Silica is a glass former. Alumina modifies a glass. Fluxes reduce melting temperature. The Unity Molecular Formula (UMF) systematically relates these chemistries to each other in a useful glaze chemistry shorthand UMF shows the ratio of glass formers to fluxes and can be used to predict glaze behavior.

For more information about UMF and the importance of the flux ratio:

Glazy Stull Chart

Glazy Stull Chart

Original Stull Chart

Original Stull Chart
 

The Glazy Stull Chart overlay was inspired by the work of R.T. Stull and Matt Katz.

Note: The Stull chart was created for glazes with a flux ratio 0.3 R2O: 0.7 RO and fired to cone 11. There may be variations in the chart for glazes with different flux ratios fired at different temperatures.

This chart is a work in progress and will be revised.

For more information, see R.T. Stull's original article in Transactions of the American Ceramic Society, Volume 14, pages 62-70.

Also see Matt Katz's Introduction to Glaze Formulation Online, Robust Mid-Temperature Glazes and How Glazes Melt.

 

Zooming in the chart

Zooming in on the Stull Chart

The chart can be zoomed in by clicking and dragging a rectangle with your mouse. On mobile devices, just pinch and zoom into the area you want to zoom. To reset the zoom, click the "Reset zoom" button in the top-right corner of the graph.
R2O:RO Ratio

The R2O:RO Ratio Scale shown in the Stull Chart.

The ratio of R2O to RO oxides has implications for glaze stability and expansion.

Matt Katz has demonstrated that a 0.3 R2O: 0.7 RO ratio produces the most robust glazes at a range of temperatures.

Again, see Matt Katz's Introduction to Glaze Formulation Online, Robust Mid-Temperature Glazes and How Glazes Melt.

As of March 2016 glaze limits are being removed from Glazy in favor of the Stull chart. The Stull chart seems to be a more reliable and descriptive guide than glaze limits. The glaze limit table chart below is retained for reference.

Glaze Limit Formulas are general guidelines for stable, functional base glazes. There are a wide range of published limit formulas, and a lot of controversy surrounding them. You will notice that many non-functional, decorative glazes will exceed glaze limits. If your functional glaze recipe exceeds a limit range it does not necessarily mean that your glaze is not safe, rather that you should pay attention to those oxides and perhaps consider modifying the recipe. Conversely, you should not assume that your glaze is stable simply because it falls within a glaze limit range.

RO/R2O (Fluxes) R2O3 (Stabilizers) RO2 (Glass Formers)
Name KNaO Li2O PbO ZnO CaO MgO BaO SrO Al2O3 B2O3 SiO2
Δ5-6 Val Cushing Glossy 0.05 - 0.6 0 - 0.5 0 - 0.6 0 - 0.15 0.05 - 0.6 0 - 0.1 0 - 0.15 0 - 0.15 0.1 - 0.3 0 - 1.0 1.5 - 4.0
Δ5-6 Val Cushing Satin 0.05 - 0.35 0 - 0.15 0 - 0.4 0 - 0.3 0.05 - 0.7 0 - 0.35 0 - 0.35 0 - 0.35 0.2 - 0.4 0 - 0.5 2.0 - 3.5
Δ5-6 Val Cushing Matte 0.05 - 0.3 0 - 0.1 0 - 0.2 0 - 0.4 0.05 - 0.8 0 - 0.45 0 - 0.5 0 - 0.5 0.2 - 0.5 0 - 0.5 2.0 - 3.0
Δ5-6 Hesselberth & Roy 0.01 - 0.03 0 - 0.2 0.2 - 0.6 0 - 0.3 0 - 0.2 0.25 - 0.4 0.15 - 0.35 2.5 - 4.0
Δ9-10 Val Cushing Glossy 0.05 - 0.5 0 - 0.4 0 - 0.15 0.05 - 0.8 0 - 0.15 0 - 0.15 0 - 0.15 0.2 - 0.5 0 - 0.5 2.0 - 6.0
Δ9-10 Val Cushing Satin 0.05 - 0.4 0 - 0.2 0 - 0.4 0.05 - 0.8 0 - 0.5 0 - 0.5 0 - 0.5 0.25 - 0.6 0 - 0.4 2.0 - 5.0
Δ9-10 Val Cushing Matte 0.05 - 0.3 0 - 0.1 0 - 0.5 0.05 - 0.9 0 - 0.6 0 - 0.6 0 - 0.6 0.25 - 0.8 0 - 0.2 2.0 - 5.0
Δ9-10 Hesselberth & Roy 0.1 - 0.3 0.3 - 0.7 0 - 0.4 0 - 0.3 0.3 - 0.6 0 - 0.3 3.0 - 5.0

There are many articles concering glaze limits, some notable ones are:

Similar Base Recipes

If two recipes consist of the same base materials (not including additional materials) in the same amounts (within 1%), then they are considered similar.

  • Two Leach 4321 glazes (40% Feldspar, 30% Silica, 20% Whiting, 10% Kaolin) may be similar even if their additional oxides differ. For instance, Leach White is a 4321 glaze with added Zircopax, while Leach Celadon is a 4321 glaze with added iron. These two glazes are considered similar by Glazy.
  • A Leach 4321 glaze recipe with 40% Potash Feldspar is similar to the same glaze recipe with 40% Custer Feldspar because the materials or their parent materials are the same.
  • A Leach 4321 glaze recipe with 40% Potash Feldspar is not similar to the same glaze recipe with 40% Soda Feldspar because the materials (and their parent materials) are different.
  • A Leach 4321 glaze recipe with 39.5% Feldspar and a Leach 4321 glaze with 40% Feldspar are considered similar because the material amounts only vary by %1.

Similar Unity Formulas

Two recipes are considered to have similar unity formulas if their oxides do not differ more than .05 For this comparison, K2O and Na2O are combined (KNaO).

Recipes that have similar unity formulas do not necessarily exhibit similar fired characterisitics.

Exporting a recipe

Recipes can be exported as a Recipe Card- an image containing the recipe photograph, name, and recipe details.

Recipes can be exported to your favorite glaze calculation software. Currently, only Insight and GlazeChem are supported, but hopefully more export formats will be added in the future.

To export a recipe, simply select from the menu on the recipe page. The Recipe Card image can be saved on your computer or shared with others online. The downloaded glaze calculation software text file can be imported into your program.

09/2016 Note: This is a new feature. If a recipe material is not listed, that material's data has not yet been added. Some materials only have GHS ratings, while others only have HMIS ratings. Warnings differ between systems and countries. Glazy does not take any responsibility for accuracy of linked SDS information. Contact your ceramics supplier for safety information for your specific materials.

You must wear a NIOSH certified mask whenever mixing ceramic materials. All materials should be handled and fired in well-ventilated areas to avoid the risk of inhalation.

Lack of a warning does NOT mean that a material is safe.

From hazard warnings alone it is difficult to determine when and how a given material may be harmful. Some materials might be relatively safe to use when making glazes, but toxic when volatized during firing or leached from a non-durable fired glaze. Other materials might be relatively safe in a durable fired glaze, but extremely toxic in raw form in the studio. It is up to the individual to understand the risks of each material and take necessary precautions.

GHS Pictograms

Explosive
Explosive
Flammable
Flammable
Oxidizing
Oxidizing
Compressed Gas
Compressed Gas
Corrosive
Corrosive
Toxic
Toxic
Harmful
Harmful
Health Hazard
Health Hazard
Environmental Hazard
Environmental Hazard