Two small marks, two different stories
A buyer opens a carton, turns a pan over in daylight, and finds a dark pinpoint on the rim. Another pan has a tiny crater in the glaze near the outlet. In an email both get described the same way — “defect on the surface” — and both get answered the same way, which is where the trouble starts. A crater and a dark dot are produced by different mechanisms, sit in different places, and support different claims.
The useful discipline is to stop describing the mark and start locating it. A defect that sits in the glaze, a defect that has opened the glaze to the body underneath, and a defect that sits in the body behind an intact glaze are three separate conditions. Each has its own cause, its own likely frequency across a run, and its own answer when a buyer asks what happens next.
What a pinhole is
A pinhole is a small opening in the glaze where the glassy layer did not close over during firing. The glaze melts and flows to form a continuous film, and while it is molten, gas from the body or from the glaze itself is still trying to escape. Most of it gets out before the film seals. Where a bubble reaches the surface late, it leaves a crater that the glaze does not have enough time or fluidity left to fill.
That description already tells a buyer where to look for the cause. Pinholes cluster where the body is thick, where the glaze is applied heavily, and where a piece sat in a part of the kiln that ran cooler or hotter than the rest. They are a firing and application condition rather than a material failure, and they are usually measured as a count over an area rather than judged one at a time — which is why a single pinhole and a field of them are different conversations.
What a speck is
A speck is a particle. Something that is not glaze and not body arrived in the wrong place and stayed there through firing. The common sources are iron-bearing contamination in the body mix, dust or scale falling onto a glazed surface before it goes into the kiln, and residue carried on handling equipment. The particle does not melt at glaze temperature, so it stays as a discrete dark point, sometimes with a slight halo where the glaze pulled away from it.
Iron is the reason this defect is watched so closely in white ware. A particle of iron in the body will often bleed through the glaze during firing and appear as a dark spot on a finished face, even though nothing ever touched that surface after glazing. That is the case a buyer most often misreads: the mark looks like dirt on the glaze, it does not clean off, and the assumption is that the factory glazed over a dirty surface. Sometimes it did. Often the particle was inside the body from the start.
Where each one sits
The three positions are worth separating before anything else is decided, because the position is what the claim actually rests on. The drawing below places them on one wall section.
Three readings follow from the drawing. A pinhole is a route, because the glaze is interrupted and the body is exposed at the bottom of the crater. A speck in the glaze is a blemish, because the glaze film around it is still continuous. An iron point in the body is a blemish seen through the glaze, because the glaze above it was never interrupted at all. Same size on the shelf, three different answers.
Why position decides the claim
A buyer’s real question is almost never “what is this mark” but “what do I do about this consignment”. The position answers it. A field of pinholes across a run points at glaze application and kiln conditions, and the answer is a sorting standard agreed in advance: how many marks over how much surface is acceptable, assessed on which faces by which light. That is a specification, and it belongs in the order, not in the complaint.
An isolated iron point on a face that is seen after installation is a different matter, because it will not clean and cannot be polished away without cutting through the glaze. And a pinhole on a surface that stays wet, or on a rim where water sits, deserves a closer look than one on a hidden face — not because the ceramic behaves differently, but because that is where a buyer’s own customer will notice it.
Photographing a defect so it can be assessed
Small marks are the hardest thing to photograph and the easiest to dispute. A useful submission has four parts, and none of them needs a measuring tool:
- A whole-fixture view, so the position on the pan is unambiguous.
- A close view with light raking across the surface at a low angle. Pinholes show as shadows; specks show as points; flat flash hides both.
- The model code, the batch code, and the carton number the piece came out of.
- A count, not an adjective. How many marks, on which face, over what area of the consignment inspected. A rate is answerable; “several” is not.
What a photograph cannot do is establish depth. Whether a mark is a crater that reaches the body or a particle sitting in an unbroken film is a question for the piece itself, under a light, by someone whose judgement is on the record. Any answer given on a photo alone should be treated as provisional by both sides.
What our own line controls, and what it cannot
Both defects are controlled where the ceramic is made, which is why the plant layout matters to a buyer. Casting, glazing, firing and QC run in the same buildings at Chaozhou, in our own tunnel kiln, so a speck can be traced to a glaze batch or a handling stage rather than disappearing into a supply chain. We have run these lines in Guangdong since 2021, on a 51,000 m² site with 285 people, and all 89 models in the range are vitreous china.
What no factory can offer is zero. A firing process that melts glass over clay will produce the occasional pinhole, and a mineral body will occasionally carry a particle. The honest control is inspection before packing plus an agreed acceptance standard, and that is what a QC record is for. Our body absorption is recorded at 0.14% under AS 1976 and published; that figure is about the body, not about surface marks, and it should not be used to argue either side of a speck claim.
Writing the claim so it can be closed
Defect claims close quickly when they contain four things: the model and batch, the inspection rate, the standard being applied, and the remedy asked for. They stay open for weeks when they contain a photograph and a phrase. If the standard was never agreed, say so and propose one — that is a faster route to a settlement than arguing about individual marks, and it produces something both sides can use on the next container.
Frequently asked questions
Are pinholes a hygiene problem?
A pinhole is an interruption in the glaze, so the body is exposed at the bottom of the crater. Whether that matters depends on where it is and how the fixture is cleaned. On a face that stays wet it deserves attention; treat it as a surface condition to be assessed, not as a self-evident health claim.
Can a pinhole be told from a speck in a photograph?
Often not. Both appear as a dark point at normal viewing distance. Raking light helps, because a crater casts a shadow while a particle sits proud or level with the film. Depth is the deciding fact, and depth is established on the piece, not on a screen.
Why does a speck not clean off?
Because it is not on the surface. A particle of iron in the body can bleed through the glaze during firing, and a particle of contamination can be sealed under the glaze film. In both cases the glaze is continuous above it, so no cleaner reaches it.
Does one pinhole justify rejecting a container?
Not on its own, and not without a standard. What justifies action is a rate: how many pieces carry marks, on which faces, out of how many inspected. Agree that rate before production and the question of a single mark mostly disappears.
What causes pinholes during firing?
Gas escaping through molten glaze after the film has begun to seal, usually where the body is thick, where glaze was applied heavily, or where a piece sat in an uneven part of the kiln. It is a firing and application condition, addressed by the kiln curve and the glaze line.
What should a defect claim include?
The model code, the batch code, a whole view and a raking-light close view of the mark, the inspection rate rather than a description, the acceptance standard being applied, and the remedy requested. Keep the piece available until the claim is closed.
Related on this site
Documents and certificates
- Catalogues, technical drawings and the AS 1976 report
- Send a defect claim with the model and batch code
Related reading
- Firing, glaze and what the kiln does to a surface
- What QC checks before a carton is closed
- Glaze Crazing: What Causes the Lines