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PinholesPerhaps 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.
CrazingGlazes 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.
ShiveringWhen 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/CreepingWhen 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.
BlistersGlaze 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.