In Fig. 30 are shown the limits of area traversed by porosity-graphs of the fire clays. The fire clays are grouped into three classes according to their rate of decrease in porosity.
Number One Fire Clays—The writers of Clay Reports have heretofore failed to recognize that of two clays having similar ultimate chemical compositions and similar ultimate fusion periods, one can be used in No. 1 fire brick, while the other would fail as a first-class fire brick and the one failing as a fire brick material would be the only one that could with success be used ih the stoneware industry. Sev eral examples of the foregoing were noted in the examination of the Illinois fire clays. In fact, the case is not an uncommon one.
In fire brick, maintenance of an open structure through the entire heat range used in the various ceramic industries is essential. On the other hand, in stoneware, closeness of structure at comparatively low temperatures, or early vitrification followed by a long fusion range is absolutely required. It is evident, therefore, that a classification of re fractory fire clays (so called because they withstand heat equivalent to cone 27 or more without failure) should take account of this dif ference in their manner of fusion. This essential difference in the be havior of fire clays is recognized in a tentative scheme of classifica tion presented by the present writer and Mr. Moore.' It will be noted from Fig. 30 that these clays show comparatively little decrease in porosity from cone 010 to cone 11. This decrease averages from 7 to 15 per cent of the initial porosity and in no case does it exceed 17 per cent.
The specific as shown in Fig. 31 remains fairly constant from cone 010 to cone 3 and then, even in the purest clays, it begins to decrease slightly. This decrease in specific gravity in the No. 1 fire clays, even when the porosity remains very is considered as evidence of the influence of the adsorbed or cementing salts which, while constituting but a very small part by weight of the whole, are nevertheless potent factors in causing fusion.
Number Two Fire will be noted that while the decrease in specific gravity of this group of clays is about the same as that shown in the No. 1 fire clays, the porosity shows a. much larger decrease. The
earthy vitrification and slow fusion is quite pronounced in this group. permitting their use in the paving brick, sewer pipe, stoneware and terra-cotta industries, but not in the manufacture of No. 1 fire brick.
Number Three Fire Clays-1n Figs. 27 and 28 are shown the limiting area of porosity and specific gravity curves of a class of clays which, in the judgment of the writer, ought to be put in a different category from the preceding group, or number two fire clays. Heretofore, both have been classed together indiscriminately in ceramic and geological literature, as number two fire clays, but they are not the same. Clays of this class differ from the .No. 1 and No. 2 fire clays, in that they seldom have a fusion point exceeding cone 16 or 17, fuse in a very irreg ular manner, and exhibit a much larger decrease iii specific gravity owing probably to the presence of iron in nodular form as sulphides or carbonates.
Fire Clays in conclusions may be summarized as fol lows: First, While all the types of fire clays here tested maintained the same range in porosity up to cone 010, there is a marked differ entiation of each at cone, 08. Second, From cone 08 to about cone 1 the No. 2 and No. 3 fire clays traverse a common area, but at cone 1 the No. 3 type begins to fuse more rapidly, until when cone 7 is reached, the No. 3 fire clays have fused sufficiently to be wholly differ entiated from the No. 2. Since the porosity curves in Fig. 27 are composite curves showing the limits of variation in the few clays tested, it is possible that broader limits will be determined when more and a larger variety of clays are tested, yet the data here presented are sufficient to demonstrate that where chemical analysis and fusion period determinations have failed, this method of differentiation has proved successful. Fourth, Differentiation of fire clays on the basis of specific changes will hardly be possible on account of the limited differences between the areas traversed by the specific gravity curves of each type of fire clay, yet as is shown in Fig. 28 the specific gravity curves parallel and diverge from one another at about the same temperatures as do the porosity curves in Fig. 27.