Co-efficient of Thermal Expansion

The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking.

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Details

Co-efficient of thermal expansion (CTE) is a measure of the reversible volume or length change of a ceramic material with temperature. The more the expansion during heating the more contraction must occur while cooling it back down. Expansion values are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which is 0.00000065). Typically the power-of-ten is dropped, so a number might simply be: 6.5. Higher numbers indicate higher expansion.

It might not seem like a CTE of 0.00000065 in/in/degree C is much but in a brittle material like ceramic it is. The phenomena of thermal expansion usually comes to the attention of a potter, for example, as a crazed glaze (having a crack pattern that is a product of the glaze being stretched onto the ware). This is a serious problem - crazing impacts food safety and ware strength. The opposite problem, shivering, is where a glaze is under compression; this is even more serious since sharp flakes of glaze can pop off ware into food and drink. Some people promote the idea of having a lab measure the CTE or your clay body and glaze to get a better fit. That is not practical (see more on that a couple of paragraphs down).

Thermal expansion is also the basic phenomenon behind why ceramic ware often cracks when suddenly heated or cooled. True flameware bodies have almost zero thermal expansion and some can withstand a flame directly on the surface. Bodies high in free quartz particles (by virtue of the percentage of silica in the recipe and not being too vitreous) have the highest expansion. Vitreous porcelains (where the feldspar glass has dissolved much of the silica) and bodies made from low-expansion minerals (like phyrophyillite, mullite) have the lowest expansions.

When heated from room temperature to 2000F fused silica (non-crystalline) exhibits a CTE of almost zero whereas quartz mineral, having the same chemistry, has an amazingly high expansion of 1.5%! By comparison fused alumina (at 1400C) is 0.9% and stabilized zircon 0.8% (also very high). It is possible to fit a glaze to an alumina body by formulating it to have a very high thermal expansion. Likewise, it is possible, although much more difficult, to fit a glaze to an ovenware body by formulating it to have an extremely low expansion.

With glazes, thermal expansion is mainly a product of their chemistry. But there is more. The degree to which all the material particles are dissolved in the melt, the degree to which the melt has become homogeneous and the degree to which phase separation and crystallization have happened on cooling in the kiln also affect the CTE. Understanding a specific glaze is thus a study of all the variables against their CTE, something that can only be done over time.

Thermal expansion of clay bodies is more complicated than for glazes. It is a product of the complex microstructure of fired ceramic (grains of unchanged minerals, others that have melted and flowed, others that have altered their crystal form, others that have reacted together to create new mineral species). That micro-structure is a product of many factors (e.g. the degree to which a body is vitrified, the firing schedule, the particle size and shape of the materials, the distribution of particle sizes) determine its complex microstructure and thus its thermal expansion. Does refiring ware affect CTE? Yes, it doubles the heat treatment on the piece. Like understanding glaze CTE, a history of data that relates all the variables to measured CTEs is needed.

So how do you get a glaze to fit a clay body? Glaze fit is dealt with on many pages on this web, we recommend physical stressing to reveal misfit (crazing or shivering), calculation to move the glaze in the direction needed to improve fit and then mixing and testing the new recipe. The cycle is repeated until fit is demonstrated.