Tests on clay materials available in Illinois coal mines . rning, but those that showed a black core of any considerable sizeat the end of the ten-hour period may be eliminated as commercialpossibilities for any type of ware. An exception may be noted in clays which remain quite porousat temperatures well above those usually used for oxidization anddo not vitrify. In burning some materials of this sort, ground coalis actually added to the clay to aid in the burning of the ware. Insuch cases only porous common brick may be made. Clays that crack or warp badly in burning can not be used forany w
Tests on clay materials available in Illinois coal mines . rning, but those that showed a black core of any considerable sizeat the end of the ten-hour period may be eliminated as commercialpossibilities for any type of ware. An exception may be noted in clays which remain quite porousat temperatures well above those usually used for oxidization anddo not vitrify. In burning some materials of this sort, ground coalis actually added to the clay to aid in the burning of the ware. Insuch cases only porous common brick may be made. Clays that crack or warp badly in burning can not be used forany ware save possibly common brick, but these should be discardedwhere any better material is available. BEHAVIOR OF CLAYS DURING BURNING 13 Pitting due to granules of iron compounds or due to the slakingof lime lumps in the burned clay when exposed to moist atmosphereis undesirable in any ware but does not eliminate a clay as a possibilityfor common brick and hollow ware unless it be excessive. Front brickand pavers must be sound and free from such 06 M ^2 5 3 5 TEMPERA TV RES EXPRESSED IN CONES Fig. 1.—Curves showing changes in porosity of paving and building-brickclays with progressive intensity of heat treatment. The most important criterion in determining the commercialavailability of a clay lies in a proper interpretation of the porosity-temperature relation in burning. An excellent discussion of this hasbeen given by The decrease in porosity indicates the progressof hardening and vitrification of the clay, the range of safe burning -Jurdy, Ross (., Paving brick and paving brick clays of Illinois: 111. State Bull. 9, 1908. 14 CLAY MATERIALS IN ILLINOIS COAL MINES temperatures, and the point of overburning. It will be noted in thecurves that many of the clays show an abrupt drop in porosity withina short range of burning temperature and a subsequent increase inporosity after the minimum has been reached. The rapid decreasein pore space is due to
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcoalmin, bookyear1917
