Most glaze faults look dramatic and mysterious. In practice, they come down to a few repeated slips: a dusty rim, a glaze mixed too thin, a bucket that needed stirring, a kiln opened too early. Crawling, pinholing and shivering are the faults you will meet most often, and each one has a short, knowable list of causes. Work through them in order and you will fix most of what goes wrong in your studio.

Start with a clean bisque

A bisque-fired pot looks clean and feels dry, but it holds dust from sanding, grit from the shelf and oil from your hands. Glaze cannot grip any of that. Wipe each pot with a damp sponge or rinse it under the tap, then let it dry completely before glazing — glazing a damp pot dilutes the glaze from underneath and gives you a patchy coat.

Grease is harder to see and just as damaging. Hand cream and cooking oil repel glaze in the same way dust does. Wear clean gloves, and once a pot is glazed, hold it by the foot. A thumbprint in a dry glaze layer will crawl open in the kiln and leave a bald patch you cannot repair.

Get the thickness right, then keep it consistent

Every glaze has a thickness it likes. Too thin and it fires dry, matt and scratchy, or lets the clay show through at the edges. Too thick and it runs onto the shelf, pools in the base of the pot, crawls or shivers off the rim. As a rough guide, aim for a dry coat of about 0.3–0.5mm — thin enough that you can still see the texture of the pot through it.

A glaze hydrometer is a worthwhile buy. Many dipping glazes work well at a specific gravity near 1.4–1.5, but your recipe's own notes matter more than any general figure, so test, record and adjust to suit your clay. Consistency counts for more than the exact number: dipping for the same count of seconds every time is what makes a glaze repeatable.

Dip, pour or spray — but pick one

Mixing application methods on the same pot is a reliable way to produce crawling and patchy colour. If you dip the inside and pour the outside, do it the same way on every pot in the batch, and keep the pour moving so the glaze does not build up in one place.

Keep the bucket in order

Short habits, big effect:

• Stir before every session. Heavier materials settle within hours. The first pot dipped in a settled bucket gets a watery coat; the last gets a lump of flint.
• Sieve regularly. An 80-mesh sieve catches dried flakes, lumps and stray grit.
• Wipe the rim. Dried glaze falling back into the bucket turns into lumps and pinholes.
• Label everything. Two similar-looking browns can fire completely differently.

Crawling: when the glaze pulls back

Crawling leaves bare clay between islands of glaze, and it usually happens early in the firing. The usual suspects are a dusty or greasy surface under the glaze, a coat applied too thickly, a glaze dried too fast under a fan or in a draught, or glaze applied over wax resist, shellac or a filled crack. Recipes with a high proportion of raw clay are also prone to it, because the clay shrinks as the coat dries and tears the layer apart.

Thin the glaze slightly, wipe the bisque properly, and slow the first part of the firing so the glaze has a chance to sinter onto the pot before it hardens. A small addition of CMC gum can help a glaze cling to smooth or vertical surfaces.

Pinholing and blistering

Pinholes are small craters where a bubble has burst and the glaze has not healed over. Blisters are the same fault on a larger scale. Common causes are dust, organic material in the clay or glaze, a bisque fired too low so carbon is still burning out beneath the glaze, glaze applied too thickly, and a firing that ends before the surface has smoothed.

Try these in order:

1. Bisque fire a cone higher if pinholes keep appearing on the same clay.
2. Slow the ramp through the range where carbon burns out, so those gases escape before the glaze seals over.
3. Soak at the top temperature for ten to twenty minutes to let the glaze flow and heal.
4. Vacuum the kiln and dust the shelves. Debris from the last firing lands on the next batch.

Cooling slowly through the last few hundred degrees helps as well. A fast cool can freeze a bubble in place before the glaze has time to level.

Crazing and shivering: a question of fit

Crazing is the fine web of cracks that appears across a glaze. Shivering is the opposite: glaze flakes away from the clay, usually at rims, edges and the foot. Both are fit problems — the glaze and the clay body expand and contract at different rates as they cool.

You cannot fix either by glazing thinner or thicker; it is a chemistry issue. If you mix your own glazes, the answer usually lies in reducing the glaze's expansion for crazing, and increasing it for shivering. Test on small tiles made from your own clay before committing a whole batch, and change one thing at a time so you know what worked.

Some potters welcome crazing on raku and purely decorative pieces. On tableware it is a compromise: the cracks hold food and moisture, and a crazed surface is harder to keep clean. Shivering matters more. Those flakes are sharp, and they come off on hands and lips, so if a glaze shivers, do not sell the pot as functional ware. If you are unsure whether a glaze is suitable for food use, ask your supplier.

Firing and loading slips

A good glaze can be ruined in the last hour of work. Glaze that runs onto the shelf leaves a chipped foot and a messy shelf; kiln wash, a thin tile or stilts under the pot will contain the damage. Leave three to five millimetres of bare clay at the foot, more for runny glazes, and keep pots from touching each other.

Resist opening the kiln early. A sudden change in temperature can dunt a pot — you will usually hear the crack happen — so leave the lid shut until the kiln is properly cool. And check your cone. Plenty of faults blamed on a glaze are simply an under-fired kiln.

A short check before every firing

Most of the faults above disappear with five minutes of checking:

1. Bisque wiped, dried fully, and handled only with clean hands or gloves?
2. Coat even, roughly 0.3–0.5mm, with no heavy patches in the dips?
3. Bucket stirred, sieved and the rim wiped clean?
4. Foot free of glaze, and pots not touching anything?
5. Shelf washed, kiln dusted, correct cone in place?
6. Slow start, soak at the top, lid left shut until cool?

Keep a notebook beside the kiln. Record the glaze, the thickness, the firing and the result, and within a few batches you will know your own materials well enough to identify the cause of a fault almost before you take the pot out of the kiln.

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