National Geographic : 1964 Sep
few minutes. Exposure to the blizzard of neu trons in the reactor causes no visible change within the glass. Neutron bombardment, however, changes the structure of a minute fraction of the atoms of many elements in the sample. The new forms, radioisotopes, lose radioactivity on an individual timetable of decay, each with the emission of characteristically different radia tion. Modern counting devices permit us to separate these different radiations and to count their intensities individually. The radi ation intensity from an isotope indicates the scarcity or abundance of the element. Neutron activation, as this technique is called, can detect and measure quantities of elements as minute as a few parts per million million. This is an amount expressed by a decimal number in which 11 zeros precede the first digit. Neutron activation is one of the few methods of sufficient sensitivity to measure the rare earth ingredients of glass, such metallic elements as cerium, europium, lanthanum, yttrium, scandium. Chemistry Unknown to Ancient Artisans The ancients of the Mediterranean and the East were intelligent and practical people. Yet when I speak of cerium and lanthanum, of antimony and manganese, of lead oxides, soda, and lime, you must not think that the artisans of those times understood the chem istry of these ingredients as we do. Identification of elements and compounds lay far in the future. Industrial chemistry in antiquity, though tremendously impressive, was strictly a trial-and-error affair. Glass makers simply knew that certain substances in nature-in rock, sand, earth, or ashes gave special properties to their products. "That yellow rock you brought in from Phoenicia last time was just the thing," a glassmaster might remark to an arriving mar iner, holding up a bowl fresh from the oven. "And see what a sparkle that sand from the mouth of the Belus has given this wine glass." One would be tempted to conclude that the ancient glassmaker's lack of true chemical knowledge was a severe handicap. Actually, through endless experimentation, these pio neers were able to develop incredibly sophisti cated glassmaking techniques, fully under standing the results if not the causes. For our program, Ed Sayre and his helpers needed from me an extensive series of speci mens, a good cross section of ancient accom plishment. Analyzing these samples would establish the chemical ingredients of ancient 358 glass from a wide selection of sites and peri ods. The work has thus provided a back ground of knowledge against which specimens can be compared in the future. In the Brookhaven program, a glass speci men should bring with it a clear archeologi cal origin. It should have been excavated by trained archeologists. It should be the prop erty of a responsible institution and bear an acquisition number. We at first preferred that it be an object of clear, rather than colored, glass. We favored colorless glass because we thought that the oxides added to impart a desired color might inject complicating variables. As it turned out, however, some of our colored samples have given us more and better information than many of the clear ones. Ideally, a specimen for analysis should come from a complete piece or from a frag ment large enough to allow an expert to vis ualize the shape of the object and its decora tion. But small fragments have yielded some of our most useful results. "See these three fragments of Roman gold band glass?" Ed said to me on a recent visit to Brookhaven. "One is from Pompeii, one has a doubtful origin-you bought it in Rome -and one came from Susa, the ancient city of Esther and Ahasuerus in Iran." The pieces Ed held were of a fine type of first-century glass. They were remnants of vases, jars, or bottles made by an elaborate technique that produced a mosaic of colored bands, each band layered in an intricate way, and some of them holding flakes of gold leaf. The effect was kaleidoscopically rich. Same Factory Made Far-traveled Glass "We've found that the amount and kind of impurities which came along with the ma terial added to give color are identical for each fragment," Ed told me. The light blue bands were colored by cop per, apparently introduced in the form of bronze; all three specimens had tiny percent ages of copper, tin, and lead in almost the same ratios customarily found in bronze arti facts. We could picture the glassmaker melt ing down a chunk from an old and battered ornamental bust to give the right color to his glass mix. What Ed reported indicates the strong probability that the three original objects came from the same factory. The fact that two of the pieces were found 2,000 miles apart shows how active in Roman times was the international trade in luxury glassware.