Begin With the Fault, Not the Fix

Nothing takes the shine off unloading a kiln quite like finding a batch of cracked mugs or a glaze that looks like orange peel. The temptation is to change three things at once — a new glaze recipe, a longer soak, a slower ramp — and simply hope something sticks. Resist it. Almost every firing fault traces back to a small handful of causes, and if you change everything you learn nothing at all. Work through the possibilities one at a time, keep notes, and you will usually identify the culprit within two or three firings.

Before you touch the schedule, do a proper visual post-mortem. Where exactly is the crack? Is the blistering on the top shelf only, or throughout? Does one side of the kiln look under-fired? Those small observations narrow the field faster than any amount of guesswork, because they tell you whether you are chasing a materials problem, a loading problem, or a kiln that is quietly lying to you about its temperature.

Cracking and Dunting: Usually Thermal Shock

Cracks that appear during or after firing are most often the kiln telling you it moved too quickly through a critical temperature. Quartz inversion at around 573°C is the classic culprit, and it happens in both directions — on the way up and, crucially, on the way down. Many potters slow the climb through that band but then let the kiln cool naturally, which is precisely when hairline cracks appear along rims and bases. Slow the cooling through 600–500°C and you will prevent a great deal of heartbreak.

Other common causes worth ruling out:

• Trapped moisture in thick or uneven sections, especially on work that was not properly bone dry. A slow water-smoking stage up to 120°C, then a gentle climb, solves most of this.
• Poorly compressed joins or coils that were never properly welded, which open up as the clay shrinks.
• Cristobalite inversion around 220–230°C on cooling, a frequent cause of dunting in ware that has already been fired once and is being re-fired.
• Opening the kiln too early, particularly below 200°C, when the ware is still vulnerable.

Blistering, Pinholing and Bubbling

If the glaze surface is erupting rather than settling, the question is almost always about heat work in the final stretch. Blisters form when the glaze is molten but the gases beneath it are still escaping faster than the surface can heal. Push the top temperature too far and the glaze simply boils. Ramp too quickly through the last 100°C and you arrive at top temperature without giving the melt enough time to smooth over.

Practical fixes:

• Add a soak of 10–20 minutes at top temperature so bubbles can burst and the surface can heal.
• Try a drop-and-hold: fall 30–50°C after the peak, then hold for 15–30 minutes to encourage a smoother, more even finish.
• Check glaze thickness. Much beyond a millimetre on a vertical surface invites crawling and pinholes.
• Make sure bisque ware is dust-free and free of fingerprints. Oils and dust burn out messily and leave pinprick holes behind.

Uneven Results: Kiln, Load or Schedule?

When one pot looks glossy and another looks dry, resist blaming the glaze. A kiln packed too tightly, with ware pressed against the elements or shelves stacked only a couple of centimetres apart, will always fire unevenly. Leave a finger's width of space between pieces, keep work clear of the elements, and avoid placing large flat shelves directly above small pieces that need radiant heat.

Every kiln has a natural gradient — usually a little cooler at the bottom, particularly in older top-loading models. If the bottom shelf consistently under-fires, you can compensate by placing work that needs more heat there, or by slowing the final ramp so the whole chamber equalises before the soak begins. Witness cones in three positions — top, middle and bottom — will tell you far more about what is really happening than the controller display ever will.

Check the Thermocouple and Elements

A kiln that reports the right temperature but delivers the wrong one is a common and maddening situation. Type K thermocouples drift with age, especially where they routinely exceed 1100°C, and a worn couple can read 40°C or more out. If your glaze consistently behaves as though it were fired hotter than the controller claims, the thermocouple is the first thing to suspect. Replace it on a sensible cycle rather than waiting for outright failure, and always confirm with witness cones.

Elements are the other silent offender. Measure their resistance with the kiln cold and isolated, and compare the readings against the specification. Look for sagging, flaking, or a bright hot spot that glows sooner than the rest. As elements thin they deliver less heat, so the first symptom is usually a firing that takes noticeably longer than it used to. If your schedule used to finish in nine hours and now takes twelve, the elements are telling you they are tired.

Keep Records and Trust Your Cones

Finally, write it down. Note the date, the load, the schedule, the ambient temperature in the workshop and where the witness cones bent. After a dozen entries you will have a personalised reference that no generic firing chart can match. Combine that with an honest check of your thermocouple and elements every few months, and the mysteries of cracking, blistering and uneven glaze results shrink to a manageable list of variables you can actually control.