Residual Quartz

Residual quartz is the crystalline quartz that remains undissolved in a fired ceramic body. It is not the same thing as the total SiO₂ reported in a body chemistry. Much of that SiO₂ may be chemically combined in clay minerals, feldspars and other silicates, and some quartz added to the recipe may dissolve into the developing glass phase during firing. Residual quartz refers specifically to the quartz crystals that survive the firing.

This distinction is extremely important in porcelain and stoneware because residual quartz can have a major influence on the thermal expansion of the fired body. Quartz has a relatively high thermal expansion, and it also undergoes the well-known alpha-beta quartz inversion at about 573°C. Thus, a body containing appreciable residual quartz normally has a higher overall thermal expansion than the same body after more of that quartz has dissolved into the glassy phase.

This is one reason silica is deliberately added to many porcelain recipes. A typical porcelain might contain 20–30% added silica, much of it as quartz. Although some goes into solution during firing, enough normally remains to act as a relatively refractory aggregate within the fired matrix. That residual quartz helps establish the thermal expansion of the body and can therefore be important to glaze fit. Removing the silica from a porcelain recipe does not simply remove an inert filler; it can dramatically lower body expansion and make a previously fitting glaze craze.

The amount of residual quartz is not determined by the recipe alone. It depends on particle size, firing temperature, firing time, the amount and chemistry of the liquid phase and the degree of vitrification. Fine quartz dissolves more readily than coarse quartz. Higher temperatures and longer firing generally permit more quartz to enter the melt. A body that is fired repeatedly can therefore continue to lose residual quartz as additional silica dissolves into the glass phase.

This helps explain why refiring can change glaze fit. If a porcelain or vitreous stoneware is already highly fired, another firing may dissolve additional quartz without producing an obvious change in appearance or porosity. Yet the fired body's thermal expansion can decrease. A glaze that was adequately compressed after the first firing could thus move closer to crazing after subsequent firings. Highly vitreous bodies, including sanitaryware and porcelains subjected to repair firing, deserve particular attention in this respect.

Residual quartz also presents a mechanical complication. Individual quartz grains change dimensions abruptly as they pass through the quartz inversion. If grains are large, the surrounding glassy matrix must accommodate that movement, creating localized stresses and potentially microcracking. This is why coarse silica particles can contribute to dunting and why quartz particle size matters, not just the percentage in the recipe. Nevertheless, ceramic bodies can contain substantial quartz without simply cracking apart around every particle: particle size, distribution, matrix elasticity, porosity and the compressive stresses that develop during cooling all influence whether those stresses become destructive.

Stonewares commonly contain residual quartz even when no silica has been intentionally added. Natural clays often contain quartz as an accessory mineral, sometimes in significant amounts. Thus two stoneware bodies having similar porosity and similar calculated chemistry can have quite different thermal expansions because their mineralogy and residual quartz contents differ.

For this reason, calculated oxide chemistry alone cannot reliably predict the thermal expansion of a fired clay body. Glaze expansion calculations are useful because glazes are largely glassy. Fired bodies are multiphase materials containing glass, mullite and other crystals, residual quartz and sometimes cristobalite. Their thermal expansion is therefore a product of the proportions, particle sizes and thermal behavior of all of these phases.

A useful way to think about residual quartz is this: silica in the recipe is a starting quantity; residual quartz is what survives the firing. It is the latter that matters most when considering quartz inversion, body thermal expansion and its contribution to glaze fit.

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By Tony Hansen
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