Remember that a very small addition of a particular element might give you a property you want, but increasing the amount only slightly can ruin your progress. As an inconvenience to glaze formulators, other elements interfere.
One other thing: You should use the minimum amount of binder. Sometimes it’s best to use several binders in small amounts rather than just one binder. Binders can be purified clays or organic compositions like gums or resins. Like I say, a mixture is usually best. None would be better.
To learn how to formulate gazes from scratch or using frits you can find it in my book Ceramics: Industrial Processing and Testing. You can read the contents at http://www.tjbooks.com/ceram.htm
Firing Conditions
First, from my experience glazes fire better in periodic kilns than in tunnel kilns. Matching the tunnel kiln cycle to the periodic kiln's cycle may not give the same results. The spatial configurations are different as is the atmosphere.
The firing curve naturally has heat up and cool down periods.
In between these slopes is a flat or modified soaking time/temperature.
During heat up, the binders are removed from the glaze.
They must be completely removed and they must not be reduced to carbon during heat up (preheat). This does not mean that you can’t approach reducing conditions. Some compounds like MnO and FeO can greatly improve melting although they are often in the glaze only in tiny amounts as impurities. These compounds do not form under oxidizing conditions. Anyway, do not reduce the binders to carbon. It’s near impossible to remove for many glazes during the rest of the firing.
There are a number of instruments to determine the nature of the burnout for a particular binder. Thermal gravimetric analysis (TGA) and Differential Thermal Analysis (DTA) come to this old head.
The preheat or heat up is usually a production standard for tunnel kilns. Therefore if you are not getting what you are looking for, you must adjust the burners or heating input in this zone. You don’t want bubbles from binder forming all during the soak period. You have enough bubble problems without that.
During the soak phase and at all higher temperatures the glaze is changing composition. That is, the glaze is loosing volatile elements because of the high temperatures. This can be complex as when one component of a glaze is removed it may increase the volatility of other components.
For you chemical engineers, this would be somewhat like steam distillation.
When you lose volatile elements you also increase glaze viscosity in the molten state. For this reason, and others I suppose, fast firing is often better than a long firing cycle. In fact, I’ve seen glazes that required fast firing.
Cooling is important to both the glaze and ceramic ware. You can cool quickly to just above the silica phase transition and then cool slowly through the transitions. This may not be good for the glaze which is trying to get rid of the bubbles created after the glaze melted. This can put you between a rock and a hard place. You have to be able to control the whole cooling zone of a tunnel kiln from the minute the ware comes out of the hot zone.
John T. Jones, Ph.D. (tjbooks@hotmail.com, a retired VP of R&D for Lenox China, is author of detective & western novels, nonfiction (business, scientific, engineering, humor), poetry, etc. He coauthored Ceramics: Industrial Processing and Testing. Former editor of Ceramic Industry Magazine. He is Executive Representative of IWS sellers of Tyler Hicks wealth-success books and kits. He also sells TopFlight flagpoles. He calls himself "Taylor Jones, the hack writer in some ezinemagazine.com articles."
More info: http://www.tjbooks.com
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