Test Tiles: The Cheapest Firing You Will Ever Do

Every glaze decision you make on a real pot is a gamble unless you have already run it past a test tile. Roll a slab about 6mm thick, cut it into tiles roughly 5cm by 7cm, and stamp or incise a code into the clay while it is still soft. A hole near the top lets you wire tiles to a kiln shelf so they do not wander. Bisque them alongside your pots, then treat each tile exactly as you would treat a pot: same dipping time, same number of brush coats, same position in the kiln.

Make two or three versions of each test. A flat tile shows how a glaze pools and settles. A vertical tile shows runs, drips and the way a glaze thins over an edge. A textured tile — pressed with coarse linen, or given a few sprigs and scored lines — reveals breaking, where a glaze thins to a paler tone over high points and darkens in hollows. That breaking behaviour is often the most beautiful thing about a glaze, and it is invisible on a smooth, flat surface.

Colour Comes From Chemistry, Not From the Pot

A glaze is a base recipe — silica, alumina and a flux such as feldspar, whiting, zinc or a leadless frit — plus colourants. The base decides whether the surface is glossy, satin or matt. The colourant decides the hue, and small amounts go a very long way.

• Cobalt at around 0.5 per cent gives a strong blue; 2 per cent is close to navy.
• Copper at 2 to 5 per cent reads green in oxidation, but can flash oxblood red in reduction.
• Iron from 1 to 10 per cent moves through honey, amber, rust and near-black tenmoku.
• Rutile and titanium produce creamy speckles and streaky beige, especially over texture.
• Chrome-tin blends give reliable pinks and maroons; tin or zircon opacify a clear base.

Commercial stains are more predictable than raw oxides and are usually added at 5 to 10 per cent. Layering is where the real fun starts, but order matters: glaze A over B is not the same as B over A, because the lower layer fluxes the upper one differently. Write down which went on first.

Thickness Is the Variable You Control Most

Most glaze faults are thickness faults. As a rough guide, aim for about 1mm of dry glaze. Under 0.5mm and the surface looks thin, patchy and dry. Over 1.5mm and you invite running, pinholing and crawling.

Get into the habit of measuring rather than guessing. Dip for a counted three seconds and note it. Three brushed coats of a glaze mixed to single cream is roughly equivalent to one dip. Check specific gravity with a hydrometer — most dipping glazes sit around 1.40 to 1.50 — because a glaze that has stood open for a week will be thicker than one you mixed yesterday, and your three-second dip will lay down far more material than it did last month. If a glaze has settled, sieve it, adjust the water, and retest before committing a shelf of pots.

Heat Work, Soak and Cooling

The same glaze at two different temperatures is two different glazes. Earthenware firings around 1000 to 1080°C leave copper bright and iron orange. Stoneware firings at 1220 to 1260°C mature the glaze properly, deepen iron to brown-black and can turn a cheerful copper green into something duller and more olive.

A soak of 10 to 20 minutes at top temperature evens out the melt and helps heal pinholes. Cooling matters just as much: a slow, controlled cool encourages crystal growth and matt, speckled surfaces, while a fast cool keeps glazes glossy and colours brighter. Atmosphere counts too. Electric kilns fire in oxidation and give repeatable, clean colour. Gas reduction shifts copper towards red and can make iron speckle and blush, but it is far harder to repeat exactly. Note where each tile sat in the kiln — top shelf, bottom, near an element — because position alone can change a result.

Reading the Surface: Common Faults

• Pinholing: usually a thick coat, a dusty bisque or too fast a firing. Sponge and dust every pot before glazing, and slow the last hour.
• Crawling: glaze pulling back into islands. Often too thick, or applied over an oily, dusty or burnished surface.
• Crazing: a fine web of cracks, caused by the glaze shrinking more than the clay body. Common on earthenware; check fit with a test tile before blaming the recipe.
• Running: a glaze that flows off the pot. Sit it on a waster tile or stilts, and thin the application next time.

Notes That Actually Repay You

A glaze notebook is only useful if it records the things that change. For each test tile, log the glaze name and batch date, specific gravity, application method and time or number of coats, the clay body, the firing programme with top temperature and soak, the kiln position, and a photograph taken in daylight in the same spot every time. Keep tiles in a labelled box by code, and stamp the matching code into the foot of every pot before it is bisque fired, so a successful result six months from now can be traced back to a single tile. Then it is not luck — it is a recipe you can repeat.