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25 April 2005
The purpose of this note is organize and present information on the role of silica (SiO2) in pottery. Initially this investigation is driven from a desire to understand the nature of the 'cristobalite hump' seen in stoneware claybody dilatometry curves. The first thing that appears to be necessary is to understand the various forms of silica. Secondly, it will be necessary to determine which forms exist in a fired piece of pottery, and in what quantities.
Of primary interest in functional pottery, is glaze fit. If a glaze fits the body it neither crazes nor shivers. Such a glaze increases strength of the ware. If a glaze and a body do not fit each other, either or both may be modified to achieve good fit. Fit is a problem of mismatched COE in the body and the glaze. Dilatometry performed on the body and the glaze will show the expansions at various temperatures, and overlaying the two charts will demonstrate the likelihood that they are well suited to one another.
The clay body dilatometer curve is not straight. Additionally, it will possess one (porcelain) or two (stonewares at cone 10) noticeable humps. The lower temperature one is the cristobalite hump, or inversion, at or about 220° C, but extending up to 280 ° C in some cases. The higher one is the quartz inversion. These inversions usually (for stoneware and porcelain pottery) occur below the set point of the glazes. They thus are instrumental in ensuring sufficient compression to prevent a glaze from crazing, but must not induce so much compression as to precipitate either shivering or dunting.
In the chart below is the dilatometer curve for Laguna Soldate 60 fired to cone 10 (5 o'clock) in oxidation. The two inversions are clearly visible. The primary focus of this paper initially is the lower, the cristobalite inversion. Reduction of this hump will tend to lower the overall expansion of the body, reducing the chances of shivering or dunting.
FIXME - supply a photograph of this from the soldate 60 chart. - explain it.
Numerous references to the forms or phases of silica may be found. It is difficult, however, to find a complete list. The table below lists the forms, that I am currently aware of, with a short description.
| Phase | Description | Forms at | Inverts at |
|---|---|---|---|
| &alpha-Quartz | The most common crystalline form of silica, this is the well known clear, smoky or pink crystal. | 573° C | |
| &beta-Quartz | This is a higher temperature form of quartz. It appears with a volume changing reversible inversion at about 573° C. | 573° C - 1470° C ?? | |
| &alpha-Cristobalite | This is low temperature cristobalite. It inverts reverisbly to &beta-cristobalite at about 225° C. At this point the &beta phase is said to be disordered rather than pure cristobalite. | 225° C | |
| &beta-Cristobalite | This is high temperature cristobalite. At this point the &beta phase is said to be disordered rather than pure cristobalite. This appears to arise from the fact that there is some irregularity in the orientation of silica tetrahedra within the lattice. FIXME - confirm this. | 225° C - 1470° C ?? | 225° C |
| Tridymite | Tridymite is a relatively rare form of silica which is said to possess nine subdivisions or forms1. I do not yet have a list of these though they are said to be the reason that the cristobalite hump in clay bodies is not sharp, but rather gradual over the range of 200° C to perhaps 280° C. | ?? | ?? |
| Coesite | |||
| Stishovite | |||
| Perovskite ?? | |||
| Silica |