Why Mix Your Own Glaze?

Buying a ready-made glaze is easy, but mixing your own gives you control over colour, texture and, crucially, what is actually in the pot your dinner sits in. Most commercial glazes are perfectly safe, but once you understand the basics you can build a palette that is entirely your own. The catch is that a glaze which looks beautiful can still release metals into food if it is badly formulated or underfired. That is not a reason to avoid it — it is a reason to be methodical.

What Makes a Glaze Safe to Eat From?

A food-safe glaze is one that is chemically stable after firing, does not craze into a network of fine cracks, and does not release harmful amounts of metal into acidic food. Three things govern this:

• A durable glass structure. Plenty of silica and alumina, with enough flux to melt at your firing temperature but not so much that the glaze stays soft and soluble.
• Safe colorants. Iron oxide, tin oxide, zirconium, cobalt and small amounts of copper are fine at typical levels. Never use lead in any form, and avoid barium carbonate, which is toxic and leaches. If you have inherited a mystery bucket of glaze from a car boot sale, do not put it on tableware.
• Full firing to maturity. An underfired glaze is porous, weak and far more likely to give up its ingredients to a lemon wedge.

Boron, usually added as a frit rather than as borax, is the workhorse of mid-range stoneware glazes because it melts well without demanding extreme temperatures.

A Simple Starter Recipe

This is a clear, glossy mid-range base, suited to stoneware fired around cone 6 — roughly 1220°C. Work in parts by weight:

• Potash feldspar — 30
• Boron frit — 25
• Quartz (silica) — 20
• Whiting (calcium carbonate) — 12
• China clay (kaolin) — 10
• Zinc oxide — 3

Take that total of 100 and add water to make a slurry about the consistency of single cream. Roughly 1.4 to 1.5 litres per 2 kg of dry material is a sensible starting point; adjust until it coats the back of a spoon thinly. Add colour slowly: 1–2% iron oxide gives a soft amber, 2–4% tin oxide opacifies to a milky white, and a whisker of cobalt at 0.2–0.5% produces blue. Record every addition, along with the date.

Mixing Safely: Dust Is the Enemy

Raw glaze materials are fine powders, and the real risk is inhalation. Silica dust is a serious respiratory hazard over years of exposure, so treat it with respect.

• Wear an FFP3 mask and nitrile gloves whenever you handle dry materials, and mix in a well-ventilated space — outdoors or with proper extraction.
• Never sweep up spills. Wipe them with a damp cloth and rinse that cloth outside.
• Keep dedicated buckets, sieves, scales, spoons and a whisk. They should never go back into the kitchen.
• Label every container with the recipe, the date and any colourant added. Store dry materials in sealed tubs, out of reach of children and pets.
• Sieve the mixed glaze through an 80 to 100 mesh sieve to remove lumps, and stir thoroughly before each use.

Testing on Tiles Before You Commit

Never glaze a finished pot with an untested recipe. Make up a batch of flat test tiles from your usual clay body — about 5 cm by 8 cm, with a small hole so you can thread them onto a wire — and glaze each with a different thickness or colour variation. Fire them in the same kiln you will use for your pots.

Once fired, run these checks:

• Acid test. Leave a wedge of lemon and a small pool of vinegar on the surface, covered, for 24 hours. Wipe clean and look for dulling, pitting or a shift in colour.
• Crazing test. Rub a washable marker across the glaze and wipe it off after a minute. Ink that lingers in fine lines means the glaze is crazed and not food-safe.
• Thermal shock. Pour boiling water into a glazed bowl and watch for cracks appearing.
• Scratch test. Drag a metal knife across the surface. A durable glaze resists shallow marking.
• Dishwasher test. Run a tile through a few cycles and check for dulling or blistering.

Only when a recipe passes every one of these should you mix it in larger quantities and commit it to your best bowls and mugs.

Applying and Firing Your Glaze

Aim for a glaze layer about 1 mm thick — thick enough to be opaque and glassy, thin enough not to run. Dip quickly, or pour over the piece, and wipe the foot ring clean with a damp sponge before firing. Leave the base unglazed and remove any glaze from the kiln shelf contact points. Fire slowly to the temperature your clay body and recipe were designed for; the difference between cone 5 and cone 6 can be the difference between a durable surface and a leaching one.

Keep a notebook of every glaze you mix, with the recipe, firing schedule, results and any changes you made. A glaze that works in your kiln, at your firing, on your clay is worth far more than any recipe borrowed from a book — and once you have two or three reliable food-safe bases, the whole palette opens up.