Ceramic Glaze Defects
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Ceramic glaze defects include things like pinholes, blisters, crazing, shivering, leaching, crawling, cutlery marking, clouding and color problems. So many things can go wrong it can seem amazing at times that it is even possible to successfully fire anything! For hobbyists and potters, a few small pinholes or uneven coverage are not an issue, but in industry one tiny defect can route a piece to the reject pile.
Defects are seen differently depending on who you are. A worker on a production line sees a defect as a maladjustment on one of the machines (e.g. the kiln, the application or decoration equipment). He/she consults trouble-shooting books (made by the equipment manufacturers) to find a sample picture that looks similar to the problem being experienced and then tunes the machine as instructed. When the adjustments don't work the problem is deemed to be caused by an upstream issue.
The technicians in charge of batching and maintaining the rheology of the slurry have different trouble-shooting books (made by the mixing equipment manufacturers, material and additives suppliers), the text beside the pictures explains how the problem might be related to the batching errors, mixing and milling issues, specific gravity, additive balance, viscosity, thixotropy, etc of the slurry. If none of the suggestions work it goes upstream to the glaze supplier.
The technicians at glaze supply companies use the university textbooks, they understand about glaze chemistry and the materials that supply it. They understand how to compare, evaluate and use frits. They know how to balance the tradeoffs between getting the desired visual or physical property in the fired result and what sacrifices and weakness are intrinsic to getting that. They know what changes to make in the recipe to fix the problem.
Often a glaze defect on a production line is a combination of issues at multiple steps.
A potter needs to know all of the above, to be able to decide what stage of production is the issue and fix it there.
A hobbyist just expects everything to work, glaze is like paint. He/she is unlikely to even read the label on the glaze jar. Hobbyists simply buy another jar and try that. If that fails they buy one of a different brand. If that fails they take up painting or weaving! The hobby ceramic supply industry is set up so that glazes are brushed on and are highly likely to work on the clay bodies hobbyists use. So most people can blissfully make ware while seldom encountering glaze defects.
This website is targeted mainly at potters, so it is about knowing every stage of production. Potters use frits much less than industry, so they are open to more glaze defects. However, they also have control of every phase of production and can thus compensate as needed. For example, they fire periodic kilns, which means they have the freedom to fire higher and employ much longer firing cycles, this gives even raw glazes time and temperature to melt and time to heal defects (on a controlled cool).
Glaze defects are more visible with certain glazes
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Reactive glazes, especially darker colored types like the one on the left, distract the eye with their visual character, this tends to hide defects (like pinholes, blisters, bare spots, scratches). But matte glazes, especially whites like the one on the right, make tiny defects in the glaze more visible. Their mixing, application and firing require much more attention to detail to produce a defect-free surface.