Pinholes
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 clay 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.
Crazing
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.
Shivering
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.
Crawling/Creeping
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.
Blisters
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.
Settling-Out
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.
