What the lines actually are
An owner looks into a pan that was fitted last season and sees a web of fine lines in the surface, running in several directions, none of them reaching the rim. The instinct is that the ceramic has cracked. In most cases it has not. What has happened is that the glaze — the thin glassy layer fired onto the body — has developed a network of shallow fissures inside itself, while the body underneath has stayed whole.
The distinction is not academic. A crack in the body is a structural event: the pan is no longer a continuous shell, and nothing about it can be assumed afterwards. Crazing is a surface event, confined to a layer that is bonded to the body rather than part of its structure. The two look similar at arm’s length and behave completely differently over the life of the fixture, which is why the first question in any such complaint should not be how bad it is, but which layer is being looked at.
Why crazing happens at all
A glazed ceramic is two materials fired into one object. The body and the glaze have different thermal expansion behaviour, and they cool together from the kiln. A glaze is formulated so that, once cool, it sits slightly in compression against the body — squeezed, in effect, by the body as it contracts. That compression is what makes a glaze tough and hard to damage. When the balance is wrong, the glaze ends up in tension instead, and a glaze in tension relieves itself by cracking into the network that the owner sees.
Three things push that balance out of line. The first is glaze fit: the formulation itself, which is a decision made at the glaze mill and tested before a run. The second is how much glaze went on and how evenly — a heavy application in one area and a light one beside it leaves the layer fighting itself. The third is the firing and cooling profile, because the rate at which the piece comes down through the cooling zone decides how much stress is locked in. All three sit inside the factory, which is the reason crazing is a factory conversation and not a site one.
Telling crazing from a crack
No photograph settles this on its own, whatever the angle. What settles it is a short sequence of physical checks that an installer, a surveyor or a technician can run in a minute, and the answers point one way or the other:
- Run a fingernail across the line. A crack in the body has an edge that catches; a crazing line is a fissure within the glaze and normally does not.
- Look at the pattern. Crazing spreads as a network over an area. A body crack is usually a single line, or a line with one or two branches, and it starts somewhere — a fixing hole, the rim, a point of impact.
- Look at the far side. A crack through the wall is visible on the inside face as well, or shows as a damp line. Crazing stays on the glazed face.
- Watch it over time. Crazing that has stabilised stays as it is. A crack lengthens, and the pan may move or weep at the outlet.
If the answers are mixed, stop guessing and put the piece in front of someone whose judgement is on the record — the installer who fitted it, or a surveyor both sides have agreed to. A supplier cannot classify a defect from a phone photo, and a buyer should not accept a classification made that way either.
What a section drawing makes obvious
The reason the two faults get confused is that both are lines on a white surface. Cut through the wall and the difference stops being subtle, which is what the drawing below sets out.
Read the drawing as a set of four statements rather than a picture. The glaze layer is a thin band bonded to the body, and it is where crazing lives. The ceramic body is the structural shell, and it is where a crack lives. The crazing path stays inside the band and stops at the interface. The crack path crosses the interface and continues through the wall. Once those four are separated, the question of which one a pan has becomes a question of where the line stops.
When it does not matter
Crazing that is confined to the glaze and has stopped moving is, in most commercial settings, a cosmetic question rather than a functional one. It does not open the body, it does not change the flush, and it does not alter the mounting. Many markets handle it inside a visual standard agreed at order stage, which is the only place it can be handled fairly — a standard written after the goods land is a negotiation, not a specification.
There is one thing to establish rather than assume, and that is whether the fissures reach the body. That is settled by inspection of the piece, not by inference from a photograph, and it is the difference between a surface condition and a route for water into the ceramic. Where a project needs the answer in writing, the honest form is a statement of what was inspected, by whom, and on what date.
When it does matter, and who decides
Crazing matters when it appears across a large share of a consignment, when it appears soon after installation, or when it keeps spreading. A pattern like that points at glaze fit or at the firing cycle, both of which are production decisions, and both of which sit with the factory rather than the site. That is a supplier conversation, and it should start with the batch code so the answer can be tied to a run rather than to a range.
It also matters when the lines arrive after a specific event: steam from a shower directly above the pan, a cold feed hitting a warm surface, water treatment changed on site. Those point at thermal cycling on site, which is a different conversation with a different answer. The decision about which of the two it is belongs to whoever inspects the piece — a technician from the factory, the installer, or an independent surveyor named in the contract. It does not belong to whoever is answering the email.
What to photograph before writing to anyone
Most crazing claims arrive as one flash photograph of a white surface, which shows that something is there and nothing else. A claim that can be answered needs a small, disciplined set:
- The whole fixture, so the position of the lines on the pan is clear.
- A close view with light raking across the surface at a low angle, which is the only way crazing shows in a photograph at all.
- The model code and the batch code from the carton or the piece itself.
- The installation date and what the fixture has been exposed to since.
Keep the piece. A crazing judgement made on a photograph is an opinion; the same judgement made on the pan, with the batch code beside it, is a finding.
What we can put in writing
Our own position is narrow and checkable. Bodies are fired in our tunnel kiln at Chaozhou, with casting, glazing, firing and QC in the same plant, so a glaze question can be traced back to the run that produced it rather than to a supplier further up a chain. We have run ceramic production in Guangdong since 2021, on a 51,000 m² site with 285 people, and every model in the range — 89 of them — is vitreous china.
Two documents support that. The AS 1976 test report records water absorption of 0.14%, and it is published rather than held back. The ceramic body carries a lifetime warranty. What the warranty covers is the body, and what settles a crazing question is the visual standard agreed with the order — so if glaze appearance is going to be inspected on arrival, write the standard into the order, and ask for the batch code to be printed and recorded. Both are simple requests, and both make the conversation after delivery much shorter.
Frequently asked questions
Is crazing the same thing as a crack?
No. Crazing is a network of shallow fissures inside the glaze layer. A crack passes through the ceramic body and through the wall. A fingernail catches a crack but not a crazing line, and a crack normally starts at a single point rather than spreading over an area.
Can crazing be polished out or repaired?
Not in any way that restores the original glaze. A fired glaze is glass bonded to the body; there is no site repair that returns it to that state. If a piece falls outside the agreed visual standard, replacement is the practical route, and the batch code is what supports it.
Does crazing mean the pan will leak?
Crazing alone does not breach the body, so it is not a leak path in the way a crack is. Whether water reaches the ceramic behind the glaze is a separate question, answered by inspection. A pan that weeps at the outlet is a different fault with a different cause.
Why do the lines appear months after installation?
Because glaze stress can sit in a piece and relieve itself later, often after repeated thermal cycling from hot water, steam or a cold feed. Late appearance is worth reporting with dates, since the timing is part of how the cause gets identified.
Is crazing covered by the ceramic warranty?
The warranty covers the ceramic body. Crazing is a glaze condition, judged against the visual standard agreed when the order was placed. Where a claim is made, it is assessed on the piece and its batch code, not on a description of the lines.
What should I send with a crazing claim?
A whole-fixture photograph, a close view with light raking across the surface, the model and batch codes, the installation date, and a note of what the fixture has been exposed to. Keep the pan itself available for inspection until the claim is closed.
Related on this site
Documents and certificates
- The AS 1976 test report, catalogues and technical drawings
- Certificates held, and the markets they cover
Related reading
- What a vitreous china body is, and why it is specified
- How firing decides what the glaze looks like
- Carton Marks and Shipping Labels