The top samples are 10 gram GBMF test balls melted down onto porcelain tiles at cone 6 (this is a high melt fluidity glaze). These balls demonstrate melt mobility and susceptibility to bubbling but also color (notice how washed out the color is for thin layers on the bottom two tiles). Both have the same chemistry but recipe 2 has been altered to improve slurry properties.
Left: Original recipe with high feldspar, low clay (poor suspending) using 1.75% copper carbonate.
Right: New recipe with low feldspar, higher clay (good suspending) using 1% copper oxide.
The copper oxide recipe is not bubbling any less even though copper oxide does not gas. The bubbles must be coming from the kaolin.
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