Why mix your own clear glaze?

A clear glaze is the quiet workhorse of a home studio. It sits over slips, oxides, stains and sgraffito, and it either lets your decoration sing or smothers it under a milky, crazed film. Buying a ready-made clear is perfectly sensible, but mixing your own gives you control over fit, finish and firing range — and it works out far cheaper once you are glazing a kiln load every few weeks. The catch is that a clear glaze has nowhere to hide. Every lump, every miscalculation and every mismatch with your clay body shows up immediately.

The good news is that reliability comes from process rather than talent. Weigh carefully, sieve properly, test on tiles, and you will end up with a glaze you can trust batch after batch.

Start with a simple, well-tested recipe

Resist the temptation to invent a recipe from scratch. Begin with something straightforward and adjust it slowly. A classic mid-range clear for stoneware, fired around 1200–1240°C, looks roughly like this by dry weight:

• Boron frit — 70 parts
• Kaolin (china clay) — 15 parts
• Silica (quartz) — 15 parts

That gives a hard, glossy, reasonably durable surface with a decent melting range. For earthenware fired nearer 1080–1120°C you will need more flux, usually in the form of additional frit or a small amount of a boron source. Frits vary considerably between suppliers, so keep a note of the exact one you use — swapping brands can shift the melting point enough to ruin a glaze that used to work perfectly.

You can add 0.5–1% CMC gum if you brush rather than dip, and a few drops of saturated Epsom salt solution if the slurry settles out hard.

Weighing and measuring decide everything

Use digital scales that read to 0.1g for a 1000g batch, and a finer 0.01g scale for anything under 1%. Measure each material into its own container, or weigh directly into a tared bucket, working from the largest quantity down to the smallest. Tiny additions vanish into a bucket full of frit if you are careless, and 0.5% of the wrong thing can cloud a clear glaze completely.

Record every batch in a notebook: date, recipe, supplier, batch number, water added. If a glaze misbehaves three months later, that record is the only way back to the version that worked.

Wear a dust mask when handling raw materials, especially silica and any metallic oxides. Mix in a well-ventilated space, and never eat or drink in the glazing area.

Sieving, water and specific gravity

Add water gradually — roughly 40–45 parts water to 100 parts dry material is a sensible starting point. Stir to a smooth slurry, then pass it through an 80-mesh sieve, and ideally again through a 200-mesh sieve. This is not fussiness: unsieved lumps create specks, crawling and pinholing, and no amount of careful application will fix them.

Let the glaze slake overnight, then remix and check its specific gravity, aiming for around 1.40–1.45 for dipping. Weigh 100ml in a syringe or measuring cylinder: 140–145g means you are in the right zone. Too thick and the glaze goes on lumpy and crawls; too thin and it dries patchy and pale. Adjust with small amounts of water or dry mix, never in a panic.

Test on tiles before the kiln load

Make eight to ten test tiles from the same clay body you actually throw with — press them from a slab so they have both flat and vertical faces. Number each one on the back with an oxide or a pencil. Apply glaze at three thicknesses: one coat, two coats, three coats. Include tiles with underglaze, slip, oxide washes and commercial stains so you can see how the clear behaves over decoration.

Fire the tiles in the positions they will occupy in a real load, and use the same firing schedule, including any soak or controlled cooling. A clear that crazes on a slow-cooled tile will almost certainly craze on a pot.

Reading the results and adjusting

Hold each tile under a strong light. Draw over the surface with a permanent marker, wipe it off, and look for ink sitting in fine cracks — that is crazing. Check edges for shivering, the surface for pinholing, and the colour of your decoration through the glaze.

Then change only one thing at a time:

• Crazing: increase silica slightly, reduce flux, or slow the cooling through 600–500°C.
• Cloudiness: often too thick an application, underfiring, or an excess of zinc or tin in the recipe.
• Pinholing: glaze too thick, dusty bisque, or a soak that is too short at top temperature.
• Crawling: thick application, greasy or dusty bisque, or inadequate sieving.

Retest after every adjustment. Two or three rounds of this and you will have a clear glaze that behaves the same way every time — which is, in the end, all any of us want from a glaze.