InstructionsThough many are unaware of it, poor glaze fit can reduce the strength of a fired ceramic piece to as little as one-fifth the strength of a similar piece with ideal glaze fit. While good glaze fit seldom occurs by accident, it can be planned for and controlled. Some ceramic artists use glaze fit to induce crazing as a decorative technique (crackling) while others artists may want to avoid a "crackle" glaze.
First, let's make sure we're all talking about the same thing:
Glaze Fit: The difference in the amount of shrinkage per unit temperature (experienced upon cooling) by a fired clay body and the shrinkage per unit temperature of the glaze fired onto the same body. The scientific measure of this shrinkage is called the coefficient of thermal expansion (CTE). Thus, glaze fit is the difference between the CTE of the glaze and body.
Coefficient of Thermal Expansion (CTE): The distance any material expands per unit of length upon heating (or shrinks upon cooling) one degree of temperature. A typical CTE of fired stoneware is about 0.000007 inches (in scientific notation, 7×10-6) per inch per °F. A typical CTE of porcelain is ~6×10-6/. While the numbers indicating CTE stay the same whether one measures length in inches or millimeters, it is important to note the temperature scale (°F or °C) and keep that consistent for both clay and glaze. A higher CTE means a material shrinks more, a lower one means it shrinks less.
Crazing: Cracks in a glaze, seen as lines that appear to be across the glaze surface. It occurs as a result of glaze tension.
Shivering: The spontaneous breaking away of bits of a glaze from the fired clay body underneath. Shivering will occur first on rims and other outside edges, as a result of excess glaze compression.
Now, let's get on to the more interesting version of this discussion: Tight Pants and a Fast Car
If a fired clay body shrinks less than the glaze (the CTE of the clay is less than the CTE of the glaze), the glaze is in tension. The best way to visualize this is to think of a tight-fitting pair of pants, where the human body is the clay body and the pants are the glaze. If a glaze shrinks more than the clay body, the glaze is in tension (the pants are too tight). A glaze has enough strength to handle a little tension. However, it doesn't stretch very well at all. If the glaze is in too much tension, it cracks—literally pulls apart from itself. This is most likely to occur on surfaces where the tension accumulates across a distance until the tension exceeds the strength of the glaze and it cracks (crazes).
Both glass and fired clay are brittle. Brittle materials are very strong in compression. Visualize the Lamborghini dealer who supported an entire 3100-pound car on only four teacups! However, it takes a lot less force (per unit area) to pull a brittle material apart than to crush that same material. Thus, we say brittle materials are "weak" in tension. All crazing occurs when a glaze is in tension.
Since a glaze is a thin layer of glass melted and "frozen," on the surface of a fired ceramic body, the glaze is a brittle material bonded to another brittle material. If the glaze and the body shrink and expand at exactly the same rate, we say they fit. Both seldom do, except by conscious effort of the artist.
If a fired clay body shrinks more than its glaze (the CTE of the glaze is less than the CTE of the body), the glaze is in compression. Excessive glaze compression will force pieces of the glaze to literally pop off the body. We call this shivering and it doesn't occur nearly as often as crazing, partly because it takes greater CTE mismatch to cause shivering. Visualize this by thinking about the pants again. If the pants are way too loose they may fall down, so there is a weak resemblance to shivering in both meanings of the word! A little glaze compression may be good, say 15%, because it actually makes the fired ware stronger. However, if there is too much compression the glaze shivers.
Troubleshooting Glaze Fit
Detecting crazing is not necessarily easy, especially with dark, opaque, or matt glazes. Use bright light and a simple, inexpensive, 10-power magnifying glass (often called a hand lens) to spot craze lines.
Understanding the real causes of crazing and shivering are a requirement to achieving a desired glaze fit or, in the case of crazing, misfit. There is a lot of misleading folklore regarding crazing. For example, many believe crazing is caused by cooling ware too fast. While rapid cooling through the quartz inversion temperature between 1100° and 1000°F (593-538°C) may damage ware, rapid cooling below 1000°F (as long as it doesn't cause damage due to thermal shock) cannot cause crazing. Rather, it simply reveals the CTE mismatch that will eventually cause a glaze to craze. All slow cooling accomplishes is possibly slowing down the occurrence of crazing so it isn't so dramatic when one opens the kiln.
In functional ware, shivering is dangerous. Potters don't want to have small bits of sharp glass popping off their pots into people's food or drink. The most likely place to observe shivering is on rims and edges. The sharper the edge or rim, the more likely shivering is to occur there if a glaze is in compression.
In order to change glaze fit, it is first necessary to understand whether to increase or decrease the CTE of the glaze, body, or both. There are only two ways to change glaze fit. One is chemical; change either the glaze recipe or the body recipe so the CTE of the glass in the glaze or body changes. In almost every case, it's easier and more effective to adjust the glaze recipe than the clay body recipe. The second way to change glaze fit is to increase or decrease the vitrification (amount of glass formed) in the clay body. Firing to a higher temperature dissolves more silica from any clay body containing quartz. The glass formed will have a lower CTE than the quartz did. Firing hotter will lower the CTE of the body, typically pushing it farther from the CTE of the glaze increasing the likelihood of crazing. To correct shivering you either have to increase the CTE of the glaze so it expands/contracts more, or reduce the CTE of the body, so it expands/contracts less. To avoid changing your clay body recipe, you will need to fire hotter. Note that if the first (or bisque) firing is hotter than the glaze firing, often the case for low fire, then the first firing is the one you would need to adjust. Underfiring, of course, will tend to promote crazing but reduce shivering, assuming the glaze still melts fully. Adjusting glaze recipes to fix glaze fit is a topic much larger than this page. Summarizing briefly, though, adding clay or silica, or magnesium and lithium fluxes (low CTE materials) will lower glaze CTE. Adding materials rich in high CTE fluxes, such as sodium and potassium, will raise glaze CTE.