Why Bother Mixing Your Own?

There is nothing wrong with a tub of ready-made glaze. It is reliable, it is quick, and it gets you from greenware to the kiln shelf without a headache. But at some point most potters look at their shelf of finished pots and think: I want a surface nobody else has. That is the moment raw materials start to look interesting.

Mixing your own glazes is a bit like cooking from scratch. You learn what each ingredient does, you can adjust a recipe to suit your clay body and your firing, and you can make twenty litres for the price of a small jar of commercial glaze. It is also a genuinely absorbing craft in its own right — half chemistry, half intuition.

What Actually Goes Into a Glaze

Almost every glaze is a combination of three things: something that forms glass, something that melts it, and something that keeps it in check.

• Glass formers: silica (flint) is the backbone. It gives a durable, hard surface but melts at a temperature far above most studio kilns, so it needs help.
• Fluxes: these lower the melting point. Feldspar, whiting (calcium carbonate), talc, zinc oxide and frits all do this job. Frits are pre-melted, ground glass — they are useful because they are insoluble, predictable, and do not release soluble nasties into your bucket.
• Stabilisers: alumina, usually brought in by china clay or kaolin, stops the glaze running off the pot and gives it resistance to scratching and attack by acids.

Most fritted, lead-free recipes sold by UK suppliers fall into this structure. A simple stoneware base might be roughly 40 parts frit, 25 parts feldspar, 20 parts silica, 10 parts china clay and 5 parts whiting. Every one of those numbers is a lever you can pull later.

Colour, Opacity and Texture

Once you have a base that melts nicely at your firing temperature, colour comes from metal oxides and carbonates, added in small percentages by dry weight.

• Cobalt carbonate: 0.5–2% gives a strong blue. A little goes a very long way.
• Copper oxide or carbonate: 1–3% for greens, turquoise in alkaline glazes, and a deep blood red in reduction.
• Iron oxide: 2–8% for ambers, rusts and tenmoku browns.
• Rutile: 3–6% for that lovely streaky, mottled quality where titanium meets iron.
• Opacifiers: tin oxide or zirconium silicate at 5–10% to turn a transparent glaze milky.

Start with single-colour tests before you get ambitious. Two oxides in one bucket can surprise you, and not always pleasantly.

Weighing, Sieving and Storing

Accuracy matters more than you might expect. A set of scales reading to 0.1g is essential, and a 100g dry batch makes a sensible test size. Weigh the dry materials into a lidded container, add water gradually to roughly the consistency of single cream, then sieve the whole lot through an 80 or 100 mesh sieve — twice is not overkill. This breaks up lumps and removes the coarse particles that cause speckles and pinholes.

Aim for a specific gravity of about 1.40 to 1.50 for dipping. You can check this cheaply with a 100ml syringe: weigh 100ml of well-mixed glaze, and if it comes out around 145g you are in the right zone. If the glaze settles hard in the bucket, add 1–2% bentonite to the dry recipe, or a few drops of a flocculant, to keep it suspended.

Label everything with the recipe, the date and the batch number. And please take dust seriously: oxides and frits are respiratory hazards. Wear a proper mask when weighing powders, work in a ventilated space, and never eat or drink in the glazing area.

Testing on Tiles Before You Commit

This is the part people skip, and it is the part that saves pots. Before a drop of new glaze touches a piece you care about, make test tiles from the same clay body, rolled thin and stamped with a recipe code. Fire them on the same schedule, at the same temperature, in the same part of the kiln.

Judge each tile on a few specific things. Does the surface melt smoothly, or is it dry and underfired? Has it crazed — fine cracks across the surface — or shivered, where it has flaked away at the edges? Both mean the glaze does not fit the clay, and you will need to adjust the silica or flux. Are there pinholes, blisters or crawling? Check whether the glaze runs onto the shelf, and if so, reduce the flux or add alumina.

For anything intended for food or drink, use only known lead-free materials and stick to recipes you can trust for durability — and have finished ware tested properly if it is for sale. A simple line blend, mixing two recipes in ten steps, will tell you more in one firing than a month of guessing.

Keep a notebook, or at least a scruffy folder of tile cards. The recipe you nearly threw away is often the one you want three years later.