Clay is impor tan t in the construction indus t ry both as a building material and as a foundation for structures. Buildings and utensils made of clay date back to the earliest periods of m a n ' s civilized development, and the use of clay is int imately associated with his history. Sun-baked clay structures were built during the pre-European period of American history. In the southwest of the Uni ted States by about 500 B.C. pot tery was made from sun-dried clay. Bricks were made from the same material, which is durable under semi-arid conditions, and great communi ty houses were built. Bricks were commonly used in Europe at the t ime of the early European set t lement of North America and fired bricks were produced a t an early date. The widely scattered and rather sparse population of this period favoured small-scale production designed to meet local demand. As a result of the increase in the density of population and concomitant exploitation of resources, extensive scientific surveys were carried out and available raw materials catalogued. W i th the development of unders tanding of the t rue nature of clay it became possible to correlate composition with the behaviour of clay on firing. By comparing the effect of different brick-maklng processes on one type of clay it was shown tha t the effect of manufac tur ing methods m a y be less than had been supposed (Butterworth, 1949). Semi-dry pressed bricks were found to be only slightly more porous t ha n wirecut or stiff-plastic pressed bricks when the same clay was used. Differential thermal analysis was employed as a guide not only to the mineralogical composition of the material bu t also to indicate its ceramic properties. This was possible as sufficient experience had been gained in trial runs in which the clay was examined on the thermal analysis equipment and the curve then correlated with its ceramic behaviour. Certain general principles emerged from this work. I t was found tha t montmoril lonite or illite confer high plasticity and high shrinkage and tend to be non-refractory, with a short vitrification range, while a luminum hydroxide or kaolinite confer refractory properties and have a long vitrification range (Grim and Rowland, 1944). An al ternative approach to taking a natural ly occurring clay and a t tempt ing to correlate its behaviour on firing with its mineralogy is to s tudy the effects of heat t reatment on artificial mixtures. For example, studies have been made on co-precipitated gels of silica and alumina. Of considerable practical importance to the building indust ry has been the production of lightweight aggregate. I t was found tha t certain clays on firing have the property of swelling or "bloating", becoming light in weight yet s trong and durable. Much work has been done to produce commercially feasible lightweight aggregate in areas where the clays are of the non-bloating variety. The addition to such clays of compounds of iron, alkalies, alkaline earths, carbon, and flowers of sulphur often induces bloating. The essentials for success are tha t a viscous melt mus t form during evolution of gas.