All Glossary

Stull Chart

Stull charts attempt to explain graphically, on an x-y coordinate system, specific fired properties of selected mixes of oxides. It is a visual way to understand the relationships between glaze chemistry and fired results. 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). Annotations typically highlight zones having specific properties (e.g. matteness, gloss, crystallization, etc). The focus of a chart must be clearly understood for it to be valid and valuable.

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 glaze help. Why? Definitely not because Stull's chart is flawless. It's not! But people perceive a position on the graphic (map) much more easily than imagining where a glaze "falls" in a bunch of numbers. My students 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 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).


Generating Stull Charts requires a lot of organized testing and record keeping. An account at insight-live.com is one way to do that.

Related Information

Stull chart showing the SiO2-Al2O3-(0.7CaO+0.3KNaO) system

This picture has its own page with more detail, click here to see it.

The original Stull chart - Chart 1 from Transactions of the American Ceramic Society, Vol. XIV (1912). Stull mapped test glazes by their SiO₂:Al₂O₃ molar ratio and identified the regions that predict surface quality.

If you really want to "get into the weeds" of glaze chemistry, the https://stullatlas.app tool, by Ryan Lack, takes Stull's insight and applies it to a modern dataset of real-world glazes, giving you an interactive map of the glaze chemistry landscape. The primary dataset comes from Glazy. It contains thousands of community-contributed recipes with calculated UMF values, cone temperatures, surface types, and firing atmospheres. Each point on the chart represents a real glaze recipe that someone has mixed, fired, and documented.

Why does fluid-melt G3948B not produce an iron red

This picture has its own page with more detail, click here to see it.

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.

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