Chemical analysis and points of fusion of a few of the fire clays from which curves were drawn are as follows: 'Chemical analyses were not made of all the clays of the No. 1 and No. 2 type and none of the No. 3. From the few that were made, how ever, it is evident that refractoriness and slow fusion are not always de pendent upon tht3 proportional content of alumina and silica, for the two No. 2 fire clays have on the average higher A1.03 and lower SiO3 content that the No. 1 fire clays. This is directly contrary to our past teachings and contrary to what might be expected from Segar's Al203 SiO= curve, as shown in figure 19, page 208.
Paving and Building Brick Clays—The standardization of tests for first-class paving brick clays has been and perhaps will be for some time the subject of consideration by ceramic investigators. The pyro-physi cal and chemical tests here reported can be said to give negative rather than positive information, in that they very effectively differentiate the clays they cannot from those that may be utilized in paving brick man ufacture. Judging from the results so far obtained, they fail, how ever, to differentiate the paving brick clays one from another in re gard to their comparative quality. For example, we have not been able to distinguish by these tests between the clays of 14 per cent and the 24 per cent type, measured in per cents of loss in the rattler test, nor between the clays that preserve their maximum strength through a wide heat range and those which attain and preserve their maximum strength only within a very narrow heat range.
The cause of failure of the pyro-chemical studies in this respect is, no doubt, to be found in the fact that inherent strength is not wholly a function of rate of vitrification or development of vesicular structure. As shown in earlier pages, physical tests on the raw clays failed to dif ferentiate paving from building brick clays. The pyro-chemical studies here reported are the only ones that give any clue to cause of toughness Or strength of the burned ware.
Pyro-chemical studies similar to those here outlined, together with a determination of the maximum strength and the range of temperature in which this maximum strength is developed, would enable the 'observer to properly classify and differentiate paving brick clays. This, however, amounts to a sub-classification of the paving clays on a basis different from that of the main sub-division.
The striking differences between the building and paving brick clays are apparent from figures 32 and 33. Earlier vitrification, irregularity
in decrease of porosity and specific gravity, apparently larger quantity of vessicular glass formed within the mass, or at least a more notable due to volatilization of certain constitutents, probably the sol uble and adsorbed salts, are the distinguishing features of the strictly building brick class.
Sufficient evidence is at hand to warrant the statement that any clay which vitrifies to a porosity of 2 or 3 per cent before cone 5 is reached, in the heat treatment prescribed in this method of burning test pieces, will be too brittle for use as paving brick material, no matter how little vesicular structure is developed. The fact is, however, that -it will be a rare case in which vesticular structure is not strongly developed if the clay shows an early' and rapid rate of vitrification.
In figures 32 and 33 are shown the upper and lower limits of areas that were traversed respectively by the porosity and specific gravity curves of clays that either are being or can be used for the purposes in dicated in the figures.
The boundary limits shown in these figures are those obtained in these tests, and, therefore, may not show exactly the true limits of the several areas. They indicate, however, approximately the relative man ner in which the clays used for the several industries behave in fusing.
All clays used for paving and sewer brick may be used for building brick, but what are here defined as strictly building brick clays cannot be used for paving or sewer brick. All paving brick clays can be used in the manufacture of sewer brick, but the sewer brick clays cannot be used for paving brick. The points of differentiation are; first, the paving brick clay fuses more slowly and decreases less in specific gravity; second, the sewer (and side walk) brick clay fuse more rapidly but maintain their shape through a considerable range of heat treatment before failing; third, those clays which are fit only for building brick vitrify rapidly and fail as soon as, or before they are completely vitri fied. The sewer brick clays can be brought with safety to complete vitrification without much danger of Toss except perhaps from "kiln marking" while those clays which are fit only for building brick bloat and become spongy as well as soft almost as soon as vitrification takes place.