Why fast fire glazes employ zinc: A melt fluidity comparison tells us

Thursday 16th July 2015

I am comparing two glazes using 10-gram GBMF test balls. I am using 10-gram GBMF test balls (they simply melt down onto a tile). The top two tiles show them at cone 6, the bottom two at cone 1 (yes, most cone 6 insight-live.com/glossary/129">boron fluxed glazes are already melting at cone 1).
Left: G2926B clear boron base glaze.
Right: G3814 zinc-fluxed clear base.

Two things are clear:

Zinc is a powerful flux (it only takes 5% in the recipe to yield 0.18 molar of ZnO), whereas it takes 25% frit to yield 0.33 molar of boron.

Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1 (bottom left), whereas the zinc one has not even started (bottom right). This is very good for fast fire because more gases of decomposition from the body can pass before it melts, producing fewer glaze defects.

Pages that reference this post in the Digitalfire Reference Library:

Zinc Oxide, ZnO, GBMF Glaze Melt Fluidity - Ball Test, Fast Fire Glazes, Melting Temperature

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