Glass

Glass is an inorganic solid material that is usually transparent or translucent, and it is characteristically hard, brittle and impervious to the natural elements. Commercial glass is ordinarily produced from sand, limestone and sodium carbonate, which are raised in temperature until molten and then cooled quickly so that visible crystals do not form. The material is formed when sand as silica, soda as alkali and lime are fused at high temperatures.

Glass begins as a fusion of silica sand, limestone and sodium carbonate brought together at high temperature. Crystalline silica on its own melts only above 1,700 °C (3,092 °F). Introducing soda as a flux lowers that threshold sharply, and the lime contributed by limestone gives the finished material its resistance to water. The classic soda-lime recipe settles at roughly 75 percent silica, 10 percent lime and 15 percent soda.

Not every glass follows the soda-lime formula. Borosilicate glass, the material of domestic cookware, has an expansion coefficient only one-third that of ordinary glass. Lead crystal tableware substitutes lead monoxide as the principal flux in place of much of the sodium oxide.

The earliest glass objects known to archaeology are Egyptian glass beads dating from about 2500 BCE. Glass use expanded sharply in the Late Bronze Age between 1600 and 1200 BCE in Egypt, Mycenaean Greece and Mesopotamia, where it was often opaque and coloured. Core-formed and cast glass vessels were first produced in Egypt and Mesopotamia as early as the fifteenth century B.C., but only later became common on the Italian peninsula.

The earliest glass vessels were core-formed, with opaque dark molten glass wound around a clay core attached to a metal rod. The Uluburun shipwreck off the coast of Turkey yielded 175 unfinished glass blocks called ingots, most of them cobalt-coloured deep blue with some purple and turquoise. Fragments chipped from three Uluburun ingots were reported in 2010 to be Egyptian in origin, based on the concentration of trace metals.

The Amarna Letters, cuneiform tablets excavated at Tell el-Amarna in Egypt, include references to glass among the correspondence of ancient kings. Kings shipped materials to other rulers expecting goods or loyalty in return, and Late Bronze Age inventories record the exchange of ivory, gems, wood, animals and people.

Crucible evidence suggests raw materials were melted at around 900 degrees Celsius, while finishing and even colouring required at least 1,000 degrees Celsius. Analysis of Late Bronze Age glass beads unearthed in Greece found Near Eastern or Egyptian chemical signatures, indicating that Greece imported glass from both regions. Isotope work reported in 2020 traced antimony in the earliest glass to stibnite ores from present-day Georgia in the Caucasus.

In the Bronze Age hierarchy of materials, glass sat slightly beneath silver and gold and was valued as much as precious stones were; King Tutankhamen's tomb contained a decorative writing palette, two blue-hued headrests made of solid glass and a funerary mask carrying blue glass inlays alternating with gold. Colour was imparted to ancient glass with materials such as cobalt for dark blue and lead antimonate for yellow, while crucibles excavated at Amarna, Qantir and Lisht — holding cobalt blue, predominantly red and mainly turquoise glass fragments respectively — suggest that these workshops specialised in one colour.

Glassblowing developed in the Syro-Palestinian region in the early first century B.C. and is thought to have come to Rome after the area's annexation in 64 B.C. Rome asserted control over the two dominant glassmaking centres of the late first millennium, Syro-Palestine and Egypt, in 62 and 30 B.C.

Ancient glass served for personal adornment, for inlays on wooden and ivory furniture, for architectural embellishment and as containers for scented oils and perfumes.

Beyond tableware, the Romans used glass for mosaics and decorative panels, windows, jewellery, mirrors, game pieces, magnifying glasses and even medicine and toothpaste. The Portland Vase, made during the reign of Augustus, depicts the marriage of Peleus and Thetis from Greek mythology and is the most famous example of cameo glass. By the 4th century CE the art of glass cutting reached its zenith with the diatreta or cage-cups, including the Lycurgus Cup carved from green and red glass. Colourless glass first appeared in the Roman glassmaker's repertoire in the late 1st century CE and became highly sought after by Roman households.

A glassblowers' guild existed in Venice from 1224, and in 1291 the glasshouses moved across the lagoon to Murano, where they have remained. The capture of Constantinople by the crusaders in 1204 and by the Ottomans in 1453 brought an influx of Byzantine glassworkers. Venetian glass grew out of close ties with Syria and the Middle East, and by the fourteenth century at least twelve glassworks were producing everyday objects.

In the 15th century Venetian efforts concentrated on perfecting cristallo, clear glass approximating rock crystal, and by the 16th century colouring and decolourizing techniques were mastered. Angelo Barovier's invention of clear glass made Murano the unrivalled leader by the middle of the fourteenth century. Techniques included millefiori, calcedonio and latticinio. Filigree was invented around 1527 by Filippo Catani della Sirena, incorporating rods with threads of milk glass into the crystal. Murano reached its high point in the 16th century.

Despite restrictions on the migration of workmen, many Venetian glassmakers defected, notably to Altare near Genoa, and their guarded techniques became common knowledge across Europe.

Two regions became noted for the quality of their glass, the Belus River in Phoenicia and the Volturnus River in Campania. Isidore of Seville preserved the legend that natron merchants shipwrecked by the Bellus River cooked on lumps of natron, whose fusion with shore sand produced the first glass. At the time of the Arab conquest in the seventh century A.D., glassmaking had flourished in Egypt and western Asia for more than two millennia, and Islamic production from the seventh through the fourteenth century was greatly innovative, witnessing glorious phases such as superb relief-cut glass and spectacular gilded and enameled glass. About 1675 the English glassmaker George Ravenscroft discovered that adding lead oxide to Venetian-type glass produced a solid, heavier glass that became a favourite for fine tableware, and English lead crystal and Bohemian glass later displaced Venetian glass in the European tradition.

Stained glass is coloured glass used for decorative windows and other objects through which light passes, coloured by metallic oxides while the glass is molten. Medieval sheets of glass were coloured with metallic oxides — copper for ruby, cobalt for blue, manganese for purple, antimony for yellow and iron for green — then annealed slowly in a kiln. The 167 stained glass windows of Chartres Cathedral, built between 1190 and 1220 CE, are the most complete group surviving anywhere from the Middle Ages.

The Gothic Revivalists of the mid-19th century returned to earlier methods and developed so-called antique glass, which remains the basic material of stained-glass windows today.

In 1952 Sir Alastair Pilkington came up with the idea of the float glass process, and on 20th January 1959 it was announced to the glass-making world. The process pours molten glass onto a bed of molten tin in a float bath to create a perfectly flat sheet of uniform thickness.

There are now around 500 glass float lines in the world and the float process is considered the world standard for high quality glass manufacture. Approximately 90 percent of all plate glass is made by float.

Glass can be recycled endlessly with no loss in quality or purity. In 2018 US glass generation was 12.3 million tons, 4.2 percent of municipal solid waste, while the container recycling rate was 31.3 percent, or 3.1 million tons of glass containers recycled. Furnace energy needs fall by nearly 3 percent for each 10 percent increase in cullet.

One half of the 2009 Nobel Prize in Physics was awarded to Charles K. Kao for transmitting light in optical fibres. The achievement rests on a class of materials that had to be made with extraordinary care: high-purity, ultratransparent oxide glasses developed for fibre-optic telecommunications systems.

• 1600–1200 BCE
  Late Bronze Age expansion
  Glass use expanded sharply in Egypt, Mycenaean Greece and Mesopotamia, where it was often opaque and coloured.^{3A Brief Scientific History of Glass · Smithsonian Magazine ↗}
• 62 and 30 B.C.
  Rome takes the glassmaking centres
  Rome asserted control over the two dominant glassmaking centres of the late first millennium, Syro-Palestine and Egypt.^{2Wondrous Glass: Roman Glass-Making · Kelsey Museum of Archaeology, University of Michigan ↗}