Crazing is a network of fine cracks in the glaze surface, like a spider's web. Pinholes are tiny pits or holes in the glaze. Both are glaze defects that let dirt and bacteria collect, and both make a WC or basin look cheap.
Crazing can appear months after installation, so it is a latent defect. Pinholes are usually visible when the piece leaves the kiln.
What causes crazing
The glaze and the ceramic body expand and contract with heat and moisture. If the glaze shrinks more than the body as it cools, it ends up stretched, and it cracks to relieve the stress.
Moisture can cause crazing later. If the body is underfired and absorbs water, it swells slowly over time and pulls the glaze apart. That is one reason a low water absorption matters for vitreous china.
Common causes on the factory floor include a poorly matched glaze recipe, underfiring, uneven glaze thickness and rapid cooling in the kiln.
What causes pinholes
- Gas escaping from the body or glaze during firing, leaving small craters.
- Dust, oil or dried slip on the surface before glazing.
- Air bubbles in the glaze slurry.
- A glaze layer that is too thin or applied unevenly.
- Refiring pieces to repair defects, which can create new pinholes.
How crazing is tested
Sanitaryware standards include a crazing resistance test that stresses the glaze and then checks for cracks, usually with a dye. A similar idea applies to wall tiles, where glaze crazing is also a common issue. Our article on crazing in wall tiles explains the tile version.
Ask for a test report on crazing for each model. A report from the factory's lab is useful for routine checks. A report from an accredited lab carries more weight. See how sanitaryware is tested for the full set of tests.
How to check at inspection
- Inspect each sampled piece under good, even light from several angles.
- Wipe a dark ink or marker over suspect areas, then clean it off; ink stays in crazing lines and pinholes.
- Pay special attention to the bowl, the rim, and visible surfaces of basins.
- Agree in the contract how many pinholes of what size are acceptable on each surface, and where.
- Reject refired pieces that show visible repair marks if your contract says so.
What a good factory does
A well-run plant controls glaze recipe, thickness and firing curve, and keeps records of each kiln run. It grades pieces after firing and separates first quality from seconds. During an audit, ask to see the grading area and the rejected pieces. Our guide on auditing a sanitaryware factory lists what else to check.
Next step
We inspect glaze quality before loading and write clear glaze defect limits into your order. To discuss your range, book a free sourcing review.