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.