CHEMISTRY IN PREHISTORIC AMERICAN ARTS
Simple chemistry, as a result of long observation rather than as a sciace, urns of considerable importance to primitive people who employed it in the production of their arts. Pottery of prehistoric America illustrates a practical comprehasion of the selection and preNration of clays and tempering material that, under their crude firing, would produce shapely water-proof vessels. Even more exact knowledge i s postulated by the use of mineral &nts i n decoration, for many of these change color in firing. Certain plant juices were used in combination with hematite to produce a certain black pigment. Through chemical tests we can distinguish several types of black N i n t s which prme to be churacteristic of definite areas and of aid to the archaeologist in locating @.eces of pottery.
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The importance of chemistry in the advances made toward the material comfort and progress of our modern civilization has almost obscured the part played by that science in the development of primitive man. The prehistoric nomad was compelled to roam over a large territory to obtain his food of nuts, berries, and game. He developed the art of basketry, but he knew nothing of pottery. He could not have used clay vessels had he discovered how to make them, for fragile pottery will not endure extensive transportation. Increase in population and the consequent decrease of area over which an individual might hunt his food was probably responsible for the beginning of agriculture. Agriculture, in turn, was directly responsible for the sedentary life adopted at that period and for the development, first, of pottery; next, of organized community life necessary for the control of irrigation systems and for the direction of people gathered together in villages; and, finally, of certain other arts, crafts, and saences. The farmer built his crude shelter and storehouses a t the edge of his field, and from the clay in his dooryard the women of his family learned to make pottery. The germ of the idea must have slowly made its way into the mind of some individual who idly rolled a ball of clay between her hands and then pushed her thumb into the center to form a depression. Yet we must concede genius to that one who saw practical possibilities in the lump of clay with the hollow center, for how many others had idly pushed their thumbs into balls of clay and seen no more! Progress in the art of ceramics was undoubtedly slow; many generations must have passed before it was discovered that the clay vessel could be fied and so made impervious to water. Hand-modeled vessels were crude in shape, but eventually the women found that the clay could be rolled into ropes or fillets, which when coiled upon themselves and pressed tightly together, would form a vessel of symmetry and beauty. 35
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Origin of Ceramics Pottery in the Pueblo area of the southwestern United States may have been an indigenous art, or i t may have developed here after the first idea had filtered up from the civilizations of Mexico where pottery had been made even a t a considerably earlier period. We cannot give the exact age of the art in the Pueblo area, but we do know that i t shortly followed the introduction of sedentary life based upon the cultivation of corn, a plant which had previously been brought up from Mexico or from Central America. The culture was neolithic, although later in date than that period of culture on the European continent. Primitive woman, of course, knew nothing of chemistry when she selected a certain clay for the pottery paste and another, or perhaps a vegetable extract, for the paint with which to decorate the vessel. Through accident and experiment she learned the type of material she must use to obtain certain desired results, and through the same trial-and-error method she learned the preparation of her materials. Nevertheless, she utilized her knowledge of the properties of substances in the manufacture of articles of beauty and utility. There were many chances of variation within the range of the possible material, and the differences in composition of clays coming from various regions led to the individuality of pottery cbaracteristic of any single area. The imprint of the individual potter never disappears from the potsherd, but that does not obscure the characteristic general style of a people or a period. And just as chemistry was used, although unwittingly, in the manufacture, decoration, and firing of pottery, so through chemistry can we now discover something of the details of its manufacture and of the extent of influence of the people who made it. The presence of the modern Pueblo Indians, descendants of the prehistoric people, offers opportunity for study of their primitive methods of pottery manufacture, methods that must closely resemble those of their ancestors. Materials Used in Potterj Baskets had been made and used in the southwest for some time before pottery was made, and i t is possible that clay used as lining for parching baskets may have baked and broken away from the basket in a bowl shaped crust which might have suggested the making of unbaked clay vessels. The early types of true fired pottery in Arizona and New Mexico are of gray or white paste. Clay, composed chiefly of silica and alumina, combined with water to form a hydrated silicate of aluminum, originally coming from decomposed granite or feldspar and deposited in beds by running water, also commonly contains mica, soda, potash, lime, and iron oxides. Kaolinite, the common white clay, is comparatively free from iron oxides. Although originally light gray in color, it becomes lighter during firing in
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direct proportion to the amount of carbonaceous matter, invariably present, which is oxidized a t high temperature or by extended firing. Vessels to be fired were first dried in the shade to drive off slowly the excess water in order to prevent warping or cracking of the walls; they were then stacked over the coals and subjected to the heat of a fire built beneath and around them. The temperature obtained in such an open fire may have averaged from 500' to 700°C. Mineral temper, which had been added to loosen the texture of the clay, allowed the escape of the constitutional water without the clay being fused or vitrified during the hour or more of firing and so . giving the vessel a bad shape and surface. Decoration of Pottery By the time these people had learned to fire their vessels, they had also learned to decorate them in crude and simple geometric designs, the earliest types apparently copied from those in common use upon their coiled baskets. The materials used a t this time for paint may have varied somewhat according to district, as in later periods, but uncompleted studies seem to indicate that a carbon paint or vegetable extract was characteristic of much if not of all of this early ware in northern Arizona. The custom of coating the surface to be decorated, or even of both surfaces, with a thin "slip" or wash of white clay greatly improved the appearance of vessels and made them less permeable to moisture. As the art of ceramics developed, pottery types became more specialized. Clays with greater content of iron oxides were used in areas where kaolinite clays were scarce or where more color was desired for variety. The inhabitants of the San Juan drainage in Utah, Colorado, New Mexico, and Arizona, and those of the Little Colorado and Upper Gila drainages of Arizona made black on white ware, hut those of the Little Colorado area and soine of those of the San Juan learned to make polychrome wares as well, using orange or red background slips. In southern Arizona the people specialized in red on buff and in the Gila polychrome ware, the black and white over a red-brown slip. In general the pottery pastes of northern Arizona (see Figure 1) were light in color, those of central Arizona somewhat darker, and those of southern Arizona (see Figure 2) distinctly brown in color. Variations of each of the pottery types show differences depending on locality and period of development. Pastes and Pigments Used in Pottery The chemical composition of the pastes was of great importance to the potter, for, in firing, the impurities in the clay change color, due to oxidation or dehydration, and some of the pastes would not hold the color of the designs superimposed upon them. Perhaps the most common impurities of the clay are iron oxides, which may fire to yellow, red, brown, or black,
FIGURE
1.-MIDDLE GILAA N D KAYBNTAPOTTRRY Carbon m i n t u.as used.
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FIGURB2.-TULARUSARUCK
O N WHITE\%'ARI: FROM SOWHIIRN ARIZONA Black paint is of carbon and iron oaidc.
depending on the kind and amount of the oxide present and the conditions under which it is fired. The clay used by the Hopi is gray in color when ground for use, but in firing it changes to a pleasing yellow, due to the presence of a small amount of yellow ochre, 2FezOZ.2Hz0,which changes to red or orange in spots that are over-fired. Limonite is ground and mixed with water to form the slip of their red ware, the 2Fez03.2Hz0 losing two molecules of water in the process and so leaving the red iron oxide color. These people claim to use no red clays or pigments directly except for body paints, but other Pueblo peoples do use red slips. Orange wares, red wares, and buff wares of the prehistoric days owe their shades to varying percentages of iron oxides; the brown pastes and slips are discolored by extraneous material, such as carbonaceous matter. Qualitative chemical tests check the ingredients of pastes, slips, and paints, but as the material from every deposit varies, such tests have seemed of little value except in definitely locating the area from which some vessels with distinctive paste came. This has been essayed with some success on the European continent but no use of it has been made in America, as colors and paints have served effectually for the same purpose. The composition of black paints, especia'ly, has been found to be so nearly constant in different areas as to serve materially as a distinguishing characteristic and thus as an indication of culture relationships. Chemical Composition of the Black Pigments The study of the constituents of pottery paints, especially the black, presented difficulties to the archaeologist, for analysis of the thin layer of
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paint laid over the slip necessitated separation of that paint from the slip itself, or the use of some test whereby the paint could be tested while still on the vessel. Observation of the materials employed by modem Pueblo potters offered leads and checks to the chemical examination. Pieces of pottery decorated in black were heated to redness in an open-draft electric muffle or with an oxygen torch and were kept a t that temperature for from five minutes to one-half hour. Care was taken that the surface was not vitrified. I t was observed that one type of black coloration burned off the surface even on short heating; the others were unaffected. This one type appeared to be carbon smudge, as was also shown by other tests for carbon. This coloration is not a true paint, but colors the entire interior surface of one type of ware through deposition of carbon or soot particles in the porous surface of the vessel during firing Such an effect is today secured by San Ildefonso potters who fire a vessel once, take it from the fire, and cover it with smoldering manure. Prehistoric potters might have used almost any carbonaceous matter for this process. Quantitative and qualitative tests indicate that while some of the FezOaof the red or yellow surface of the vessel is changed to Fez04 during this firing of the vessel in the reducin~atmosphere, the amount changed was entirely insufficient to account for the blackness. The amount of carbon detected was sufficient. Those sherds of pottery on which no change had been noted in black paint after heating were treated with a drop of hydrofluoric acid which was allowed to trickle across the surface. After the acid had dried, the sherds were again heated. Of these, one type showed the paint removed, a second showed the black gone but a brown stain left, and a third type showed no effectwhatever. The paint which burned off after the acid treatment was discovered to be carbon paint which had been applied as a vegetable extract, the carbon of which was protected by a thin, transparent, incipient glaze. That paint which left only the red stain after heating was evidently carbon paint to which bad been added ground hematite, the type of paint used by the Zunis and Hopis today. The Guaco, or Bee plant, or a species of the Sophia is used as a basis for the modern paint. The plants are boiled in water for several hours, until only a thick dark liquid is left. The fluid is dried on corn husks and stored until needed for use, when it is dissolved in water. This used alone would produce the carbon paint; combined with enough ground hematite to thicken the solution, i t produces the third type of paint. The Hopi extract, without the added Fez03, analyzed* as follows:
* Chemical analyses given here and the methods of testing pottery paints were worked out by F. G . HAWLEY,chief chemist of the International Smelter, Miami. Arizona.
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PREHISTORIC AMERICAN ARTS Volatile and combustible Ash . . . . . . . . .
. . .
. . . .
71.5% 28 5
Partial analysis of the ash gave: Silica . . . . . . . . . . Alumina. . . . . . . . . Iron oxide . . . . . . . . Lime . . . . . . . . . . Magnesia Alkalies (KnO)and (NazO)
. . . . . . . . .
...
........... ...
8.2% 4.2 7.2 9.5 1.1 32.5
Carbon dioxide combined with the alkalies and lime is also present. Although the black color of the carbon paint is due to the carbonaceous matter, that material would all be burned away when the painted vessels were first fired if i t were not for the alkali carbonates, sodium carbonate and potassium carbonate, the prinapal constituents of the ash, and the silica and a little soluble alkali contained in the pottery clay which is available on the surface of the vessel. These silicates and carbonates melt in the firing and form a very thin silicate film over the carbon particles of the paint, a film too thin to be called a glaze and invisible, but which protects the carbon from coming in contact with oxygen and so being consumed. If the vessel is not kept surrounded with burning fuel, producing a reducing atmosphere during the time in which the film is being formed over and around the carbon particles, the carbonaceous matter bums out before i t has been covered. Occasionally the carbon of the third type of paint will slowly burn off when oxygen diffusesthrough the film; this leaves the paint reddish brown in color, for when the carbon is gone, the ground hematite is visible. The film of fusible silicates aids in sintering the carbon of the carbon-and-iron paint to the slip and so prevents its washing or rubbing off the vessel. Red iron oxide slips also contain silica and alkalies and so sinter to the clay wall of the vessel in firing, but white paint, which contains only a trace of alkalies, is sometimes easily removable from the surface of red vessels. Chemical tests for manganese oxides showed that these were the main constituents of the fourth type of black color. The oxides were probably obtained from marshy places where such oxides accumulate. Being black in original appearance, it might well have suggested itself to the people as pottery paint. Some iron oxide is usually present. True glazes, green and black, were used in the late prehistoric but preColumbian period in the White Mountain district of the Little Colorado drainage. Their period has been definitely dated through correlation of pottery types and tree ring counts from roof timbers by Dr. A. E. Douglass and E. W. Haury, of the department of astronomy, University of Arizona. Both a lead-copper glaze and a silicate glaze, which seems to have differed in little but relative proportion from the incipient glaze of the black paints,
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were used, but the secret of the preparation of any glaze material has been lost by the modem descendants of these people. Coloring Matter Employed in Other Arts Mineral pigments seem never to have been used on basketry splints, for nature furnished cream, black, green, and brown material. In a few graves near Flagstaff, Arizona, however, remnants of baskets painted with thick clays of rich blues, reds, and yellows were found by an Arizona State Museum expedition. As the people had no wool until sheep were introduced by the Spaniards, they spun and wove their native cotton, and this yarn was sometimes colored by rubbing red clay into it. Designs on belts and sandals of yucca fiber may have been of vegetable dyes, such as the Navajos used on their rugs before the modern vogue for aniline dyes, but final investigation of such coloring matter awaits the future. The chief use of color, however, was in the production of pottery, that art which offeredleast limitations and most variation technically, and which provided necessary as well as artistic household receptacles.
