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Glaze
Faults |
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| Perhaps the
most common of all glaze defects, pinholes are
tiny holes in the glaze surface which penetrate
all the way through to the body. They are
caused by gases escaping from the caly body
during the firing cycle, after originating from
tiny pieces of organic matter, such as charcoal,
which is present in the clay. |
The
most common remedy in low fire ceramics is to
ensure that the piece is bisque fired 2 cones
hotter than it is glaze fired (ie when glaze
firing to cone 05, bisque fire to cone 03.)
Other possible remedies include: a longer firing
cycle with 15 minute soak at the peak
temperature, changing the peak temperature, a
slower cooling cycle, a thinner glaze coating,
using a glaze with more flux, and decreasing the
amount of zinc or rutile in the glaze if it is
present. |
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| Glazes that
have crazed show a fine pattern of cracks in the
surface of the glaze. Sometimes they are easier
to detect by breathing on the piece and fogging
the glaze surface. Crazing is the result of a
mismatch between the coefficient of expansion of
the glaze and the clay body. When the glaze has
too high a coefficient of expansion relative to
the clay body, crazing will occur. |
The
solution is to reduce this difference in
expansion. This can be done by changing to a
lower expansion glaze or adding a relatively low
expansion material such as silica to the
existing glaze. Alternatively, you can switch
to a higher expansion clay body. |
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| When a glaze
shivers it cracks and pieces of glaze peel right
off the piece, often a the edges of a piece.
This is another mismatch in expansion
coefficients, but this is the opposite of
crazing. In this case the glaze expansion is
too low relative to the clay body. |
One
solution is to increase the expansion of the
glaze by adding a material such as a high
expansion frit. |
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Crawling/Creeping |
Solution |
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| When a glaze
crawls or creeps it will tend to mound up and
expose an area of bare bisque. This often
happens in corners where glaze has built up too
heavily or has not flowed all the way into the
corners. Glaze can crawl on firing due to the
presence of dust, grease or other dirt on the
bisque. Sometimes crawling is a defect of the
glaze itself due to the use of materials that
have been too finely milled. In can also be
caused by shrinkage and the consequent cracking
of the glaze as it dries before being fired.
Also when underglaze is applied too heavily it
may lead to crawling of the covering layer of
clear glaze. |
One
general solution to crawling is to thin the
glaze with by adding water or by applying less
glaze. |
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| Glaze blisters
look like little craters (appr 1/8 inch
diameter.) |
Possible causes of blistering are: too thick a
layer of glaze, insufficient drying of the
glazed piece prior to firing, or too dense a
clay body which traps air in the piece. |
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| This is the
most common problem for glazes prior to firing,
which may also result in firing problems. When
a glaze settles out some of the heavier
components of the glaze settle to the bottom of
the container. If you try to use this glaze
without thoroughly remixing you will be applying
a glaze with key ingredients missing. A glaze
stays in suspension due to the presence of
various types of clays, such as bentonite,
and/or gums, such as CMC. One common cause of
settling out is the addition of too much water
to the glaze, which dilutes the effect of the
suspending agents and allows some of the glaze
ingredients to settle out. Another possibility
is the growth of bacteria which will consume an
organic gum, such as CMC, and will lead to loss
of suspension. To prevent bacteria growth do no
return used glaze, which has been poured out of
the original container, back into the original
container. Also do no introduce possibly
contaminated objects, such as brushes, into the
original container. Storing glaze in a hot or
sunny environment may also encourage bacteria
growth. Freezing can also destroy the action of
CMC, so brushing type glazes in particular
should not be shipped by unheated freight in
winter when the possibility of freezing exists. |
If
a glaze has settled out, but has not gone rock
hard in the bottom of the container, it can be
re-suspended by the careful addition of epsom
salts. Epsom salts can be readily purchased in
most drug stores. First you need to create a
saturated solution of Epsom salts by dissolving
them in a cup of warm water until no more will
dissolve. Then add this solution slowly and
carefully to the glaze while continuously
stirring the glaze. It should require less than
approximately one teaspoon of epsom salt
solution per gallon of glaze. The quantity will
depend on the severity of the problem. |
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