Why Glaze Chemistry Repays a Little Study

Most potters learn glazes the hard way: a recipe looks lovely on someone else's pot, you mix it up, fire it, and it crazes, crawls or runs off the shelf. Glaze chemistry has a reputation for being intimidating, but at studio level it comes down to three materials doing three jobs — silica, alumina and fluxes. Once you understand what each one contributes, a glaze that misbehaves stops being bad luck and becomes something you can adjust deliberately. All you need is a notebook, a stack of test tiles and the discipline to change one variable at a time.

Silica: the Glass Former

Silica, added as quartz or flint, builds the actual glass network. Without enough of it you get a glaze that is soft, easily scratched and prone to leaching colourants into food. Most mid-range recipes sit somewhere between 20% and 35% silica by weight.

• Too little silica: the glaze is glassy and runny, scratches easily and often crazes because the network is weak.
• Too much silica: the melt stiffens and refuses to mature, giving a dry, satin, under-fired surface.
• Plenty of silica, balanced with flux: durability, hardness and a lower tendency to craze.

Silica is also your main lever on thermal expansion. Broadly speaking, the more silica in the formula relative to the fluxes, the less the cooled glaze wants to contract — and less contraction, when the numbers are close to the clay body, means less crazing.

Alumina: the Backbone

Alumina is the quiet workhorse. It comes in through kaolin and ball clay, some feldspars, and alumina hydrate where extra is needed. It stiffens the melt, raises the melting point, and gives the fired glaze its hardness, chemical resistance and resistance to scratching. A glaze with almost no alumina may be beautifully glossy but it will be soft, and it will mark in the dishwasher.

Two ratios are worth writing on the inside of your glaze cupboard. In a typical glossy stoneware glaze, the molar ratio of silica to alumina is about 6:1 to 10:1. Drop towards 4:1 and the surface turns matte and buttery; push past 12:1 and you risk a glassy, brittle, runny glaze. The second ratio, flux to alumina, runs roughly 1.5:1 to 3:1 for a glossy finish. Higher flux with low alumina gives fluidity and a strong craving to run off the pot.

Fluxes: Lowering the Melting Point

Pure silica melts around 1700°C, which is well beyond any studio kiln. Fluxes do the work of bringing that melting point down to a temperature your kiln can reach and your clay can survive. They behave very differently from one another.

• Sodium and potassium (feldspars, some frits) are powerful fluxes and give bright, glossy melts — but they raise thermal expansion sharply, which is a common cause of crazing.
• Calcium and magnesium (whiting, dolomite, talc) melt less aggressively but produce harder, more durable glazes with lower expansion and better resistance to acids.
• Boron, usually from a frit rather than raw borax, does the heavy lifting at earthenware temperatures around 1020–1100°C. Useful and reliable, but a high-boron, low-alumina glaze is a classic crazer.
• Zinc and lithium are potent and temperature-sensitive; zinc encourages mattness and crystalline surfaces, while lithium is expensive and can shift certain colouring oxides.

Because a bucket glaze is mixed by weight while the kiln works by molar proportions, two recipes with identical percentages can behave quite differently once one uses soda feldspar and the other potash feldspar. That is why recording which materials you used matters as much as recording the amounts.

Crazing, Shivering and Glaze Fit

Crazing is a fit problem, not a mixing problem. The glaze contracts more than the clay body beside it as the kiln cools, and the resulting tension cracks the surface in a fine web. If you see crazing, work through these in order:

• Increase silica and alumina slightly, keeping the rest of the recipe the same.
• Swap part of the sodium or potassium flux for calcium or magnesium — replacing some soda feldspar with whiting is a classic adjustment.
• Reduce boron if you are firing at earthenware temperatures.
• Apply a little thinner; a heavy glaze layer crazes more readily than a 0.3–0.5mm coat.
• Check the firing itself. Over-firing pushes fluxes harder; very fast cooling can also leave a glaze under stress.

Shivering is the opposite fault — the glaze contracts less than the body and flakes off along rims and sharp edges. It is rarer and usually means you have over-corrected, with too much silica and alumina or a heavily magnesian glaze.

Testing, Records and Reliable Results

Nothing replaces fired evidence. Make a dozen test tiles from a flat, well-compressed strip of your actual clay body, glaze them with the same thickness and fired together in the same load. Then check them hard: a boiling water to ice water quench reveals incipient crazing; a drop of ink or a weak dye wiped over the surface finds every crack; a dishwasher cycle or a slice of lemon will tell you whether the glaze is truly mature or quietly leaching.

Keep a simple log: recipe, specific gravity, glaze thickness, firing schedule, soak time, cooling rate, and what the tile did. Cooling matters more than many potters expect — a slow cool through the final 100°C reduces stress in both glaze and body. Finally, remember that an under-fired glaze is never a durable one, no matter how good the chemistry looks on paper. Get the melt fully mature, and the numbers will have a chance to do their work.