When a glaze "settles out" some or all of the heavier components of the glaze sink to the bottom of the container. If you try to use this glaze without thoroughly re-mixing it you will be applying a partial glaze with key ingredients missing. A glaze stays in suspension due to the presence of various types of suspenders, such as ball clay, bentonite and CMC gum. One of the common causes of settling out is the addition of too much water to the glaze, which dilutes the effect of the suspension agents causing some of the heavier glaze ingredients to drop out of suspension. Another possibility is the growth of bacteria which will consume any organic materials, such as gum. This is of particular concern in the case of brushing glazes because gum is usually a large part of their suspension system. To prevent bacteria growth do not return used glaze, which has been poured out of the original container, back into that container. Also do not introduce potentially contaminated objects, such as brushes, into the original glaze container. Storing glaze in a hot or sunny environment may also encourage bacteria growth. Freezing can also destroy the action of gum, although in most cases frozen glazes can be gradually thawed and show no ill effects. If a glaze has settled out, but has not gone rock hard on the bottom of the container, it can be re-suspended by the careful addition of Spectrum's 1071 Suspender. After a glaze has settled out and been re-suspended it is generally a good idea to filter it through an 80-mesh screen to remove any coarse particles.
Glazes that have crazed show a fine pattern of cracks in the surface of the glaze, just the same as a crackle glaze although the latter is intended and the former is not. Sometimes the cracks are easier to detect by breathing on the piece and fogging the glaze surface. Crazing is the result of a mismatch between the coefficients 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 expansions. This can be achieved by lowering the expansion of the glaze by adding a relatively low expansion material, such as silica, or by using a higher expansion clay body. You should be aware that delayed crazing can occur hours or even days after the piece has come out of the kiln if the expansion mismatch is close to the limits where crazing will occur. Delayed crazing can also occur over time as porous, exposed clay takes on moisture from the atmosphere – causing the pot to expand slightly, and cracking the glaze surface. Because of the porous nature of earthenware clay, it is not recommended to make earthenware vessels with a “dry foot” ring, but rather to glaze them entirely and fire on stilts. This is not generally a problem with stoneware or porcelain as they are vitreous when fired to maturity.
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 the opposite condition to crazing where the expansion coefficient of the glaze is too low relative to the expansion coefficient of the clay body. One solution is to increase the expansion of the glaze by adding a high expansion material, such as a crackle glaze. If producing both castware and ware from pugged clay, be careful to test your glaze on both. Casting slip often has a different coefficient of expansion than pugged clay and as such it is possible to have a glaze that fits handbuilt or thrown pieces, but shivers off of slip cast pieces.
CRAWLING OR 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 because the coat is too thick or because it has not adhered properly to the bisque surface. Improper adhesion can be from bridging, such as in corners, or from the presence of dust, grease, finger oils, or other dirt on the piece. Be careful to clean the piece thoroughly before glazing. Sometimes crawling is a defect of the glaze itself caused by the use of materials that have been too finely ground. Materials that have too fine particle size create an excess of surface tension, which tends to pull the glaze apart. Crawling may also be result of a heavy application of glaze, which is allowed to dry too fast, producing cracks in the unfired surface of the glaze. This will then lead to crawling when the glaze is fired. Putting on thinner coats of glaze and allowing the glaze to dry thoroughly between each coat can resolve this problem. Another possibility is too heavy a layer of underglaze or stain under the glaze. Glaze requires a porous surface to adhere to, so anything that completely fills the pores in bisqued clay, and leaves nowhere for the glaze to seep into can result in crawling.
One of the most common glaze defects is pinholes, tiny holes in the glaze surface that penetrate all the way through the glaze to the body. Pinholes are caused by gases that escape from the clay body during the firing cycle. The gas originates from tiny pieces of organic matter, such as charcoal, which is present in the clay and for some reason has not managed to completely burn off during the bisque firing. The material then attempts to off-gas while the glaze is melting, and gets trapped in the glaze as the surface turns from liquid to solid. For earthenware, the best remedy is to ensure that the piece is bisque fired 2 cones hotter than it is glaze fired (i.e. when glaze firing to cone 05, bisque fire to cone 03). Other possible remedies include: a slower bisque firing cycle to give the carbon more time to burn out; a 15 minute soak at the peak temperature to keep the glaze in a liquid state a little longer; lowering the glaze firing temperature by 1 cone (for earthenware only); or using a glaze with more flux. **
Glaze blisters look like little craters in the glaze surface and may have sharp edges. Possible causes of blistering include: insufficient drying of the piece between glazing and firing; too dense a clay body that traps air in the piece; gas forming impurities in the glaze or body; over-firing the glaze; and a firing cycle that is too fast. The most common type of blisters comes from overfiring. More often than not, people fire without witness cones now. This means that they are unaware of differences in temperature from top to bottom in their kiln (even with the advent of kiln controllers, most kilns are not the exact same temperature from top to bottom). If firing a glaze with a range of firing temperatures (cone 4-6 for instance), then firing to the middle temperature (cone 5 in this example) will ensure that hotter spots in your kiln will not exceed the rating of the glaze. **
** always be sure to analyze a glaze defect carefully. Pinholes result in tiny craters with soft edges, whereas blisters result in slightly larger craters with sharp edges. The remedies for each are very different, and mixing up the two will likely worsen the problem.