Sometimes it is better to replace the base in a production glaze recipe

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 problems have emerged, especially a 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 accrue. 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). This 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 mix of calcine and raw Ravenscrag Slip also enable control over the slurry and application properties. I later tuned this recipe a little more and settled on G3933EF.

Related Pictures

Sourcing Li2O from spodumene instead of lithium carbonate comes at a cost

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 to mix this recipe in 2022 was $17.84/kg vs. $10.40/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.

A better cone 6 oatmeal glaze using Ravenscrag Slip

Left: G3933EF oatmeal based on Ravenscrag Slip.
Right: G3933 oatmeal based on a mix of G2934 matte and G2926B glossy base glazes.
Both have the same added colorants. The Ravenscrag version features several advantages. Most importantly much less tendency to crawl. It has better application properties, the slurry needs less water and it is naturally thixotropic. It has an extra option for adjusting properties: Changing the ratio of roast-to-raw Ravenscrag clay. It is responsive to cooling differences - more matte on slow cool versions of the C6DHSC schedule (e.g. 150F/hr), more glossy on faster cools (e.g. 250F/hr). And, its recipe is adjustable (e.g. raising the MgO if a more persistent matte is needed). And, it looks and feels way better, interacting with dark bodies for richer color and varying in tone more for thinner and thicker sections.

Links

Glossary	Base Glaze 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.
Glossary	Glaze Chemistry 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.
Materials	Ravenscrag Slip 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.

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