G3948A has stopped working. It is early 2023 and we 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 an error in restocking of our TriCalcium Phosphate container: Pyrophyllite was put in by mistake. When that ran out completely and the empty jar was restocked the glaze suddenly began working again!
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.
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.
G3948A Iron Red glaze: Can you help?