Why the Chemistry Matters at the Bench

You can mix a glaze by rote for years and still be baffled when a trusted recipe turns cloudy in a different kiln. The potters who troubleshoot quickly are almost always the ones who understand what each ingredient is doing. Glaze chemistry is not an academic exercise; it is the difference between guessing and knowing which knob to turn. Once you can picture silica, alumina and flux as three colleagues with very different temperaments, most surface defects stop being mysterious and start being solvable.

The Three Pillars: Silica, Alumina and Flux

Almost every glaze can be described as a glass former, a stiffener and a melter. Get the balance between them right and the surface behaves.

• Silica (usually from flint, quartz or feldspar) is the glass former. It builds the durable, glossy network that makes a glaze a glaze. Too little and the surface stays soft and soluble; too much and it will not melt properly at your top temperature, leaving a dry, matt, underfired look.
• Alumina (from clay, feldspar or alumina hydrate) is the stiffener. It raises the melting point, increases viscosity and controls how the glaze flows and crystallises. It is the difference between a runny glaze that pools off the rim and one that holds its place on a vertical pot. Alumina also helps a glaze resist crazing by reducing its expansion.
• Fluxes (lead in older recipes, plus sodium, potassium, calcium, magnesium, barium, zinc, lithium and boron) are the melters. They break down the silica network so it softens at kiln temperatures. Each flux has its own character: boron melts early and widely; calcium gives durability and a stiff, glossy surface; sodium and potassium are strong but increase expansion; magnesium promotes matt surfaces and opacity.

Why the Ratio Matters More Than the Recipe

Two recipes can look completely different on paper and behave almost identically in the kiln, because what matters is the molar ratio of flux to alumina to silica, not the percentages of raw materials. This is why potters use a unity formula, or Seger formula, which reduces a recipe to a standard set of numbers. In a typical mid-range glossy glaze you want roughly three to five times as much silica as alumina, and enough flux to bring the melt down to your firing temperature. When you learn to read that balance, you can swap a feldspar, adjust for a different clay body, or substitute a frit without everything falling apart.

The ratio also explains why raising the temperature does not simply "melt it more". A glaze fired hotter will dissolve more alumina from the clay body beneath it, which stiffens the melt and can actually make it less runny than you expect. The chemistry is a moving conversation between glaze and pot.

Common Defects and What They Are Telling You

Most surface problems are the three pillars out of balance, or a firing problem that mimics one.

• Crazing (a web of fine cracks) usually means the glaze is contracting more than the clay body as they cool. Fluxes like sodium and potassium push expansion up. Reduce them, add a little more silica or alumina, or check that your glaze fits the body at all — crazing is a fit problem before it is a recipe problem.
• Shivering (glaze flaking off, often at edges) is the opposite: the glaze is contracting less than the body. It is less common and more serious, and usually means too much silica or alumina in an already tight fit.
• Pinholing (small craters) is often gas escaping through a glaze that has already sealed over. Slow the glaze's melt with a little more alumina, bisque fire slightly higher, or hold at the top temperature so bubbles have time to rise and break.
• Crawling (glaze pulling back into islands) points to a dusty, greasy or over-smooth surface, or to a glaze with too much clay and too little flux — the layer shrinks before it can bond.
• Dunting (cracks that appear after firing) is a fit problem at the other extreme, where the glaze compresses the body so hard it splits. Again, adjust the flux balance or re-formulate for the body.

Practical Steps for Adjusting a Glaze

When something goes wrong, change one thing at a time and record it. Keep a notebook with the recipe, the firing schedule, the clay body and the result, because a defect on one body is often perfect on another.

• To make a glaze less runny, add a small amount of alumina or reduce the strongest flux.
• To make it more glossy and melted, nudge the flux up or add a little boron, and check you are not over-firing.
• To fix crazing, lower the flux expansion by swapping some sodium or potassium for calcium or magnesium, or add silica.
• To fix pinholing, slow the melt with alumina, hold longer at top temperature, or apply thinner coats.
• Always test on a tile of your actual clay body before committing a pot.

Chemistry will not make you a potter, but it will stop you blaming the kiln. Learn the three pillars, keep notes, and every failed glaze becomes a clue rather than a loss.