|Monthly Tech-Tip |
Stull charts attempt to explain graphically, on an x-y coordinate system, specific fired properties of a range of oxide mixtures. These graphs are generally used to predict how to achieve a specific result. For example, a graph may display results for a constant amount of flux with varying amounts of SiO2 and Al2O3 (the latter corresponding to the x and y of the graph). Notations with arrows typically overlay the chart and explain zones of matteness, glossy, crystallization, etc.
Chazo Chazim Mehmeti, who developed the UMF Glaze Excel Stull Chart creator says this (slightly edited):
My opinion, based on years of teaching, is that the Stull chart is the best in the history of the glaze help. Why? Definitely not because Stull's chart is flawless. It's not! But people perceive the position on the graphic (map) much more easily than imagining where a glaze "falls" in a bunch of numbers. My student make comments like this: "Making glazes with the Stull chart and CBC has become a very interesting video game."
-First of all, users more easily understand the role of the fluxes in the glaze. By adding one they see how "the point" immediately drops.
-Advanced users can easily see the difference in the same gram amount of BaO vs. Li2O and better understand the role of molecular weight.
-Users more easily understand the composition of kaolin when the point moves diagonally upwards as a product of kaolin having more silica than alumina.
As already mentioned, there is no chart that can be exchanged for knowledge, but still, Stull charts are extremely helpful tools. In my spreadsheet I also have a chart I call "Chazo's Boron Curve", I developed it with some help from two of my friends (Prof. Dr. Sc. David Brčić, and Dr Sc. Viktor Bojović). I created the "CBC" to improve the precision of the Boron line (especially at low temperatures, eg. cone 04).
Stull chart showing the SiO2-Al2O3-(0.7CaO+0.3KNaO) system
Chazo Chazim Mehmeti made this Stull chart to help explain why my G3948B does not produce iron crystals. It plots the formula amounts of Al2O3 (vertical axis) vs. SiO2 providing one lens through which to view the chemistry of multiple iron red glaze recipe candidates that work and don't work. His argument is that, among other necessary things like the presence of P2O5, MgO and CaO, the amounts of SiO2:Al2O3 must be within certain bounds (this chart conveys both the ratio and amounts). He points out that the relationship is sensitive enough that one should use the closest possible chemical analysis of the materials (not generic ones) when plotting points on the chart. Over a year of testing with his students, the ones fitting in the small zone on the chart worked every time (#2 being best). That is where my G3948A, which is working really well, also resides. The G3948B, which does not work, appears way off to the right because of its high SiO2 content. Of course, there could be other reasons for its failure, but the SiO2 issue is a good place to start.
Understanding Stull Charts by Pieter Mostert
|By Tony Hansen|
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