Digitalfire Ceramic Glossary



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Quartz Inversion


The term "quartz inversion" is used in two ways. Often, people are simply referring to the temperature 573C. More likely they are referring the phenomena that occurs there: The sudden volume change that particles experience as they pass up and down (thus it is called an inversion) through 573C. Actually it is not quite this simple. Many graphs of quartz thermal expansion vs. temperature can be found using google, however these vary so the picture is not so clear as the 573 degree designator suggests. However the general pattern shows that quartz is expanding significantly through the entire 0-600C range, but it climbs alot faster through a narrow window of temperatures centering around 550C (about a 50C range). Obviously many factors are involved, so we are best served by speaking in general terms about this topic.

Quartz inversion is not an issue with glazes because the particles dissolve in the melt and become the SiO2 part of the glass structure (glass expands and contracts on heating in a linear fashion). But with clay bodies it is different, only some of the quartz particles dissolve in the feldspar glass. This is such an issue in bodies because the quartz particles have a different thermal expansion and contraction profile than the brittle ceramic matrix in which they are embedded. The population of the quartz particles, their size and the degree to which they have remained intact as crystalline quartz determines their ability to impose their thermal expansion on the matrix as a whole.

Quartz inversion becomes the greatest concern when there are sufficient large particles of free quartz available and heat is not evenly distributed through a piece of ware in the kiln. When this happens parts of it are hotter or colder (because of differences in thickness, rapid temperature rise or fall, dampening against the kiln shelves, uneven exposure to draft or radiant heat, remnants of variations from earlier parts of the firing). Only the quartz particles in the part of the ware within the critical temperature window are changing in volume. Thus, to the degree to which a piece is unevenly heated the expansion/contraction moves through it as a wave. Another major determiner of whether ware cracks as it passes through quartz-inversion temperature window is the density and homogeneity of the matrix in which the quartz particles exist and the degree to which they are bonded to the surrounding matrix. During kiln heat-up the particles generally exist in a matrix having plenty of pore space to absorb the change (unless the piece has previously been fired to a high density). But on cool-down the fired ceramic around each quartz particle is much less capable of absorbing its volume change, especially if the particle is of significant size and the temperature drops rapidly through critical range. The problem is doubly serious when ware is re-fired since the ware further densifies and the quartz particles have had opportunity to bond more into the surrounding matrix.

The cristobalite form of silica also experiences inversion, but at a much lower temperature and through a narrower range. On a graph comparing the two, the quartz line is much less dramatic, it would thus appear that cristobalite is something to be avoided.

Pictures

Example of dunting, where a crack has released the stresses produced by uneven thermal contraction during cool-down in the kiln. This usually happens by cooling too quickly through quartz inversion.

Shivering on a transparent over an engobe

Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body. But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerting added compression on the slip. This causes a failure at the slip-body interface.

An unevenly cooled tile has cracked

Example of a severely dunted cone 6 stoneware tile. This problem was deliberately created by stacking several tiles on top of this one. This set up a temperature gradient across it so that different parts passed through quartz inversion at different times.

What would happen if you made a body from 50:50 kaolin:ball clay?

It would craze glazes! This is fired at cone 6 and the crazing was like this out of the kiln. This is about as bad as I have ever seen. One might think that there is adequate quartz in this high of a percentage of ball clay to at least minimize crazing, but no so. This demonstrates the need for adequate pure silica powder in stoneware bodies to give them high enough thermal expansion to squeeze glaze on cooling to prevent crazing like this. This is also not proving to be quite as refractory as I thought, it looks like it will have about 3% porosity at cone 10.

Why did this piece came out of a decal firing crazed?

This Cone 10 matte mug has been refired to attach decals. During the refire the Quartz-containing body passed up through quartz and cristobalite inversions while the glaze did not (all of its quartz was converted to silicates during the previous glaze firing). The sudden expansion in these two zones stretched the glaze and cracked it. Had that glaze been better fitted (under some compression) it would have been able to survive.

Dilatometer curve of vitreous porcelain (red) vs. stoneware body

The 500-600C zone is the alpha-beta inversion of quartz. Notice the vitreous body experiences a bigger expansion change there. But in the 100-270C cristobalite inversion region the stoneware undergoes a much more rapid change (especially in the 100-200C zone). This information affects how ware would be refired in production to avoid cracking (slowing down in these two zones). In addition, that stoneware would not be a good choice for an ovenware body. Photo courtesy of AF

Thermal shock failure in raw ball clay much worse than the 100 mesh material

The cup on the left is raw, unground, ball clay (Plainsman A2 fired to cone 10 reduction). It cracked under a flame in only 4 seconds. The 200 mesh version on the right lasted 14 seconds (it is broken because I dropped it). It would appear that the larger quartz particles in the material on the left are imparting much less resistance to thermal shock failure.

Out Bound Links

In Bound Links


By Tony Hansen




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