Crawling

The defect of crawling is mainly due to:

  • An initial interruption in the continuity of the thickness of the glaze.
  • A successive influence of the surface tension (increased) during firing.

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There might be many causes for this problem:

  • Excessive fineness of the particles (too long milling)
  • Too thick layer of engobe
  • Too fast heating of pieces 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

 

Pinhole

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Pinholes origin is a gaz migration from the body, either trapped by the glaze, or released too late, thus the glaze has not time enough to close. Could be an excessive degasification of soluble salts like sulfates (SO₄²- ), carbonates (CO₃²- ), organics, under-firing of the glaze, under-firing of the body.

The first corrective action is an adjustment of the firing curve and cycle, in accordance of the root cause.

The remedies could come from the glaze, with an adjustment of the fusibility (Raw materials for glaze), or from the body with a reduction or substitution of material containing salts or carbon, especially clays.

 

Crazing and shivering or peeling of FFC

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

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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 action : addition of quartz (in 1% increments), to the detriment of another material (usually kaolin) or using a higher dilatation Chamotte.

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

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

Shivering or peeling is the opposite phenomenon. When the thermal expansion of the glaze is too low compared to the body’s one, this could lead to detachment of parts of the glaze from the body.

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Thus, the remedies are the opposite ones.

Increase the thermal expansion of the glaze : decrease quartz and increase feldspar.

Use lower dilatation chamottes:

 

Black spots / Stains

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If blue stains are coming from the blue pigment used for the brightening of the glaze, black spots are usually coming from unclean glaze raw materials (contamination with iron).

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

Brown stains are coming from manganese, usually from the water, but possible contamination of the raw materials could 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 just sticks a few mm away.

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

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