|Monthly Tech-Tip |
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.
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 happens a lot faster through a narrow window of temperatures centering around 550C (about a 50C range). One technician made an interesting comment about the endothermic nature of the upward process: "I am amazed at the amount of latent heat absorbed by the inversion. The temperature just stopped going up for about an hour or so, and then proceeded on up, with no change of setting". 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, and glass expands and contracts on heating in a linear fashion (that being said, quartz particles that are not small enough can remain undissolved). But with clay bodies it is different, only the smaller quartz particles dissolve in the feldspar glass. Those 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.
What does it take? Three months! Porcelains are fine grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a piece like this down through quartz inversion (~1200-900F) without dunting is only possible if done very slowly. The fact that ancient Chinese potters made very large porcelain pieces means they knew about slow cooling also (and it was a natural consequence of the heavy kilns they used). But our modern kilns cool quickly so the drop must be slowed. Peter adds an extra level of "super humanness" by actually lustre firing these pieces, that means 2 more trips through the hazardous quartz inversion territory! If you do this be prepared to fire super slow (e.g. 25 degrees per hour) through this range.
The cracks happened on heat up (since they have opened up wide). A combination of issues contribute. The kiln shelves heat-sink the wide flat bottoms, vessel walls are thick, there is some unevenness of wall thickness and only a 30-minute hold at 220F to remove glaze water from the bisque (that could have left dampness in thicker sections). Factors like these combine to produce temperature gradients within the piece. The firing schedule rose rapidly from 250-2100F (400F/hr) amplifying these gradients as it climbed. At quartz inversion these gradients produced a wave of volumetric change moving through the bisqued piece and this likely initiated a crack where stresses met at a combination sharp-contour and thickness-change, the bottom corner.
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
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.
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.
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.
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.
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.
The firing crack from the rim down has released the stresses produced by uneven thermal contraction during cool-down in the kiln. Any factor that contributes to a temperature gradient within a piece will contribute to the likelihood of dunting. Cooling too quickly through quartz inversion, for example, can cause this in almost any piece. Pieces that are thick and heavy, or have uneven cross section (with thick foot and thin walls, for example) will certainly suffer gradients, even in slow cooling. A wide, flat bottom (that is heat-sunk by the a heavy shelf) will also increase the temperature gradient between the outer walls and the inner foot. If that wide piece has vertical walls that get direct radiant heat, especially if one part is more exposed to the elements, it will start a gradient during the up-ramp in the firing. And, on the down-ramp, it will "come back to bite you" with a crack.
Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. While stoneware dunting happens between 950-1150F on the way down, this could be happening anywhere. A simple fix is to slow down the entire cooling cycle. Learn to program your kiln. Use a conservative cooling rate of about 200F/hr (even slower at 1150-950F). No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).
|Temperatures||Quartz inversion (alpha-beta) (540C-600C)|
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.
This process of printing a design (using ceramic inks) onto film-coated paper to create a waterslide transfer. On wetting, the film decal can slide off the backing on to the glazed ware.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
Dunting generally refers to firing cracks that occur in ceramic ware as it is cooled in the kiln. The reason is generally uneven cross section or too rapid cooling.
Wikipedia quartz inversion