Since the tracing of the porosity curves through the upper or paving brick clay area does not necessarily signify that they are good for paving brick manufacture, the lower limit may appear to be superfluous. It remains a fact, however, that, according to the tests here reported, a clay must have its porosity curve confined within the limiting bound aries shown in order to develop the required toughness. So far as ex perience with the Illinois clays is concerned, the curves for porosity and specific gravity in figure 29 and 30 respectively, denote quite rig idly the allowable variation in rate of decrease in porosity and specific gravity.
In the preceding discussions of physical, chemical, and pyro-physical and chemical properties of clays all of the relations between these prop erties that were known or observed have been shown. A review of these discussions reveals the following as being the most important.
1. Measurement of some of the properties failed to give results that show the factors which affect them or which are involved with them. This was made plain in case of the "individual grains" as obtained by mechanical analysis. We have seen that the methods universally em ployed to effect the physical disintegration of clay are not sufficiently intensive to produce complete disintegration. It has also been demon strated that the grains or particles so obtained do not usually consist of one mineral substance. As a consequence of this cementation of smaller particles of different substances into bundles or groups, any in ference or conclusion based on fineness of grain cannot be very general in application.
2. Ultimate analysis or gross rational analysis of clay cannot reveal qualities that affect either the "working" or "burning" properties.
3. Either ultimate or rational analysis of the several groups of grains may reveal some important relation of constitution to manifested prop erties. This, however, remains to be demonstrated. It can be said however, that such determinations will not likely become "commercial" tests of clays. On the other hand, however, it seems certain that they will be valuable for research purposes.
4. Vitrification behavior, rate of fusion or toughness of bricks, do not seem to depend within any but very wide limits or in any traceable manner upon chemical or mineralogical constitution of clay.
5. No combinations of physical and chemical properties can be said to be essential to clays from which first-class paving brick may be manufactured.
6. The most satisfactory tests tried or developed during the course of these researehes for distinguishing between clays on the basis of their commercial availability are rate of decrease in porosity and specific gravity. While even these tests, so far as can be judged by our results, do not make an absolute discrimination, the discussions and curves here given make plain the fact that such tests are the most serviceable of any so far developed. The other tests have special uses and are not to be entirely condemned.
7. Toughness of brick does not bear a consistent relation to degree or range of vitrification. Each clay has its own peculiar range and de gree of vitrification at which its maximum toughness is developed. In some clays this range is very small and in some quite large. In some clays maximum toughness is attained when the brick still shows an ab sorption of 8 or 12 per cent and in others not until the absorption has been decreased to 2 or 4 per cent. No tests other than the "rattler test" on full size brick which have been burned with different intensity of heat treatment have brought out data which bear on this peculiarity of clays.
8. The pyro-physical studies which have been described suggest a series of determinations which should be more valuable in that they ought to reveal the cause for this want of correlation of toughness and vitrification behavior. The series of determinations referred to is that of the volume changes which take place with increasing intensity of heat treatment.' The volume changes which are important are : (a) exterior volume of (b) volume of skeleton of brick.
(c) volume of open pores.
(d) volume of sealed