R.T. Stull (Ray Thomas Stull) was an American ceramic chemist who created the famous Stull Chart (or Stull map) in 1912. This graphical tool plots the molar ratio of silica (SiO₂) to alumina (\(Al_2\mathcal{O}_3\)) from a glaze's Unity Molecular Formula (UMF) to predict fired surface qualities like gloss, matte, or crazing
The Stull Chart Explained
The chart maps out how the relationship between two primary elements affects a ceramic glaze's final fired surface properties: [1, 2]
The Grid Axes: It plots the levels of Silica (SiO₂) on the X-axis against Alumina (Al₂O₃) on the Y-axis. [1, 2, 3]
The UMF Standard: The coordinates use the Unity Molecular Formula (UMF) system, which measures the proportions of molecules rather than raw weights. [1]
The Baseline: Stull developed his original map by keeping the glaze fluxes constant (specifically at a ratio of 0.3 R₂O : 0.7 RO) and firing them to Cone 11. [1, 2]
Glaze Prediction Regions
By looking at where a recipe plots on a modern digital version like the Stull Atlas app or Glazy, ceramists can accurately predict the following outcomes: [1, 2]
Matte Surface: Glazes with a low Silica-to-Alumina ratio (typically less than 4:1 or 5:1) will form a textured, matte finish due to micro-crystallization.
Bright / Glossy Surface: Glazes with a higher Silica-to-Alumina ratio (greater than 5:1, often peaking near 1:7.5) melt fully into a smooth, reflective glass.
Unfused / Underfired: Located at extreme corners where there is either too much alumina or too much silica to melt properly at the target temperature.
Crazed: Areas on the map where the thermal expansion mismatched between the glaze and clay body will result in fine spiderweb cracking. [1, 2, 3, 4, 5, 6, 7]
Modern Application
Even though it was created over a century ago for specific high-fire porcelain conditions, modern tools like the interactive web features on Glazy Help still rely heavily on Stull's boundaries. Potters frequently reference it to convert a glossy glaze into a matte glaze, troubleshoot fit issues, or stop a glaze from running off the piece
