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Glaze Defects




Crazing and shivering or peeling of FFC

Black spots / Stains

Popping / Chipping


Crawling is mainly due to:

  • An initial interruption in the continuity of the thickness of the glaze
  • The influence of increased surface tension during firing.

Sanitaryware glaze crawling

There are many potential causes of crawling:

  • Excessive fineness of the particles (too long milling)
  • Engobe is too thick
  • Heating of pieces too quickly when still wet
  • Irregularities (coarse particles or granulates) in the surface of the body; dust, oil, grease, casting marks (different permeability)
  • Re-absorption of moisture after drying
  • Soluble salts on the body
  • Insufficient or degraded binder
  • Powdery first layer of glaze; excessive thickness


Sanitaryware glaze pinholes

Pinholes are caused by the migration of gas from the body which is either trapped by the glaze or released too late meaning the glaze does not have time to close.

Pinholes can also be the result of excessive degassing of soluble salts such as sulfates (SO₄²-), carbonates (CO₃²- ) and organics, under-firing of the glaze and under-firing of the body.

The first corrective action is to adjust the firing curve and cycle depending on the root cause.

It is also possible to adjust glaze fusibility (raw materials for glaze), or to reduce or substitute materials in the main body which contain salts or carbon, especially clays.

Crazing and shivering or peeling in fine fire-clay (FFC)


Crazing consists in the appearance of network of cracks in the glaze.

Sanitaryware glaze crazing

Usually, crazing is due to improper glaze/body thermal expansion coefficient matching: the thermal expansion of the glaze is too close or higher than the body. An adjustment of the dilatation of either the body or the glaze is required.

  • Decrease the thermal expansion of the glaze: increase quartz and/or decrease feldspar.
  • Increase the thermal expansion of the body: this is the simplest solution: add quartz (in 1% increments) in replacement of another material (usually kaolin) or using a higher dilatation chamotte.
  • France: Clayrac SHD

If thermal expansion is properly adjusted, but the autoclave test still produces crazing, it is possible that moisture expansion is too high. In this case, crazing can appear after several month/years of use.

The main cause could be an excess of feldspar or alkali in the body, a too low firing temperature or poor quality chamotte.

Shivering / Peeling

Shivering or peeling is the opposite phenomenon: when the thermal expansion of the glaze is too low compared to the thermal expansion of the body. This can lead to detachment of parts of the glaze from the body.

Sanitaryware glaze peeling

The solution is therefore the opposite: increase the thermal expansion of the glaze by decreasing quartz and increasing feldspar.

For this, use lower dilatation chamottes:

  • FR Clérac: Clayrac SMD, Clayrac Epurée
  • UK: Molochite®

Black spots / Stains

Sanitaryware glaze black spots

Blue stains tend to come from the blue pigment used for brightening the glaze, whereas black spots tend to be caused by contaminated glaze raw materials (with iron).

Green spots are generated by poor quality chamotte contaminated with pyrites or copper. This means the process has to be checked: brass connectors, pipes, etc.

Brown stains come from manganese, usually from the water, but possible contamination of the raw materials should not be excluded.

Popping / Chipping

Popping or chipping is a defect where a small chip of the piece (body + glaze) is blasted during firing and sticks to the glaze a few millimeters away.

Sanitaryware glaze popping chipping

This phenomenon is usually linked to carbonate or sulfate contamination in the slip: gypsum from the mold, water contamination, calcium carbonate in a raw material and generally poor calcination of the chamotte.

  • France: Clayrac SMD, SHD, Clayrac Epurée

Still having issues with glaze defects? Have a question you need to ask us?

Please contact us. We will be only too happy to help.

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