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The glaze thickness is also the same. The firing schedule is critical with iron reds, we tried both the C6DHSC and C6IRED schedules, both of which normally produce the result on the left (with this G3948A recipe). But in both firings the result is now like the piece on the right. The one difference is this: The new batch was ball milled. My first theory was this: Iron oxide particles agglomerate and neither our propeller mixer or blender remove these agglomerates - but the ball mill does. The agglomerates must be either seeding the crystals or facilitating their growth. However subsequent failures in mixing the recipe without ball milling did not solve the problem. But neither did they melt as well as this milled version.
These two pieces were fired in the same kiln using the C6DHSC firing schedule. Fluid melts are an essential enabler of crystal growth during cooldown, that is what there are. Both contain significant Li2O to help the B2O3 achieve that fluid melt. Glaze #1, G3948A, has less iron than is typical yet works! Its high MgO/CaO are very likely key factors as to why. Glaze #2 has much more Na2O and it has both SrO and ZnO that #1 does not have. #2 is much higher in Al2O3 and has more than double the amount of SiO2. So which of all these factors is responsible for #2 having zero crystals? Very likely it is two important ones: The low CaO/MgO levels. And the high SiO2.
Chazo Chazim Mehmeti made this Stull chart to help explain why my G3948B does not produce iron crystals. It plots the formula amounts of Al2O3 (vertical axis) vs. SiO2 providing one lens through which to view the chemistry of multiple iron red glaze recipe candidates that work and don't work. His argument is that, among other necessary things like the presence of P2O5, MgO and CaO, the amounts of SiO2:Al2O3 must be within certain bounds (this chart conveys both the ratio and amounts). He points out that the relationship is sensitive enough that one should use the closest possible chemical analysis of the materials (not generic ones) when plotting points on the chart. Over a year of testing with his students, the ones fitting in the small zone on the chart worked every time (#2 being best). That is where my G3948A, which is working really well, also resides. The G3948B, which does not work, appears way off to the right because of its high SiO2 content. Of course, there could be other reasons for its failure, but the SiO2 issue is a good place to start.
This is the G3948A recipe. Iron red glazes are easy to do in high-temperature reduction but not so in medium-temperature oxidation. Most people just try a bunch of recipes they find online hoping that one of them actually fires the way it is shown in the picture! A better approach for us was to study a range of ones claiming to be iron reds looking for things in common with the chemistries and recipes. G3948A, on these two M370 mugs, is a product of that. Unlike many, the original recipe we found, G3948, did have a suggested firing schedule. It seemed strange so we just used the standard C6DHSC slow-cool schedule. That one is also ideal for the liner glazes, giving them a better gloss finish. It was not tempting to even try the original recipe (because it measured up poorly against common sense recipe limits), but it did make sense to fix obvious issues and then try it. Unlike every other recipe we have seen, this one suffers no issues with gelling of the slurry because it contains no Gerstley Borate and uses black iron oxide. It has very good application properties and requires only 80 water for each 100 powder to mix as a creamy dipping glaze. And it does not need any lithium carbonate.
G3948A has stopped working. It is early 2023 and we have mixed it again multiple times and each time it is firing this ugly brown! I have each original mix ticket and have triple-checked each. I have tested both frits using a GBMF test and they are not mislabelled. The other materials are from the same 50lb bags. In the end the problem turned out to be restocking of our TriCalcium Phosphate container with Pyrophyllite. When that ran out completely and the empty jar was restocked the glaze suddenly began working again!
Iron Red Glaze
A type of ceramic glaze, typically fired around 2200F, where iron oxide in the cooling glass precipitates out to form a striking red crystalline mesh on the surface.
The fine mineral, oxide and clay particles used in ceramics often aglommerate during storage or even in the latter stages of production. These must be broken down later.
G3948A Iron Red glaze: Can you help?