The chemical composition of ancient glass is complex, with many different compounds found in many different combinations. A variety of oxides are formed from the primary components, such as the sand, quartz, and various alkali fluxes, as well as in the colorant and opacifying materials. In addition, other oxides associated with the impurities are found with the various minerals used for coloring. Up to the 1960s, various analytical techniques were in use to identify these components, but from this time more efficient techniques became available. These include X-ray fluorescence (XRF), scanning-electron microscopy (SEM), particle-induced X-ray emission (PIXE), and various methods of spectroscopy. Each technique provides slightly different kinds of analytical information, and often several techniques are used to cross-check or add to results.34 In isolation, the chemical analysis of a piece of glass will not necessarily add greatly to our understanding. Data need to be collated from a variety of sources in order to build a picture of glass production in different regions at different times. Material found at production sites is a key part of this, as is a knowledge of trade patterns.
Analysis over the past half century has resulted in a much greater understanding of glass: for example, identifying that natron starts to be used from around 800 BC and that glass produced in central China from around 500 BC—discussed below—uses barium oxide and lead with potassium as the flux and is not of the soda-lime type found across western Eurasia and North Africa. These methods would, for example, confirm whether or not the glass for this bowl was made in the Hellenistic or Chinese worlds. Qualitative XRF analysis has been carried out on one of the bowls discovered in the tomb. Such analysis indicates all the elements present in the glass, without providing information on the quantities. This analysis showed the presence of potash, lead and barium, along with silica and various alkalis.35
Two decades ago Henderson noted that “glasses made at two different areas using the same technological tradition but with slightly different raw materials will probably contain recognizably different trace elements and will possibly have different stable isotope signatures.”36 Since then, techniques of isotopic analysis have been developed that have the potential to enhance further our understanding of glass and other material.37 The relative levels of strontium and neodymium in Hellenistic glass have distinguished two different sand sources used in making such glass.38 As Henderson points out, there might be a compositional and isotopic distinction between the sands found on the beach at the Belus River and those at Sidon, indicating the two sources and providing a means of distinguishing glass melted in the two areas.39 An analysis of these sands awaits. Trace element analysis has already provided evidence for more than one production area for Middle Hellenistic glass, one of them possibly in Italy.40 This is just the start of a potentially very rich methodology for mapping and understanding in much greater detail the sources of glassmaking, glass production, and glass trade.
In the case of our bowl, however, we are still reliant on its style, form, and dating, along with the basic information provided by the XRF analysis. The former points strongly to this being a late Hellenistic bowl produced in the Levant. The latter introduces an element of doubt into this hypothesis. Without further information the results are inconclusive but certainly leave open the possibility, as Borell points out, that this bowl was produced in China.
GLASS PRODUCTION IN ASIA
In South Asia, as noted above, faience is found in Indus valley cultures, and there are signs of glass production there in the early second millennium BC.41 The evidence remains fragmentary through the first millennium. Glass with a high aluminum content is found at Rupar in the Punjab from the beginning of the first millennium BC and a high-potassium (potash) glass from Hastinapura in Uttar Pradesh around the middle of the first millennium BC.42 From about 400 BC potash glass is also found in Southeast Asia, though the production sites of the Indian and Southeast Asian glass are still uncertain for this period. But by around 200 BC, production sites on the southeastern coast of India were producing glass beads. These, now known as Indo-Pacific beads, were traded to Southeast and East Asia as well as to Africa.43 “The Indo-Pacific bead industry produced one of the, if not the, most widespread and ubiquitous trade item of all time.”44 This glass is distinguished by its high aluminum and low lime content.
Potash glass continued to be produced in Southeast Asia, including at sites around the Tonkin Gulf in what are now Northeast Vietnam and Southwest China. A detailed analysis of the potash glass from South and Southeast Asia of this period distinguished several subgroups, suggesting different production sites, possibly including one in Southwest China.45
Elsewhere in Asia, glass beads are found in tombs in the Tarim basin, in present-day Chinese Central Asia, dating from around 1000 BC, and in central China from around the fifth century BC.46 Scientific analysis shows that most are soda-lime glass, which suggests that they were imports from West Asia, but the crudeness of some of the beads and the presence of lead and magnesium has led some scholars to suggest the possibility of the start of local glass production in Central Asia at this time.47 Glass from other Tarim sites dating from the fifth century BC onwards shows more sophistication and is of various compositions, including soda-lime. But other examples show significant levels of barium oxide and lead with potassium as the principal alkali. This glass seems to have been imported from China, as we see evidence of glass technology with these materials in central China from this time. No other place is known to have produced glass of this composition.
Both the relatively late appearance of glass in China and the use of barium oxide are features worthy of comment. By the end of the second millennium BC, potters in China were using furnaces to fire pots with ash glazes at temperatures of about one thousand degrees Centigrade and at higher temperatures within the early first millennium. Glazes are vitreous substances, and it has been suggested that the technology for glazing ceramics in Mesopotamia evolved from the glass industry.48 In China, the use of glazes seems, however, to have preceded glass.49 The ceramic technology in the Chinese region continued to develop, and by the end of the first millennium semivitreous glazed stonewares were produced that were both hard and able to hold hot liquids. This led to the production of porcelain, which, being vitreous and semiopaque, even more closely replicated some of the qualities of jade. The materials used for stonewares in southern China contained virtually no clay but instead consisted of fine mica or hydromicas. These were 6 to 10 percent potassium oxide, which, when fired, melted some of the silica in the clay body into a stiff glass and toughened the resulting stoneware.50 So the technology was available for making glass.
Possibly as early as 1000 BC the Chinese produced a synthetic pigment, called Han blue or purple, that was used for about a thousand years. It was similar in composition to Egyptian blue but was synthesized at a higher temperature. Elisabeth West FitzHugh and Lynda Zycherman have tentatively suggested that the discovery of Han blue—made from barium copper silicate—may have been a serendipitous accident of glassmaking. However, there is currently no firm evidence for glassmaking in China by 1000 BC.51 The earliest finds are potash-lime glass beads made in central China from about 800 BC. They are replaced from the fifth century along the central Yangzi valley by lead-barium and potash glass.52 Han purple is seen used to decorate glass beads found in burials from the second half of the first millennium.53
Lead-barium glass remained the predominant glass in central China up to the early first millennium AD and is found throughout China, north and northwest into the steppe and Central Asia, and south to the sea. Brill and others have suggested that the turbidity that barium produced resulted in a glass resembling jade and that this glass was produced as a jade substitute.54 The lead made the glass more brilliant and reduced its melting temperature.55 As Gan Fuxi observes, it was natural for the Chinese to use lead as a flux, as they had long experience of using it in bronze working. In addition, both lead and barium ores are found in large quantities along the Yangzi River valley.56 The use of saltpeter or potash as an alternative flux to make potash-lime glass is also not surprising given a long history of use in China.57
