Glaze defects

Once fired, ware tends to be more resistant to crazing due to better development of the glaze/body interfacial layer, which reduces stress gradients between the glaze and body.

[3] In pottery a distinction is often made between crazing, as an accidental defect, and "crackle", which is when the same phenomenon, often strongly accentuated, is produced deliberately.

[3][11][12] Regulations have existed since the late 1960s to protect consumers from the potential risk of toxic materials, mainly metals, being released from glazes into drink and foodstuffs.

The propensity for any glaze to release metal may depend on complex interactions between the formulation used, any applied decoration and the kiln atmosphere.

[1] Monitoring the level of metal release from glazed ware forms part of the quality control procedures of all reputable producers.

[1][13] Test methods are specified according to national and international standards, although testing usually involves: the ware being immersed or filled with a 4% acetic acid solution; covered and left for 24 hours at room temperature, although if cooking ware is being tested higher temperatures are needed; the acetic acid solution decanted from the ware and the concentration of leached metal measured by Atomic absorption spectroscopy.

Some of the most well recognised legislation are: across Europe 'EC Directive 84/500/EEC 1984'; for the UK 'GB Ceramic Ware (Safety) Regulations SI 1647, 1988'; and for the USA 'FDA Compliance Policy Guide 7117.06 and 7117.07 for cadmium and lead.

Blisters appear as large bubbles either just below or penetrating the surface, leaving sharp, rough edges that collect dirt.