For anyone who has opened a kiln to a pot that was fine yesterday
Pottery troubleshooting — crazing, shivering, crawling, pinholes & blisters
Because the glaze and the clay shrink by different amounts as the kiln cools. If the glaze shrinks more it is left in tension and cracks — crazing. If it shrinks less it is left in compression and flakes off — shivering. One mismatch, two directions, and the cure for one is the cause of the other.
Written by Abdelmounim El Moussaddar · published by TopDigitalKnowledge · 42 pages, 10 figures, 7 sources, every one cited inline · last verified September 2026
What everybody says
A crazed pot can be fixed by firing it differently — cooling more slowly, or adding a hold.
It cannot. Fit is a property of the two materials, not of the journey, and the source is blunt about it: glaze fit “can rarely be fixed by firing differently (that just delays it)”. A slower cool postpones the moment the glaze gives way, sometimes past the point where you unload the kiln and call it a success. The stress is still there, and it is released later — on a shelf, or in a customer’s hands.
Three test tiles, one clay, one glaze, one firing: one crazed, one perfect, one shedding glaze from its edges. Nothing was wrong with the bucket and nothing was wrong with the kiln. What separates them is fit — whether the glaze and the clay change size by the same amount as the kiln cools.
If the glaze shrinks more than the body it is left in tension and cracks. If it shrinks less it is left in compression and flakes off. One axis, two ends — which is why the cure for one is the cause of the other, and why applying the wrong fix makes things worse rather than merely failing.
The first instinct on a crazed pot is to cool more slowly or add a hold. The source is blunt: glaze fit “can rarely be fixed by firing differently (that just delays it)”. The stress is in the materials, not the journey. A piece that leaves the kiln clear and crazes a fortnight later has not developed a new fault — it has finished expressing the one it already had.
Heatwork is “the combined effect of time and temperature”, in the Orton Foundation’s own words. The same cone 6 bends at 1185 °C fired slowly and 1243 °C fired fast — a 58 °C band inside one number. And their own example: a kiln set to reach 2232 °F in one hour leaves porcelain grossly underfired, needing nineteen more.
Crawling is blamed on dust or oil on the bisque, everywhere. The source says dust almost never causes it — “besides, glaze is a mix of dust and water!” It is a drying failure: the layer shrinks too much as it dries and cracks, and those cracks become the points the melt pulls back from. If you cleaned carefully and it crawled anyway, you were watching the wrong variable.
A glaze fault is identified by looking at it, so the book is built that way: ten full-page plates showing what each defect actually looks like, and a fault finder arranged by what you can see rather than by what is wrong. The rolled edge of a crawl against the sharp fracture of a shiver is not a distinction a diagram can carry.
No, deliberately. Recipes are the one thing this subject is not short of, and a recipe you cannot diagnose is a recipe you cannot adjust. What is missing from the shelf is the reasoning, so that is what this is. It will tell you which direction to move a recipe and why, in the order the source gives.
Yes, and there is a chapter for exactly that. Fit is a relationship between two materials; if the glaze is fixed, the body is not. Testing one glaze across two or three clays settles the question in a single firing. The crawling, pinholing and blistering chapters apply unchanged whatever you are using.
The ten plates are digitally rendered illustrations of the defects described. They are there because a glaze fault is diagnosed by eye and a drawn diagram cannot show the difference between a rolled crawl edge and a sharp shiver flake. The written text and the five drawn figures carry the evidence; the plates carry the appearance, and the book says so in its colophon.
Seven, every one opened at its publisher: the Digitalfire reference library on crazing, shivering, crawling and blisters; the Edward Orton Jr. Ceramic Foundation on heatwork and its own published cone charts; and Council Directive 84/500/EEC read in full on EUR-Lex. No blogs, no forum posts, no retailer guides.
It names the document and declines to print the figures, and explains why. The FDA page confirms CPG Sec. 545.450 of November 2005 and its findings, but the action-level table itself was not readable at the publisher. Several secondary sources quote it and agree; agreement between secondaries is not a reading. The EU limits, which were read in full, are printed.
PDF only, US Letter, delivered instantly. There is deliberately no EPUB: the value is in full-page plates and a fault-finder table, and a reflowable format destroys both. It prints cleanly at home, which is what the firing log at the back is for.
42 pages · PDF
Crazing and shivering are the same mismatch in opposite directions, and no firing schedule fixes either. 7 sources, every one read at the publisher — including the European directive quoted from the directive itself.
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TopDigitalKnowledge · written by Abdelmounim El Moussaddar · last verified September 2026
Every number in this guide is traced to a named, published source.
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