Digitalfire Ceramic Glossary

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Ceramic Chemistry


There is a close link between the way glazes fire in a kiln and their chemistry. Physical properties like color, hardness, melting temperature, thermal expansion, leachability, etc are all direct products of the chemistry. Understanding the relationship between the absolute and relative amounts of the common oxides appearing in a typical glaze formula (not a recipe) to the physical presence of a fired glaze itself is the key to control. An education in glaze chemistry usually starts with a study of each of the dozen or so oxides that a fired glaze is conceptually structured from and what properties each contributes; then a study of what materials contribute what oxides and finally how to use a computer program to study existing glazes and explain their behavior and start adjusting them to alter properties or fix problems. Understanding this is well within the reach of anyone and is actually much simpler than trying to grasp the relationship between the recipe of a glaze and its behavior.


By Tony Hansen

Out Bound Links

In Bound Links

  • (Project) Ceramic Oxides Overview

    Ceramic formulation and adjustment technology can ...

  • (Glossary) Medium Temperature or Mid-Fire Glaze

    In functional ceramics this term generally refers ...


Pictures
Two glazes, same chemistry, different materials. Left has 34% Wollastonite, in other whiting sources same amount of CaO. See INSIGHT manual under: Mineralogy vs. Chemistry

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Here is Cone 6 Perkins Studio Clear and an adjustment open side-by-side in my account at insight-live. The one on the right (G2926A) substitutes Frit 3134 for Gerstley Borate (I did all the juggling of its recipe to match the chemistry of original right within Insight-live). A melt flow of the two is identical (bottom left) except that the GB version has an amber coloration from its iron (the 3134 version actually flows a tiny bit less, the other has already dripped off). Anyway, the flow test on the upper left shows G2926A flowing beside the PGF1 transparent (a tableware glaze used in industry). This extra flow indicates that this glaze is too fluid. That means it can accept some silica (the more silica any glaze can accept the harder, more stable and lower expansion it will be).

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Example of cutlery marking in a cone 10 silky matte glaze lacking Al2O3, SiO2 and having too much MgO. This is an excellent example of how imbalance in chemistry has real consequences.

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An Insight live page displaying four cone 6 matte recipes. It has been exported to a CSV file which I have opened in my spreadsheet software and reorganized to compare these 4 glazes and relate the chemistry to the melt flow tests.

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We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Each pair shows the same stain on two different base glazes (G2934 cone 6 matte and PGF1 cone 6 glossy). Why does the maroon not develop in the left pair, why is the purple stain firing blue on the right? The Mason Colorworks color chart and reference guide specifies that the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it actually is expressing a minimum, the more CaO the better). But the colorless one has 11% CaO, it should work (the maroon one has only 9% and it is working)! Likewise the purple color develops correctly in the 9% CaO but wrong in the 11% CaO base. Both stains have the same caution on the reference guide. What is going on? It is an undocumented issue: MgO. The 11% CaO base glaze is high in MgO (that is what makes it matte), that impedes the development of both colors. When you talk to tech support at Mason (or any stain company), they need to know the chemistry of your glaze to help, not the recipe.

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