Glazing
A glaze is a skin of glass grown onto clay in the fire: silica melted by fluxes, held in place by alumina, colored by metal oxides. This page opens that recipe, tours the families a studio actually fires — from transparent liners to copper red — and ends with the faults every glaze eventually shows, and how to fix them.
The glass former. Every glaze is built around melted quartz; more silica means a harder, higher-melting glass.
The stiffener. Clay and feldspar bring alumina that keeps the melt viscous — the reason glaze stays on a vertical wall.
The melters: feldspar, calcium, sodium, zinc, boron. Which flux leads decides the firing temperature — and half the character.
Metal oxides paint the glass — iron, copper, cobalt, chrome. Tin and zircon cloud it white; rutile breaks and streaks it.
Glass in its plainest form: it seals the surface and shows the body and any underglaze work beneath. The default liner for food surfaces — when it fits, it disappears.
Micro-crystals grown during cooling scatter the light: the surface reads soft, dry or buttery. A true matte is a crystal structure, not an underfired glaze — slow cooling deepens it, and cutlery marking is the price to watch.
One to three percent iron in a clear feldspathic glaze, starved of oxygen, turns jade: gray-green to blue-green, pooling darker where it runs thick. The classic of Chinese stoneware — at its best over carved porcelain.
An iron-saturated glaze, near black in depth, breaking amber to rust where it thins over rims and ridges. The breaking edge is the whole point — put crisp profiles under it.
High-alumina and feldspar-rich: milky white through flame orange, pinholed and flashed, trapping carbon in gray-black clouds when the early atmosphere is right. The moodiest glaze in the kiln — no two firings agree.
Half a percent of copper, reduced, turns blood red — oxblood, flambé, peach bloom. Miss the reduction window and the same bucket fires clear or liverish; the margin is a masterclass in kiln control.
Washed wood ash brings its own fluxes; on the pot it melts into green-gray rivulets that gather in every carving. The oldest high-fire glaze — first found where kiln ash landed on shoulders by accident.
Zinc-silicate blooms grown on purpose: fire hot, then hold in the cooling range while crystals unfurl in a melt made runny by leaving alumina out. Every pot needs a catch dish under it — and a grinder to free the foot.
A tin-white opaque glaze over red earthenware, painted with stains while still raw — the brushwork fuses into the glaze in the fire. The route to the brightest painted color in ceramics.
The glass former; 10 – 30 % of almost every recipe.
Brings alumina and keeps the slurry suspended; 5 – 20 %.
Suspension aid — about 2 % stops hard settling.
The high-fire workhorse flux — brings its own silica and alumina; 20 – 50 %.
A soda-rich cousin of feldspar that melts earlier — the mid-fire favorite.
Calcium carbonate: the classic high-fire flux for a hard, durable glass.
Calcium plus magnesium — the route to buttery satin mattes.
Magnesium: silky surfaces and lower thermal expansion — a crazing fix.
A strong mid-fire flux; in excess, the seed of crystalline glazes.
Pre-melted, ground glass: makes boron and soda safe and predictable — the melter of low and mid fire.
The potter's iron: honey at 1 %, celadon reduced, tenmoku at 10 %.
Green in oxidation, blood red in reduction; 0.5 – 3 %. Volatile at high fire.
The strongest colorant of all — 0.25 % already reads deep blue.
Impure titanium: streaks, breakup and crystal seeding; 2 – 6 %.
The opacifiers: tin the softer white, zircon the cheaper, harder one; 4 – 10 %.
The classic hazard: lead glazes leach into acidic food and drink and are strictly limited by law on tableware. Modern studio practice is simply lead-free — treat historic and imported recipes with suspicion.
Barium carbonate is toxic raw and can leach from matte or underfired glazes. For food surfaces swap it for strontium carbonate — keep barium mattes on outer walls.
Bright reds, oranges and yellows ride on cadmium and selenium. On food surfaces use only encapsulated (zirconium) stains, and only lab-tested — cadmium carries its own legal migration limit.
Above roughly 2 – 3 % copper destabilizes a glaze: it leaches itself, increases the release of other metals and can taste metallic against acidic food. Heavy copper surfaces belong on the outside.
Saturated manganese "bronze" surfaces are for sculpture, not plates — and manganese fumes during the firing demand good kiln ventilation.
Any matte or underfired surface has an incomplete glass network and releases more of whatever it contains. If a food glaze must be matte: fire it to full maturity — and test it.
The traditional Leach-style base — a gloss clear and the starting point of countless studio glazes.
A generic boron-fluxed gloss base for the electric kiln — the family every commercial cone-6 clear belongs to.
A magnesium matte — buttery where it lies thick; slow cooling deepens the surface.
Frit carries the whole melt at earthenware heat; kaolin keeps it suspended.
About three seconds in a well-mixed bucket lays an even coat — the studio default for speed and consistency. Tongs, a steady wrist, and let the gloss go off before moving.
Brushing glazes in two to three thin coats, each dry before the next. The slowest method but the most exact — for small ware, details, and studios without buckets.
Roll glaze around the inside and pour it out; sweep the rest over the outside. The way to line jugs, vases and anything a bucket will not swallow.
A spray gun builds thin, controllable layers and gradients no dip can make. Booth and respirator are mandatory — airborne glaze is silica dust.
A glaze is food-safe as a fired system, not as a jar label: well-fitted (no crazing over a porous body), free of leachable heavy metals such as lead, cadmium and barium, and tested on the exact clay body it lives on. When in doubt, line food surfaces with a proven commercial liner glaze. Full guide: food safety, testing & law →
Cone and temperature ranges are typical; recipes shift them. Fire vertical test tiles of every glaze on every body you use — so running shows before it reaches a shelf.
Glaze materials are dry powders — silica dust harms lungs. Weigh and mix wearing a respirator, and wipe surfaces wet, never dry.
Recipes are parts by dry weight and classic, freely shared starting points — not tested products: sieve, fire test tiles on your own clay body, and add about 2 % bentonite to keep the slurry suspended.