10 Common Glaze Mistakes and How to Avoid Them

Glazing is where a good pot can turn disappointing in a single firing. The crawling, pinholing, and patchy colour that show up on the kiln shelf usually began hours earlier—at the bucket, the banding wheel, or the damp cupboard. Glaze faults are rarely mysterious. They follow patterns, and once you know what to look for, most can be prevented before the lid goes down. The ten faults below are the ones potters ask about most.

Crawling: When the Glaze Pulls Back

Crawling looks like bare islands of clay surrounded by glaze with rounded, retreated edges. The glaze has pulled away from the body during firing, often because it never adhered properly in the first place or because it shrank too much as it melted.

The usual culprits are dust on the bisque, oil from fingers, a glaze applied too thickly, or a recipe that is short on clay and long on materials that shrink. I have seen crawling appear only on the rims of pots that were handled after bisque firing—hand cream and fingerprints are enough.

Prevention is mostly preparation:

• Wipe bisque with a damp sponge before glazing and let it dry.
• Handle pots by the foot or wear clean cotton gloves.
• Sieve glaze before use and stir thoroughly.
• Apply two or three thin coats rather than one heavy one, letting each dry.
• If crawling persists, check the glaze recipe with a test tile—adding a small amount of clay or a gum may help adhesion, but change one thing at a time.

Firing slowly through the early stages also gives the glaze time to settle and the body time to release gas. A fast climb can encourage crawling on tricky shapes.

Pinholing and Blistering: Gas on Its Way Out

Pinholes are small craters; blisters are larger bubbles that have burst or frozen on the surface. Both mean gas escaped through the glaze at the wrong moment.

Common causes include glaze applied too thickly, bisque fired too low so organic material burns off during the glaze firing, firing too fast, dust or debris on the bisque, and clay bodies with high sulfur content or plenty of organics.

What helps:

• Bisque to the cone your clay needs, and hold at the top for a short soak.
• Keep glaze coats thin and even.
• Slow the firing between roughly 800°C and 1000°C, where much of the gas is released.
• Add a short hold at top temperature so bubbles can smooth out.
• Clean bisque and sieve glaze.

If pinholing appears only on one part of the kiln, suspect firing rather than the bucket.

Uneven Coverage: Patchy, Streaky, and Thin in the Wrong Places

Uneven glaze is easy to spot after firing: lighter patches, visible brush marks, or a surface that feels thin and dry. The cause is usually inconsistent application, not a bad glaze.

Dipping is fast but unforgiving. If the glaze bucket is settled, the bottom of the pot gets a different mix than the top. Stir before every dip, and check the thickness with a hydrometer if you use one. Many potters aim for a specific gravity that has worked before, then adjust with water in small amounts.

A few habits make coverage more reliable:

• Wet the inside of the pot first, or dip quickly and evenly.
• Rotate the pot as you lift it, and let it drain over the bucket.
• Mark a dipping depth with tape on a stick so each pot goes in the same amount.
• For spraying, keep the gun moving and overlap passes by half.
• For brushing, use a soft brush and crisscross strokes.

Thin glaze often fires paler and rougher; thick glaze may run or craze. Aim for the middle.

Crazing and Shivering: When the Fit Is Wrong

Crazing is a network of fine cracks across the glaze. Shivering is the opposite: glaze flakes away, sometimes in sharp slivers. Both point to a mismatch between glaze and clay body.

Crazing happens when the glaze contracts more than the body as they cool. Thick glaze makes it worse. Fast cooling can also trigger it. Shivering happens when the glaze contracts less than the body, putting the glaze under compression until it pops off.

You cannot fix fit by wishful thinking. Make test tiles from the same clay you throw, apply the glaze at the thickness you intend to use, and fire them the way you fire your work. If crazing appears, try a thinner application and a slower cool. If it persists, adjust the glaze recipe—usually by increasing silica or reducing materials that raise expansion—or choose a different glaze.

For functional ware, take crazing seriously. Cracks can harbour moisture and bacteria, especially on cups and bowls. A crazed glaze is not always unsafe, but it is a warning sign worth addressing.

Runs, Drips, and Stuck Pots

A run is glaze that flows too far; a drip is a blob that lands on the shelf. Both usually come from too much glaze near the foot or a recipe that is very fluid at top temperature.

Simple prevention:

• Leave a clean margin at the foot—at least a few millimetres.
• Wax the foot ring and wipe any glaze off before firing.
• Use stilts or a waster slab if you are testing a runny glaze.
• Put catch basins under pots when you are unsure.
• Reduce the top temperature slightly or shorten the hold for glazes that move.

It is tempting to glaze right to the edge to avoid a bare foot, but a stuck pot costs more time than a wiped foot ring. Keep a dedicated shelf for tests.

Colour and Surface Surprises: Reading the Kiln, Not the Bucket

A glaze that looked right on a test tile can fire pale, dark, glossy, or matte in a different kiln. Thickness, atmosphere, and firing schedule all change the result. A thin coat of a tenmoku may be amber; a thick coat may be nearly black. Layering two glazes can produce a third colour entirely.

Record what you do. Note the clay, glaze, number of coats, firing cone, and position in the kiln. Keep a test tile for every change. If a colour disappoints, fire a small tile at the same thickness before blaming the recipe. Kiln position matters too—top shelves often run

Photo: anaterate / Pixabay