|Monthly Tech-Tip |
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.
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.
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.
G3948A Iron Red glaze: Can you help?