Most potters use glazes for years before asking the question: what is actually happening in that bucket? The answer is more accessible than it looks, and understanding even the basics changes how you work with glazes at every level.
This is not a full glaze chemistry course. It is the foundation — the part that makes everything else make sense.
What a Glaze Actually Is
A fired glaze is glass. Specifically, it is a layer of glass that has been formulated to melt at ceramic temperatures (anywhere from cone 06 to cone 10 and beyond), bond to your clay body, and create a stable, non-toxic, often beautiful surface.
Like all glass, it is built from three categories of materials:
- Glass formers (silica) — the backbone of the glaze. Silica (SiO2) is what becomes glass. Without enough silica, you do not have a glaze.
- Alumina (Al2O3) — the stiffener. Alumina makes the glaze viscous so it does not run off your piece during firing. It also adds durability and hardness.
- Fluxes — the melters. Fluxes lower the melting point of silica so it flows at ceramic temperatures. Common fluxes include calcium (from whiting), potassium and sodium (from feldspars), magnesium (from talc or dolomite), lithium, barium, and zinc.
Every glaze recipe is a balance of these three. Changing one changes everything else.
Unity Molecular Formula (UMF): The Tool That Changes Everything
Glaze chemists use a system called the Unity Molecular Formula to analyze and compare glazes. Instead of looking at percentages of raw materials, UMF converts everything into molar ratios so you can see exactly how much of each oxide is present.
You do not need to calculate UMF by hand — software like Insight, Glazy, or the built-in calculator in many pottery apps handles this automatically. But knowing how to read it matters.
A typical cone 6 oxidation glaze in UMF might look like this:
- Fluxes (KNaO, CaO, MgO) — in the R2O/RO column, totaling 1.0
- Alumina (Al2O3) — usually between 0.3 and 0.5
- Silica (SiO2) — usually between 3.0 and 5.0
The silica-to-alumina ratio (Si:Al) tells you a lot about glaze character. A higher ratio tends toward glossy. Lower tends toward matte.
Why Glazes Crawl, Pinhole, and Crater
Most glaze defects have a chemical explanation:
Crawling — the glaze pulls back from the surface during firing, leaving bare clay patches. Common causes: too much clay in the recipe (high shrinkage during drying), applying over dusty or oily bisqueware, or a glaze with too high a surface tension in the melt.
Pinholes and pitting — small holes in the fired surface. Usually caused by gases escaping from the clay or glaze during firing that cannot heal before the glaze stiffens. Solutions: slower firing through the 1000-1100°F range (where organic burnout happens), or a longer hold at peak temperature.
Crazing — a network of fine cracks in the fired glaze. Caused by a mismatch between the thermal expansion of the glaze and the clay body — the glaze contracts more than the clay during cooling and literally tears. Fix: add silica to the glaze to lower its expansion, or switch to a clay body with a closer expansion coefficient.
Shivering — the opposite of crazing. The glaze contracts less than the clay and pops off in flakes. Less common but more dangerous on functional ware.
Colorants: The Last Layer
Colorants are metal oxides added to a base glaze in small percentages:
- Iron oxide — the most versatile. Tan/amber in oxidation, celadon to dark brown/black in reduction. Percentage matters enormously (1% vs. 8% gives completely different results).
- Cobalt carbonate — blue, very powerful. 0.5% gives a pale blue. 2% is a deep blue. Use carefully.
- Copper carbonate — green in oxidation, red in heavy reduction. Unstable at high temperatures.
- Rutile — complex effects, breaks over texture, produces variegation. Beloved in cone 6 glazes.
- Manganese dioxide — purple/brown, use cautiously as it can be toxic in raw form.
The same colorant in two different base glazes will look completely different because the base chemistry affects how the colorant develops.
Where to Go From Here
The best way to learn glaze chemistry is to test systematically. Pick a base glaze you like and make line blends — vary one material at a time and fire tiles. The results will teach you more than any textbook.
Resources worth your time:
- The Potter is Complete Book of Clay and Glazes by Frank and Janet Hamer — the reference
- Digitalfire.com (Tony Hansen’s site) — free, deep, technical, invaluable
- Glazy.org — open-source glaze database with UMF analysis built in
Use The Potters Mud Room glaze library and test tile library to log your special glazed pieces and test tiles alongside the recipe, firing results, and photos. Over time you will build a personal database that becomes one of your most valuable studio assets.