Techno File: Cristobalite Dave Finkelnburg

Appears in the June/July/August 2020 issue of Ceramics Monthly.

Studio artists have discussed the merits of cristobalite for decades. However, without sophisticated industrial testing, its effects can be disastrous in small-scale production, so adding cristobalite to a clay body, or using a clay body in which it forms isn’t recommended unless one is fully prepared for the challenges it presents.

Definitions

Cristobalite: A crystal form of silica with the chemical formula SiO₂,which is formed from pure molten silica at 3101°F (1750°C). It’s used in high-temperature furnace bricks and by the ceramic industry on rare occasions to strengthen glazed ware.

Dunting: Cracking of a ceramic piece due to thermal stress during heating or cooling.

Inversion: A measurable, reversible change in volume that takes place over a specific temperature range during either heating or cooling of a material.

Shivering: Pieces of a glaze popping off the surface of a fired ceramic as it is cooling or after it has cooled, caused by the glaze being in excessive compression.

Science

Cristobalite, and far more abundant naturally occurring quartz, are polymorphs of silica. That is, they have different crystal forms but the same chemical formula, SiO₂. Both also go through a reversible volume change, known as an inversion, during heating and cooling. The volume change of cristobalite during its inversion is greater, about double the volume change of quartz during its inversion.

Both quartz and cristobalite have relatively open crystal structures that leave room for inclusion of other elements such as fluxes, aluminum, and iron. Depending on the kind and amount of these elements in the crystal network, the inversion temperature shifts up or down. This explains why studies report the inversion of these minerals occurs over a range of temperatures. The cristobalite inversion range has been reported to be between 410°F (210°C) and 536°F (280°C). The quartz inversion range has been reported between 1004°F (540°C) and 1112°F (600°C). The inversion temperature for quartz is thus roughly 1058°F (570°C). The cristobalite inversion temperature is much lower, 473°F (245°C).

Because of its desirable refractory properties, cristobalite is manufactured in industrial quantities. The process involves mixing finely ground quartz with a catalyst and heating it to around 2732°F (1500°C). Iron as well as compounds of the flux elements are used as the catalysts. Conversion to cristobalite occurs more rapidly with finer quartz.

This plate, created by Kala Stein (https://kalastein.com), suffered the effects of cristobalite while she was an adjunct professor at Alfred University. A fairly dramatic bump at 482°F (250°C) due to cristobalite volume changing 7% most likely split the plate. It was fired, then glazed and decorated, and was being heated in the glaze firing when it split. The proof is that the edges of the crack are not sharp, as they would be for a crack that occurred during cooling. The edges on this plate are rounded, because the the glaze melted after the plate failed. Stein notes that when she opened the kiln, she found the pieces of the plate spread apart as seen in the photo, like it popped apart from shear force.

In the Studio

Cristobalite formation in fired ceramics is associated with insufficient flux in the body to produce a significant glass phase in the fired work. Clay bodies that develop a strong glass phase tend to have little to no cristobalite after firing.

Even if there is sufficient flux to vitrify the ware, if the body is poorly mixed, there can be clusters of quartz grains with no flux mixed into them. Those clusters behave just like an inadequately fluxed body and cristobalite can form.

A kiln firing fundamental has always been that heating or cooling ware that contains silica sand, which is almost always present in stoneware and porcelain, should begin slowly prior to and progress slowly through the end of the temperature range for quartz inversion. The same is true for the temperature range of the cristobalite inversion if there is reason to believe there is significant cristobalite in the ware. Although it is not well documented, some high-iron clays have been reported to contain cristobalite.

How Cristobalite Can Help or Hurt

Glaze fit is an ever-present challenge for ceramic artists. Ware that swells over time because it absorbs moisture is especially problematic. Industrial plants sometimes add cristobalite to a clay body in a carefully controlled amount with the intent to have the fired ware shrink slightly during the cristobalite inversion and place the fired glaze in compression. When successfully executed, that method produces strong, durable ware. There is a risk, of course. Too much compression will result in shivering.

This method is generally not used with oven ware. While average oven cooking temperatures are below that of the cristobalite inversion, if that temperature is exceeded, a cracked pot can be the consequence of the thermal stress.

Avoiding Cristobalite Formation

The same transformation of quartz sand into cristobalite used in industry can occur in a clay body fired in a studio kiln. Quartz is usually present in the clay body and so are flux elements and sometimes iron. The conversion can begin at as low as 1634°F (890°C) but cristobalite forms primarily above 1922°F (1050°C).

The ideal temperature for cristobalite formation is between 2012°F (1100°C) and 2102°F (1150°C), according to Dr. William F. Carty, professor of ceramic engineering at Alfred University. He has said a mixture of fine quartz with a small fraction of sodium carbonate will convert to cristobalite in two hours at that temperature. Once the cristobalite forms in a clay body, it tends not to dissolve into the glass phase, notes Carty. “Once it forms,” he says, “you’re stuck with it.”

Multiple firings can increase cristobalite formation if the body chemistry is such that it will occur at all. Any step that increases the length of time the ware is within the temperature range where cristobalite forms will increase its development, provided the body chemistry is favorable to that formation.

Clay bodies that are well mixed are essential to avoiding cristobalite conversion. Clay bodies that become well vitrified and nonporous at peak firing temperature are less likely to produce cristobalite. Firing rapidly through the temperature range that favors cristobalite formation also helps prevent its formation.

Note: I know of no way to detect cristobalite in a clay body other than firing a sample suitable for testing with a dilatometer and then getting the dilatometer trace (2), which would, of course, show a bump up at around 486°F (252°C), or grinding up the fired sample and testing the powder using X-ray diffraction (XRD) to look for cristobalite peaks in the XRD graph.

the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his masters degree in ceramic engineering from Alfred University.