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.
Fast fire glazes are used in most industries now and many can fire up and down in less than two hours. The process produces more consistent results and obvious reductions in production costs. This contrasts with firings of 24 hours cold-to-cold that most potters would do.
Technicians in industry think about the 'seal over temperature' of glazes. They want this to be high enough that all gases of decomposition from body and glaze materials have already been expelled before it is reached. If this does not happen then bubbles and blisters occur. They also think about the speed and suddenness at which the melt occurs (the more quickly the better).
Low carbon bodies are also considered important. This shortens the time needed for soaking the bisque firing.
Potters use high levels of alkalis (from high feldspar content) and lots of B2O3 (from often significant levels of Gerstley Borate, Colemanite or high-boron frits). These glazes melt too early for fast fire. Instead, levels of CaO, MgO, ZnO are higher in fast fire glazes, and these are sourced from frits instead of raw materials. Not surprisingly then, fast fire glazes are taken to higher temperatures to reach the active states of these later melting oxides.
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 more about the chemistry of fast fire glazes.
Porcelain mug put into kiln at 8:45am, drinking coffee from it at 12:15! How?
One way to fast-fire is to fire a lot lower. This is Zero3porcelain 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).
Why fast fire glazes employ zinc - a melt fluidity test tells us
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.
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 same glaze is already well melted! What does this mean? Industry avoids high boron glazes (they consider 0.33 to be 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!
Graph of wall tile fast-fire frits vs. traditional
Ceramic Glazes Today Todd Barson of Ferro Corp. overviews the glaze formulations being using in various ceramics industry sectors. He discusses fast fire, glaze materials, development and trouble shooting.