How does glazing pottery actually work?

Glaze is a thin layer of glass melted onto the pot, not a paint. Why it changes colour completely in the kiln, what the temperature rating means, and the food-safety question people should be asking.

Difficulty: intermediate
Time: 30 min
Read: 5 min
Safety: warning

Short answer

A glaze is a mixture of glass-forming minerals suspended in water. You apply it to a porous bisque-fired pot, the water soaks in leaving a powder layer, and the second firing melts that layer into glass. The colour in the bucket tells you almost nothing about the colour it will be — only a test tile fired on your clay at your temperature does.

Do this first

How do I get my pottery fired if I do not own a kiln?

Glazing is the stage where potters lose work they were pleased with, because it is the least intuitive part of the process. Nothing about a glaze in the bucket predicts the result, the same glaze behaves differently on different clays, and the whole thing is decided by a firing you do not watch.

Safety

Raw glaze materials are fine powders, and many contain silica; some contain metal oxides including barium, lithium, chrome and, in older or imported glazes, lead. Never mix dry glaze powder without a fitted respirator and never in a kitchen. Do not eat, drink or smoke while glazing, and wash hands thoroughly. Food safety of a finished glaze depends on the specific glaze fired to the correct temperature on the correct clay — assume a glaze is not food safe unless the supplier states that it is and you have fired it as specified.

Step by step

Understand what a glaze is.Finely ground minerals — a glass former, a flux to lower its melting point, and a stiffener — suspended in water, with colouring oxides added. Fired to the right temperature the mixture melts and fuses to the clay as a thin layer of glass.
Note that it only goes on bisque-fired work.The first firing turns the clay into a hard porous ceramic. That porosity is what pulls the water out of the glaze and leaves an even powder layer behind. Glaze applied to raw dry clay usually falls off or causes the piece to fail.
Match the glaze temperature to the clay and the kiln.Every glaze has a firing range, and so does every clay. A glaze fired too cool does not melt properly and stays rough and unsafe; fired too hot it runs off the pot onto the kiln shelf, which is the fastest way to be banned from a studio.
Clean the bisque first.Dust, grease and even fingerprints stop glaze adhering evenly and cause crawling, where the glaze pulls back into islands leaving bare patches. Wipe with a damp sponge and let it dry before glazing.
Apply by dipping, pouring or brushing.Dipping gives the most even coat and needs a full bucket. Pouring suits insides and large pieces. Brushing is what most beginners use with commercial brush-on glazes and needs three coats in alternating directions to avoid streaks.
Get the thickness right.Too thin and the colour is weak and the surface may not be sealed. Too thick and it crawls, blisters or runs. Commercial glazes state a number of coats — follow it, because it is the manufacturer telling you their thickness.
Keep glaze off the foot.Anything glazed that touches the kiln shelf fuses to it permanently. Wipe a clean band around the base with a damp sponge, and check it twice. This is the rule studios care about most.
Test on tiles before committing a pot.A small tile of the same clay, glazed the same way, fired in the same kiln. Glaze colours in the bucket are usually nothing like the fired result, and combinations of two glazes are wholly unpredictable. There is no substitute for a test tile.
Ask explicitly about food safety.A glaze is food safe only as a combination of that glaze, that clay and a correct firing. Ask the supplier and the studio directly rather than assuming. Crazing — a fine network of cracks — makes a surface unsuitable for food even if the glaze itself is rated safe.

Tips

Keep a numbered tile for every glaze and every combination you try, with the details written on the back in underglaze pencil. Within a year it is the most valuable thing in your studio box.
Underglazes are applied before the bisque firing or on bisque and covered with a clear glaze, and they behave much more like paint — colour in the pot roughly matches colour on the pot. They are far easier to control than coloured glazes.
Wax resist painted on the foot before dipping keeps glaze off it cleanly, and is quicker and more reliable than sponging afterwards.

Common mistakes

Choosing a glaze by the colour of the liquid — Raw glaze is usually a pale grey or beige regardless of what it fires to, and a bright bucket often fires dull. Only a fired test tile tells you anything.
Glazing the foot, or too close to it — Molten glaze fuses the pot to the kiln shelf. The pot is destroyed removing it and the shelf may be too, which is why studios are strict about it.
Applying glaze over dusty or greasy bisque — It causes crawling — the glaze pulls away into islands during firing, leaving bare unglazed patches that cannot be repaired without a third firing.
Mixing dry glaze powder without a respirator — The airborne fraction contains silica and metal oxides, and it is the single most hazardous moment in the whole craft. Buy ready-mixed if you cannot mix safely.

If it doesn't work

The glaze pulled back into patches

Cause: Crawling — dust, grease or glaze applied too thickly — Fix: Clean bisque thoroughly before glazing and apply at the stated thickness. A crawled pot can sometimes be re-glazed and re-fired.

A fine network of cracks across the surface

Cause: Crazing — the glaze and the clay body shrink at different rates — Fix: It is a glaze-and-clay mismatch, sometimes deliberate as a decorative effect. Crazed ware is not suitable for food. Change to a glaze matched to that clay body.

Small sharp pinholes in the surface

Cause: Gas escaping from the clay through the melting glaze — Fix: Fire the bisque a little higher, apply glaze slightly thinner, and slow the cooling. Ask the studio, who will know their kiln.

The glaze ran down and stuck to the shelf

Cause: Fired above the glaze range, or applied far too thickly — Fix: Check the glaze and clay temperature ratings match the firing, and thin the application. Apologise, and expect to pay for the shelf.

The surface is rough and dry

Cause: Under-fired, or a glaze fired below its range — Fix: Refire at the correct temperature. An under-fired glaze is not sealed and is not food safe.

Questions people ask

Can I glaze at home and get it fired at a studio?

Often yes, and it is a common arrangement, but studios differ. Some insist you use their glazes because they know how those behave in their kiln and on their shelves. Ask before buying glaze.

What does the temperature rating mean?

Glazes and clays are sold with a firing range, often expressed as a cone number as well as a temperature. Both the clay and the glaze must be within the range the kiln is fired to, or one of them will fail.

Is a home-glazed mug safe to drink from?

Only if the glaze is stated food safe by its maker, fired to the specified temperature on a suitable clay, and the surface is smooth and uncrazed. If any of those is uncertain, treat the piece as decorative.