Whether the pottery of the aborigines was hardened by fire or cured by the simple process of drying, has been made a question. Prof. E. T. Cox, of Indianapolis, has shown by comparing the analyses of ancient pottery and hydraulic cements, "that so far as chemical constituents are concerned it (the pottery) agrees very well with the composition of hydraulic stones." He remarks further, that "all the pottery belonging to the mound-builders' age, which I have seen, is composed of alluvial clay and sand, or a mixture of the former with pulverized fresh-water shells. A paste made of such a mixture possesses in a high degree the properties of hydraulic Puzzuolani and Portland cement, so that vessels formed of it hardened without being burned, as is customary with modern pottery. The fragments of shells served the purpose of gravel or fragments of stone as at present used in connection with hydraulic lime for the manufacture of artificial stone."[7] The composition of Indian pottery in analogy with that of hydraulic cement suggests the difficulties in the way of inventing the art, and tends also to explain the lateness of its introduction in the course of human experience. Notwithstanding the ingenious suggestion of Prof. Cox, it is probable that pottery was hardened by artificial heat. In some cases the fact is directly attested. Thus Adair, speaking of the Gulf Tribes, remarks that "they make earthern pots of very different sizes, so as to contain from two to ten gallons, large pitchers to carry water, bowls, dishes, platters, basins, and a prodigious number of other vessels of such antiquated forms as would be tedious to describe, and impossible to name. Their method of glazing them is, they place them over a large fire of smoky pitch-pine, which makes them smooth, black and firm."[8]
Another advantage of fixing definite ethnical periods is the direction of special investigation to those tribes and nations which afford the best exemplification of each status, with the view of making each both standard and illustrative. Some tribes and families have been left in geographical isolation to work out the problems of progress by original mental effort; and have, consequently, retained their arts and institutions pure and homogeneous; while those of other tribes and nations have been adulterated through external influence. Thus, while Africa was and is an ethnical chaos of savagery and barbarism, Australia and Polynesia were in savagery, pure and simple, with the arts and institutions belonging to that condition. In like manner, the Indian family of America, unlike any other existing family, exemplified the condition of mankind in three successive ethnical periods. In the undisturbed possession of a great continent, of common descent, and with homogeneous institutions, they illustrated, when discovered, each of these conditions, and especially those of the Lower and of the Middle Status of barbarism, more elaborately and completely than any other portion of mankind. The far northern Indians and some of the coast tribes of North and South America were in the Upper Status of savagery; the partially Village Indians east of the Mississippi were in the Lower Status of barbarism, and the Village Indians of North and South America were in the Middle Status. Such an opportunity to recover full and minute information of the course of human experience and progress in developing their arts and institutions through these successive conditions has not been offered within the historical period. It must be added that it has been indifferently improved. Our greatest deficiencies relate to the last period named.