We are comparing the degree of melt fluidity (10 gram GBMF test balls melted down onto a tile) between two base clear glazes fired to cone 6 (top) and cone 1 (bottom).
Left: G2926B clear boron-fluxed (0.33 molar) clear base glaze sold by Plainsman Clays.
Right: G3814 zinc-fluxed (0.19 molar) clear base.
Two things are clear: Zinc is a powerful flux (it only takes 5% in the recipe to yield the 0.19 molar). Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1, whereas the zinc one has not even started. This is very good for fast fire because the unmelted glaze will pass more gases of decomposition from the body before it melts, producing fewer glaze defects.
|Oxides||ZnO - Zinc Oxide|
Fast Fire Glazes
Traditional ceramics made in industry are fired very quickly compared to what potters do. Industry must formulate bodies and glazes that fire in an hour or two.
The melting temperature of ceramic glazes is a product of many complex factors. The manner of melting can be a slow softening or a sudden liquifying.
|Tests||Glaze Melt Fluidity - Ball Test|