Fast Fire Glazes
Fast fire glazes are used in most industries now and many can fire up and down in less than two hours. Traditional alkali and boron glazes melt too early and gases of decomposition from the body cause them to bubble and blister. Fast fire glazes thus need to melt late and quickly. Fast fire glazes can also be formulated to form a crystal network early in the firing (from CaO or MgO) that is porous and stable to above 1000C (after which it collapses and melts quickly). Search for the term "fast fire" in the materials area to find frits intended for this purpose. This will help you to learn about the chemistry of fast fire glazes. Generally, they have much lower boron and sodium and higher zinc, magnesia, calcia and silica.
Porcelain mug put into kiln at 8:45am, drinking coffee from it at 12:15! How?
This is Zero3 porcelain made using Dragonite Halloysite (instead of New Zealand Halloysite). It is the L2934C recipe. It was fired to cone 03 and glazed with G2931K clear glaze (which has fired crystal clear and flawless). I fired at 1200F/hr to 1950F, held it for 15 minutes, cooled at 999F/hr to 1850F and held it for 30 minutes, then dropped as fast as the kiln would do. It has some translucency and fires with a purplish hue (the NZ burns blue-white and is more translucent).
Your boron glaze might melt alot earlier than you think
The porcelain mug on the left is fired to cone 6 with G2926B clear glossy glaze. This recipe only contains 25% boron frit (0.33 molar of B2O3). Yet the mug on the right (the same clay and glaze) is only fired to cone 02 yet the glaze is already well melted! What does this mean? Industry avoids high boron glazes (they consider 0.33 high boron) because this early melting behavior means gases cannot clear before the glaze starts to melt (causing surface defects). For this reason, fast fire glazes melt much later. Yet many middle temperature reactive glazes in use by potters have double the amount of B2O3 that this glaze has!
Why fast fire glazes flux using zinc
We are comparing the degree of melt fluidity (10 gram 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.
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