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 a 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. In other words, they are shrinking at different
rates with the clay body being too large. 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. or change your bisque to one or two cones higher. Remember, not all glazes
fit all clays. Be sure to ask for our advice, we are more than willing to take the time to help you.
Shivering:
When a glaze shivers it cracks and pieces of the glaze peel right off the piece, often at the edges of the piece. This is
another mismatch in expansion coefficients, but this is the opposite to 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. Sometimes shivering can be preventing by layering another glaze overtop the first, especially aroud the rim or sharp
edges.
Crawling: When a glaze crawls or creeps it will tend to
mound up and expose an area of bare clay. 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 in firing due to the presence of dust, grease, finger prints or other dirt on
the bisque. Too much tin oxide will cause crawling, also. Sometimes crawling can be a fault of the glaze chemicals being milled
to finely. It 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 either by adding water or by applying less glaze.
Blisters:
Glaze Blisters look like little craters, approximately 1/8" 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. Sometimes lowering the bisque and slowing down the cool will heal over blisters.
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 not return used glaze, which
has been poured out of the original container into a new batch. 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 the 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 or our Dry Suspender/Binder.
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 (super-saturated). 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, but you can add to much.
Downdraft
Vents and Their Effects on Glaze Problems: Glaze defects like pinholes, blisters and a few other small problems
are very frequently helped by the "fresh air in-contaminated air out" feature of downdraft vent systems.
