The Reversible Expansion of Refractory Materials
T201-T222 (22 pages including 9 discussion pages)
The results disclose great differences in the behaviour of different refractories when heated up to 1000°. A number of these, including calcined alumina, magnesia brick, carborundum, calcined kaolin, and hard calcined ball-clay expand regularly over the whole range. These results do not, therefore, suggest the existence of any molecular changes. The behaviour of silica (quartz) is in marked contrast, in that the inversion of α to β quartz, accompanied by a very large expansion between 500° and 600°, is succeeded by a range from 600° to 1000° through which the dimensions are practically invariant. Specimens of burnt fireclay containing free silica reveal the same change, but in a somewhat less marked degree. Some clays resemble more nearly kaolin itself, while others approach in this respect the behaviour of silica. A mixture of half fireclay and half silica containing approximately 80% of silica behaves like silica. Specimens out from glass pot fireclay mixtures were examined and were not found materially different from the fireclays. It is evident that pots made from kaolin ball-day mixtures or from clays containing no free silica (quartz) will be quite different in their behaviour on heating and cooling from pots of which the basis is fireclay, and also that great care will be necessary in the treatment of pots of the latter description in the range 500° to 600°. A remarkable feature of the results, not easily accounted for, is the contraction just below "600° on cooling clay and silica which actually exceeds the corresponding expansion on heating. With silica and fireclays this is especially marked, and suggests that a rapid cooling of ware or glass pots through the region 600-500° must be fraught with quite special risk of cracking. The behaviour of raw clays and of silica in forms other than quartz is under examination. Most refractories examined expanded roughly 0·5% up to 1000°, but magnesia brick more than twice this amount.
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