Pottery glaze, from a ready-mixed tub to a raw materials shelf

Printed prices, verified

Four questions before you buy glaze

1. What body and what temperature?. A glaze is matched to a clay body and a firing range, not chosen from a photograph. A stoneware glaze on an earthenware body, or either fired outside its range, gives crazing, crawling or a run onto the shelf.
2. Ready mixed or from raw materials?. Ready mixed costs more per litre and removes every variable. Raw materials cost less, scale to a bucket, and require scales, a respirator and a test tile discipline. Nobody should start with raw materials and nobody making a lot of work stays away from them.
3. Will it touch food?. Then it is a specific question about a specific glaze and a specific firing schedule, answered by the manufacturer's data sheet for that product. This site quotes those sheets and never generalises them, because ware that touches food is regulated and the claim belongs to whoever tested it.
4. Have you made a test tile?. On your body, in your kiln, to your schedule. A glaze behaves differently on a different clay and in a different firing, and a tile costs a few pence against a kiln load.

What is in a glaze, in four words

Glass former, flux, stiffener, colour. Silica is the glass; a flux such as a frit, a feldspar or whiting lowers its melting point into the range a kiln can reach; alumina, usually from china clay, stops the melt running off the pot; and an oxide or a stain provides colour. Almost every recipe you will read is those four jobs shared between five or six materials.

Understanding that is what turns a list of ingredients into something you can adjust. A glaze that runs needs more stiffener; one that is dry and underfired needs more flux or a hotter firing.

The colouring oxides, and how little you need

Cobalt is the strongest blue in ceramics and is used at fractions of a percent; copper gives greens in oxidation and reds in reduction; iron does everything from pale celadon to near black depending on quantity and atmosphere; rutile and ilmenite give the broken, variegated surfaces that sell pots.

These are sold in small quantities for a reason. A hundred grams of cobalt carbonate colours a great deal of glaze, and the cost per bucket is far lower than the price on the jar suggests.

Mixing safely is the whole of the safety advice

Every raw glaze material is a powder, several are silica, and some are toxic in ways that matter at the weighing stage rather than after firing: barium carbonate and the manganese and nickel compounds are handled with care and a fitted respirator.

Weigh in a ventilated space, never dry-sweep a glaze bench, keep the buckets labelled with the recipe and the date, and wash your hands before you eat. That is the entire routine, it costs nothing, and it is why this page states it rather than assuming it.

Common questions

Is pottery glaze food safe?

That is a property of a specific glaze fired to a specific schedule, and the answer is on the manufacturer's data sheet for that product. This site quotes those sheets where they exist and never describes a glaze as food safe on its own authority.

Why did my glaze craze?

Craze lines mean the glaze is shrinking more than the body underneath as it cools, so the fit is wrong. It is usually solved by changing the glaze or the body rather than the firing.

Can I use a stoneware glaze on earthenware?

Not usefully. It will not melt at earthenware temperature. Match the glaze to the range printed on the clay.

How much glaze do I need?

A bucket of dipping glaze goes a long way and keeps indefinitely if it is covered and stirred. Buy small for testing, then mix in quantity once a recipe is proven on your body.

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