Glazing Pottery Without Common Firing Faults

Reading the fault: crawling, pinholing and shivering

There is nothing quite like opening a kiln after a glaze firing. The moment of promise, the scent of hot clay, the gentle tick of cooling metal. And then you see it: a patch where the glaze has pulled back like a nervous tide, or a surface speckled with tiny craters, or a sharp flake lifting from a rim. These three faults — crawling, pinholing and shivering — are the most common reasons a glaze result disappoints. The good news is that each one leaves clues, and most can be corrected by adjusting application thickness and kiln schedules rather than abandoning a favourite recipe.

Crawling: when glaze pulls away from the clay

Crawling looks like bald patches where the glaze has retracted during firing, leaving bare clay or a thin, uneven film. It happens when the molten glaze cannot wet the clay body properly, so surface tension pulls it into beads. The usual culprits are a dusty or greasy bisque surface, glaze applied too thickly, or a glaze that has been over-sieved and lost its finer particles.

Start with the bisque. Wipe every pot with a damp sponge or a soft brush under running water, then let it dry completely. Oily fingerprints from handling are enough to cause crawling, so wear clean cotton gloves when glazing. Check your glaze’s specific gravity — aim for 1.40 to 1.50 for most dipping glazes. If it is too thick, add water slowly and stir thoroughly. Sieve to 80 mesh, not finer, as over-sieving removes the coarser particles that help the glaze bond.

• Application: dip for two to three seconds, then allow the glaze to dry before the next coat. Two thin coats are far safer than one thick one.
• Kiln schedule: fire slowly through the early stages — 100°C per hour up to 600°C — so any residual moisture and organic matter escape gently.
• Soak: a 15-minute soak at top temperature lets the glaze flow and heal small breaks.

Pinholing: tiny craters with big causes

Pinholing is the appearance of small, round holes in an otherwise smooth glaze surface. Sometimes they are mere dimples; sometimes they expose the clay beneath. The cause is gas or vapour escaping through the glaze while it is still molten. That gas can come from the clay body, from the glaze itself, or from the kiln atmosphere.

Bisque firing is your first line of defence. Fire to 1000°C with a 20-minute soak to burn out carbon and sulphur. If you bisque lower, say 950°C, organic matter may still be present and will burn through the glaze later. Glaze thickness matters too: a heavy application traps gases. Aim for 0.3–0.5 mm — roughly the thickness of a postcard. You can check with a needle tool: scratch a line through the dry glaze and look at the depth.

During the glaze firing, use a drop-and-hold schedule. After reaching top temperature, drop the kiln by 30–50°C and hold for 15–20 minutes. This reduces the glaze’s viscosity just enough to let bubbles rise and burst, then smooth over. Avoid firing too fast between 800°C and 1000°C, as this is when most carbonates release gas.

• Bisque: 1000°C with a soak.
• Glaze thickness: 0.3–0.5 mm.
• Top soak: 15–30 minutes at peak, then a drop-and-hold.

Shivering: when the glaze is too tight for the clay

Shivering is the opposite of crazing. Instead of a network of fine cracks, the glaze lifts away from the clay in sharp flakes, often at rims, edges or thick corners. It happens when the glaze is under too much compression — its thermal expansion is lower than that of the clay body, so as the piece cools, the body contracts more and squeezes the glaze until it pops off.

The fix is rarely about application alone, but thickness makes it worse. A very thick glaze on a thin rim is almost guaranteed to shiver. Keep glaze coverage even and avoid pooling. If shivering persists, you need to adjust the glaze recipe: increase its thermal expansion by adding more flux (such as a frit or feldspar) or reducing silica. Alternatively, choose a clay body with a higher expansion. Always test on small tiles before committing a whole kiln load.

Cooling rate also plays a role. A slow, controlled cool — 60–100°C per hour down to 600°C — gives the glaze and body time to accommodate each other. Opening the kiln early, while the ware is still above 200°C, can shock the glaze and cause shivering or dunting.

Application thickness: the quiet variable

Most glaze faults come down to thickness. Too thin and you get a dry, patchy surface; too thick and you invite crawling, pinholing and shivering. Invest in a hydrometer and a simple glaze thickness gauge, or make a scratch test with a needle. For dipping, keep the glaze at 1.40–1.50 specific gravity. For spraying, build up thin passes rather than one heavy coat. For pouring, work quickly and evenly.

Remember that different glazes behave differently. A matt glaze may need a thinner application than a glossy one. A glaze high in clay (like a shino) can tolerate more thickness because it shrinks less on drying. Keep a notebook: record specific gravity, dip time, and the resulting thickness for every glaze you use. After a few firings, you will have a personal chart that removes the guesswork.

Kiln schedules that let glazes settle

A reliable kiln schedule is your quiet ally. Here is a practical starting point for a mid-range stoneware glaze firing (cone 6–7, around 1220–1240°C):

• Ramp 1: 100°C per hour to 600°C.
• Ramp 2: 150°C per hour to 1000°C.
• Ramp 3: 100°C per hour to top temperature.
• Soak: 20 minutes at top.
• Drop and hold: reduce by 40°C and hold for 15 minutes.
• Cooling: controlled at 80°C per hour to 600°C, then natural cooling.

Adjust these figures to suit your clay and glaze, but keep the principles: slow early, soak at peak, and cool gently. If you are firing a kiln with a manual controller, use cones to verify the heat work. A little patience here saves a lot of chiselling later.

Glazing is a conversation between materials and heat. Listen to the faults, adjust one variable at a time, and your pots will reward you with surfaces that are as dependable as they are beautiful.