-live.com. These glazes are fired at cone 6 using the C6DHSC schedule (we are focussing on the amber glossy glaze on the insides of the mugs). Most oxides want to form silicate crystals (combine with SiO2) as the glaze cools (if the cooling ramp is slow enough), iron oxide is not the least of these. Alumina (Al2O3) stabilizes the melt, that means it helps the melt to solidify as a glassy solid, not a crystalline one (thus, it does not devitrify). Notice the two Al2O3 values (black-on-red numbers): The glaze on the right has much less. That is because Ferro Frit 3134 contains almost no Al2O3 (notice in the blue panel, only 2%). The alumina in the glaze on the left, sourced more abundantly by Frit 3195, readily releases in the melt, ready to take on its job: Stiffen it and impede the formation of iron silicate crystals during cooling to create a better glass.
You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.