LITMIR.BIZ

      FIGURE 4. Structure of crystalline solid, liquid, and amorphous materials. After Corning Museum of Glass, “What Is Glass?” December 2, 2011, www.cmog.org/article/what-is-glass.

      Glassmaking was the last to be developed of three main nonorganic human technologies (the other two being pottery and metalworking). Before glass the same materials and technologies were developed for the production of vitreous materials such as faience and Egyptian blue. Faience is a hard, highly colored glazed material often reflecting the light and is made from the same materials as glass—silica and plant ash—although fired at a temperature about two hundred degrees Centigrade lower than that required to produce glass. It also is produced on one firing, whereas glass requires two: one to produce the raw glass and then a second to work it into beads and other objects. Faience is found in Mesopotamia from around 4000 BC, in Egypt in the fourth millennium, and at an Early Harappan site in South Asia around 2700 BC.⁸

      Egyptian blue, considered to be the earliest synthetic pigment, was made by repeated high firings from silica, copper alloy filings or crushed ore, calcium oxide, and a fluxing material. It was probably developed to imitate lapis lazuli, a stone imported to Egypt over three thousand miles from the mines in present-day eastern Afghanistan.⁹ It appears from around 2500 BC, and later examples show that it was also being made in Ugarit in present-day Syria. It continued in use until around AD 400.¹⁰

      The first known manmade glass—in the form of beads—is found in northern Mesopotamia and dates to about 2500 BC.¹¹ The raw glass—known as soda-lime glass—was made from silica and plant ash fused together in crucibles in small quantities. Silica by itself has a melting temperature of over 1700ºC, but when it is mixed with an alkali, such as soda, this can be reduced to about 1000ºC. The alkali is called a flux. The resulting glass slag was then fired again and fashioned into beads by various means, such as winding the molten glass around a clay-coated wire. Glass beads came to be produced in various colors, the glass colored and made opaque with the addition of raw materials. Some of the resulting beads resembled semiprecious stones such as turquoise and lapis lazuli.¹² Raw glass from Eridu in present-day Iraq, dating from around 2300 BC, is made from silica and plant ash, combined with a cobalt-rich material to produce this blue coloring.¹³

      Tutankhamun’s death mask contains lapis lazuli along with locally made deep-blue glass. Presumably the substitutions were made because of the cost and rarity of lapis. By this time glass technology had spread into Egypt, possibly taken there by Mesopotamian craftsmen. Beads of the same composition as those on Tutankhamun’s mask are found in northern France and Scandinavia, evidence of early trading in glass.¹⁴ Some scholars argue that glass technology was also developed in the Indus valley and that some beads previously classified as stone are in fact weathered glass. The evidence is uncertain, but given the production of siliceous faience there by around 2700 BC, the development of glass would not be unexpected.¹⁵ Firm evidence for glass production across the Indian subcontinent is datable to around 1450–1200 BC, and there was contact between the Harappan cultures in the Indus valley and those in Mesopotamia by land and sea.¹⁶

      The first glass vessels, rather than beads, were also found in northern Mesopotamia and date from around 1500 BC, probably produced under the Hurrian kingdom of the Mitanni.¹⁷ They were core formed and made from silica and plant ash.¹⁸ Making vessels in this way required the fusion of large volumes of glass inside furnaces that could reach approximately 1150 to 1200ºC. The molten glass was used to coat a core of dung and clay that had been shaped around a rod. Trailed-on glass provided surface decoration. Colored glass started to be produced systematically, with the addition of antimony leading to opaque white, yellow, and turquoise, and the addition of cobalt to produce the deep blue. These required further technological innovations: the glass had to be heat treated, so that crystals of the antimony or other substance would form, and then had to be cooled slowly to develop the opacifiers. This resulted in a stronger glass. Sources and supply of the raw materials were also required, not all available locally. This period also sees the appearance of mosaic glass vessels produced in molds and decoration using marbling, further technological innovations.

      As well as the export of beads and, later on, finished vessels, unworked glass—raw furnace glass or ingots—was exported as neighboring cultures developed glass technology and were able to work glass into local products. The discovery of a fourteenth-century BC shipwreck off the coast of southwestern Turkey near Kas gives us a glimpse into the network of trade and exchange of this pre–Silk Road time across western Eurasia and Africa.¹⁹ The wreck contained, among many other items, amber from the Baltic, ostrich eggs, ebony and ivory from Africa, an Egyptian gold scarab inscribed with the name of Nefertiti, Cypriot pottery, Canaanite jewelry, pomegranates, cumin, and almonds (and a stowaway house mouse). Copper ore for the 345 ingots (each weighing twenty-three kilograms) came from Cyprus. It was probably a merchant ship of the Syro-Palestinian peoples who lived on the eastern coast of the Mediterranean, and was on its regular circular voyage from Syro-Palestine northwest to Cyprus, thence to the Aegean, and occasionally as far west as Sardinia, then back home via North Africa and Egypt.²⁰ The ship’s cargo included about 175 translucent cobalt-blue and turquoise glass ingots, about fifteen centimeters in diameter. These are known from textual sources to be called mekku-stone, and analysis has shown them to be identical to Egyptian and Mycenaean glass.²¹ Around 9,500 glass beads and 75,000 faience beads, along with others in carnelian and other semiprecious stones, were also found. Many of these would have been items for trade, but others were probably the personal possession of crew members.²² Scholars have also suggested that the one ton of tin found in the Mediterranean shipwreck might also have come from mines in Central Asia.²³

      Glass technology developed in different cultures across West Asia and Europe in the following millennium. Glass started being worked in Greece around the thirteenth to twelfth centuries BC; it was fused in northern Italy around the eleventh to tenth centuries BC; and thereafter the technology moved across Europe.²⁴ From about 800 BC there was a significant development in the composition of glass made in western Eurasia with the use of the sodium-rich minerals natron or trona largely replacing plant ash as the alkali. Natron is found on the edge of the desert northwest of Cairo at a place called Wadi el Natrun (Natron valley). Natron was a purer and denser source of the alkali required for glassmaking and, unlike plant ash, did not need prior preparation. It had been used for embalming bodies in Egypt as early as about 2000 BC, but it was relatively rare. However, it became the main source of the alkali in glass in the Levant and Europe for one and a half millennia. The Roman scholar Pliny the Elder (ca. AD 23–79) recorded one account of the start of glassmaking near the Belus River: “A ship of [Phoenician] natron merchants came to shore and when the men were scattered all along the beach preparing their meal, since there were no stones to support their kettles, they put pieces of natron from their ship under them. When these had caught on fire and the sand of the shore mixed with them, there flowed transparent streams of a new substance, and this was the origin of glass.”²⁵

      The Greek historian Strabo (64/63 BC–ca. AD 24) also wrote of glassmaking in the region, identifying another source of sand further north along the Mediterranean coast near Sidon. Excavations over the past two decades in Beirut have given evidence of both glassmaking and glass working, and thousands of glass vessels have been uncovered.²⁶ These date from the late Hellenistic period onwards and include the type seen in the bowl under discussion, namely a blue monochrome hemispherical bowl with a decoration of a single groove below the rim. They were made by casting, that is by either pouring the molten glass into or over a mold or heating the raw glass inside the mold.²⁷ Although plain bowls are found, ones with a grooved decoration are most common, the groove probably being cut by a wheel and not formed by the mold.²⁸ Large assemblages have been found

Скачать книгу