|Monthly Tech-Tip |
Almost all ceramic glazes are a base recipe with additions of colors, opacifiers, variegators, etc. Our traditional G3933 oatmeal glaze is a good example (recipe on the left). It can produce rich brown silky matte surfaces, especially on dark burning bodies. But one problem has emerged: The tendency to crawl. Much testing has yet to reveal the reason. Would it be possible to base the recipe on Ravenscrag Slip and achieve the same chemistry? Yes. And some unexpected benefits accrued. In the recipe on the right I sourced MgO (the key to the matte surface) from dolomite and Ferro Frit frit 3249 (earlier tests sourcing from talc were unsuccessful, off-gassing from the talc was puffing up the glaze with micro-bubbles). The all-new G3933E recipe has the same chemistry (I derived it in my account at insight-live.com). It is not likely to be without problems, but it looks identical (with richer color from a little more iron oxide), it does not crawl and it's recipe and chemistry are flexible. It is glossy when cooled fast and silky matte when cooled slowly. The MgO can be increased easily to get matteness with quick cool also. The mix of calcine and raw Ravenscrag Slip also enable control over the slurry and application properties.
Lithium carbonate is now ultra-expensive. Yet the reactive glaze on the left needs it. Spodumene has a high enough Li2O concentration to be a possible source here. It also has a complex chemistry, but the other oxides it contains are those common to glazes anyway. Using my account at insight-live.com, I did the calculations and got a pretty good match in the formulas (lower section in the green boxes). Then I made 10-gram balls and did a GLFL test at 2200F (notice the long crystals in the glass pools below the runways). Not surprisingly, this recipe is very runny, that's why the tiny yellow crystals grow during cooling. The new version fires very similar, perhaps better. Our calculated cost in 2022 was $17.84 vs. $10.40 per kg. But there is a practical cost: Poor slurry properties. The spodumene sources so much Al2O3 that 70% Alberta Slip had to be dropped to accommodate it! How does one use this type of glaze without ruining kiln shelves? Using a catcher glaze is one answer.
Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
A light-colored silty clay that melts to a clear glaze at cone 10R, with a frit addition it creates a good base for a wide range of cone 6 glazes.