The most interesting treatment of shells is exemplified by those with designs applied by an etching process. The Hohokam may have been the first people to discover the technique of etching, for they were using it about the eleventh or twelfth century and the earliest recorded use of the process is on a coat of armor made in Europe in the 15th century.[57] Among the Hohokam the process does not appear to have continued beyond Sedentary times. It was probably never very commonly used and the difficulty of controlling the medium may have contributed to an early abandonment. Painting and etching were sometimes combined, for an example has been found of a shell etched with geometric designs and painted with red and green pigment. [Illustration: Fig. 49—Hohokam etched shell. Sedentary period. (Courtesy Arizona State Museum)] Since shell is nearly pure calcium carbonate it is easy to see why portions exposed to the action of acid would be eaten away, but we have no way of being sure exactly what the technique used may have been. Experiments conducted in the laboratories of Gila Pueblo, however, have shown how such results could have been obtained with available materials.[31] The problem of finding a suitable acid was first considered. Obviously, for the ancient Hohokam, the problem could not be solved by running down to the corner drugstore. For the purposes of the experiment, a mild acetic acid solution was produced by fermenting juice from the fruit of the giant cactus. Portions of a shell were covered with pitch, a material which resists acid, and the shell immersed in the acid for seventy-two hours. When it was removed, the pitch-covered portion stood out in relief while the exposed parts had been partially eaten away, duplicating the effect found on the prehistoric shells. Bone tubes continued to be made, but they were plain and undecorated. Other bone artifacts include daggerlike objects with carved heads, which may have served as hair ornaments. Usually the carving represented the heads of mountain sheep or a bird-and-serpent motif. It was in the Sedentary level at Snaketown that the first objects made of metal were found. These were little copper bells, pear-shaped and split at the bottom, which very much resemble sleigh-bells. A great many identical bells are found in Mexico and it seems probable that the Snaketown examples were imported from there.[59] In the Anasazi area many copper bells were imported from the south. Most of them are dated at between 1300 and 1400 A. D., although some have been found which were brought into Pueblo Bonito and Aztec at an earlier date. THE CLASSIC HOHOKAM