Glaze Course - Lesson Summaries
Glaze Defects - Crazing Part 1
Lesson Number 22
When a glaze develops a pattern of cracks on the surface of the pot the phenomenon is called "crazing".

In the category of domestic ware crazing on the inside of a container is considered generally to be an unacceptable fault. From a health point of view cracks in the glaze surface, where food or drink are involved, are thought to be a haven for bacteria and therefore a potential health hazard. While there seems to be no significant research into this possibility the perception of a customer may none-the-less be that the health hazard exists in such cases.

Apart from potential health hazards, in the field of low temperature ceramics, a crazed glaze can allow a liquid containing pot to leak. Earthenware clay bodies do not vitrify when fired and will therefore allow water to seep through the fired pot unless the container has been "sealed" on the inside with a glaze that does not craze.

Aesthetically speaking crazing on the outside surface of a pot can either be an enhancement or a visual flaw that diminishes the beauty of the surface. For example in the case of maiolica, where coloured decoration sits on a white "canvas" crazing that has accumulated grime over time, can seriously interfere with the surface effects. On the other hand the pattern of cracks in the celedon glazes of ancient pots is admired by many. The so called "crackle" glaze is one that some potters value in there collection of finishes.

The overall strength of a ceramic form is increased where it is coated with a glaze that does not craze.

Whether you work in stoneware or porcelain or at earthenware temperatures the control of crazing is a skill that will help you to eradicate the effect from your glazes or control the phenomenon in a way which may allow you to use the effect as an enhancement where appropriate.

In this first of two sessions on crazing we will:

  • define crazing
  • look at the various forms of crazing
  • discuss the reason why crazing occurs
  • look at systems provided by cramic researchers that we can use to assist us in the manipulation of crazing effects.

Full lessons contain content and activities not listed above. More lessons will be added to this list as they are completed.
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Contact: Lawrence Ewing - 1015 Ellis Rd, Five Rivers, RD3, Lumsden, Northern Southland,
New Zealand