Tile stacking in an electric kiln - Fingers crossed!

Small-scale operations everywhere are making tile like this. Most use plastic clay intended for pottery, this introduces more drying shrinkage complicating drying them flat. Stacking them in the kiln can be a game of chance. Stacked too tightly and they crack (mostly because of quartz inversion). Stacked to loosely and most of the energy goes into heating the shelves and stackers. Using a clay with minimum large quartz particles is the best way to avoid dunting, however that is also a balance since such clays are more difficult to fit glazes to (without crazing).

Related Pictures

Tile having angular shape is cracking at vertexes

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Cracks are happening during heatup (we know this because these have widened, cooling cracks are hairlines). Assuming the cracks were not there after drying, the piece likely could have been fired successfully by employing a slower heat-up. How to solve: Smooth, compress and round those sharp concave angles. This denies cracks a place to start. Even if the concave corners could have been rounded and compressed to a tiny radius (e.g. 2mm), it would likely have helped. And rounding and compressing the sharp edges would help (there are a couple of cracks on the straight sides).

An unevenly cooled tile has cracked

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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.

It possible to make a thin flat porcelain tile from a plastic pottery body

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Yes. The body is Plainsman M370 (~ 25 silica, 25 feldspar, 30 kaolin, 20 ball clay + talc to tune maturity), a plastic throwing clay with far too much drying shrinkage to be suitable for tile. It is 3.8 mm thick fired (vs. commercial tiles are 5-7mm) and 33cm (13 in) square. It dried absolutely flat between sheets of plasterboard. We have even achieved total flat drying at this size using Polar Ice. Bisque and glaze firing were on an alumina shelf in an electric pottery kiln (at 300F/hr up through quartz inversion on the glaze firing), a completely unsuitable method for firing tile evenly top and bottom. Cooling on both firings was free-fall in a fairly empty kiln. Yet, it is flat! And flexible enough that I could lay it on the cement floor and stand on it without it breaking! Of course, to produce these consistently, special furniture that sinks minimal heat and a kiln that can evenly apply it front and back are needed. This is doable for custom applications. Of course, to compete in the commercial market, they need to be dust-pressed and there are lots of specifications to meet.

A plastic pottery clay for rolling ceramic tile:

Not a crazy idea when it can do what this can!

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This is the dolomite body recipe L4410P (a development version of Plainsman Snow). It is monoporosa tile on steroids; this body has zero percent firing shrinkage at cone 04! Predicting the final size and keeping that size consistent is much easier with such bodies. I have measured its drying shrinkage as 6% (doing our standard SHAB test). The final size needed is 20.5 cm square. Thus, I calculate the cut size to be 20.5 / (100 - 0.06) = 21.8 cm (or 20.5 / 0.94 = 21.8 cm). To keep these flat, we put them between sheets of drywall; the process takes 2-3 days. Since no change in size occurs during firing, this body has another big advantage: Tiles stay flatter during firing (a major problem with tile production). While making wall tile using a plastic pottery body is not something for industry (especially because of the space requirements for drying), for artisans working on a small scale, a body made by mixing super plastic ball clay with dolomite produces amazing working and tactile properties. The bonus is that they work so well at low temperatures, where there are so many glazing options.

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