The Iron-Red mechanism is working in one fluid melt base but not the other

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.

Related Pictures

Why does fluid-melt G3948B not produce an iron red

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.

Fix obvious issues in Glazy recipes before even trying them

Many come to Insight-live after "Glazy recipe fails". A better way is to recognize the potential in a recipe, fix it by material logic and calculation (e.g. limit recipes), then and try it. Glazy "Red Orange #111576" is a good example. It has two things I avoid: Lithium (expense) and a high percentage of red iron (slurry gelling). It is easy to fix both. Spodumene is a better source of Li₂O but it contributes lots of Al₂O₃ and SiO₂. We can "make room" for it by replacing the feldspar with frit 3110 (the latter contributes much more sodium and much less Al₂O₃/SiO₂ than the feldspar). Second, use black iron instead of red. The results using the C6IRED schedule are fabulous. And the cost is way down. Amazingly black iron does not gel the slurry at all! And it is not nearly as messy as the red. Like any iron red, this has a fluid melt so is running (although it is applied thickly). The thermal expansion is still quite low so it should not craze. And the LOI is much lower, that should minimize bubbling.

How to keep an iron-red glaze from being a bucket-of-jelly

This is G2890C, a cone 6 iron red glaze. It was so gelled that it was unusable! First I measured specific gravity (with difficulty): 1.48. That's too high, so I added water to reduce it to 1.44. Then I dripped in Darvan 811 (as recommended for iron-containing slurries). I added it until adding more did not thin it further (more was needed than for deflocculating the average non-iron-containing slurry). But it was still gelled. The only choice was to add more water, taking the specific gravity down to 1.42. That made the difference, making the slurry thin enough for both better application and preventing it going on in too thick of a layer. But there is an even better solution: Use black iron oxide, no Darvan is even needed for that.

Links

Glossary	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.

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