The Curious History of Glass: From Obsidian Knives to Fiber Optics

Glass is the most invisible technology in the world, in a literal and a metaphorical sense. The literal sense is the point: a windowpane is good when you do not notice it. The metaphorical sense is more interesting. Glass is the substrate of an enormous fraction of modernity, and almost no one thinks about it. Without glass, there is no microscope and no telescope, which means no germ theory and no Galileo. Without glass, there are no test tubes and no chemistry as a quantitative science. Without glass, there are no windows in cold climates, which means cities are dimmer and smaller. Without glass, there is no fiber optic cable, which means no internet at scale.

The history of glass is correspondingly long, and the standard textbook version of "the Phoenicians invented it accidentally on a beach" is mostly a myth. The real story is older, more distributed, and full of surprising bottlenecks.

Obsidian: the natural ancestor

Glass exists in nature. Volcanic obsidian is technically a glass: silica that cooled too fast to crystallize. Ancient peoples worked obsidian into knives and arrowheads for at least 100,000 years. The edges of a freshly knapped obsidian flake are sharper than any steel scalpel; modern eye surgeons sometimes use obsidian blades because the cleavage produces edges measured in single molecules.

Obsidian was the first traded glass. The famous obsidian sources of Anatolia, Lipari, and the central Mexican volcanoes show up in archaeological sites hundreds of kilometers away, evidence that long-distance trade in this material predates virtually every other technology of civilization.

The first manufactured glass

Manufactured glass appears around 3500 BCE in Mesopotamia and Egypt, initially as a glaze on faience beads (a paste of crushed quartz fired to form a glassy surface). True glass objects, where glass is the material rather than a coating, appear by around 2500 BCE.

The earliest glass was difficult to make in any quantity. The temperatures required to melt sand cleanly are around 1700°C, beyond what wood-fired kilns could reach. Ancient glassmakers got around this by adding fluxes (natron from Egyptian salt deposits, plant ash from coastal halophytes) that lowered the melting temperature to around 1200°C, which the kilns could just manage. The resulting glass was opaque, full of bubbles, and colored unpredictably by trace metals in the sand and ash.

For two thousand years glass was a luxury material made in small workshops, used for beads, small vessels, and inlays in jewelry. The technique of glassblowing, which made larger and thinner vessels possible, was invented in Syria around 50 BCE and spread rapidly through the Roman Empire. By the second century CE, glass was common enough that ordinary Romans drank from glass cups.

Murano and the secret of clear glass

The fall of the Western Roman Empire took most of the glass industry with it. Glassmaking survived in the Eastern Mediterranean and slowly migrated west. By the 13th century, Venice had become Europe's glassmaking capital, and in 1291 the Venetian senate moved all glassmakers to the island of Murano, ostensibly to prevent fires but really to keep the techniques secret.

The Murano glassmakers cracked the problem of truly clear glass around 1450, with a recipe called cristallo. The key was extreme purity of inputs: white sand from the Ticino river, soda from purified plant ash, manganese added as a decolorizer to neutralize iron tints. Cristallo was so far ahead of any other European glass that Venice maintained a near-monopoly on optical-quality glass for two centuries. The penalty for a Murano glassmaker leaving the island was death; the secrets were considered state-level intellectual property.

The monopoly leaked eventually. By the 17th century, England's George Ravenscroft developed lead crystal (substituting lead oxide for some of the lime, producing a softer, more brilliant glass), and France's royal glassworks at Saint-Gobain pioneered cast plate glass for mirrors. The Hall of Mirrors at Versailles in 1684 was the propaganda campaign that announced French glass had caught up to and surpassed Venetian.

The optical revolution

The single most consequential thing glass did was to enable optics. The earliest spectacles appear in Italy around 1280. By 1608, Hans Lippershey in the Netherlands filed a patent for a telescope, which Galileo refined the following year and pointed at Jupiter, irrevocably changing astronomy. The microscope's history is parallel: Antonie van Leeuwenhoek, a Dutch draper with no formal training, ground his own lenses through the 1670s and saw bacteria ("animalcules") for the first time.

The crucial constraint on early optics was lens quality. Spherical aberrations, chromatic aberrations, bubbles, and stress fringes all degraded the image. The story of optical glass for the next two centuries is the story of fighting these defects: Joseph Fraunhofer's careful melts in Bavaria around 1820 produced glass clean enough that the dark lines in the solar spectrum (Fraunhofer lines) became visible, launching spectroscopy as a science. Ernst Abbe and Otto Schott in Jena collaborated with Carl Zeiss in the 1880s to develop apochromatic lenses, finally taming chromatic aberration. German optical glass dominated the world from then until World War II, when the industry fragmented across borders.

Float glass: the 20th century revolution

Window glass through the 19th century was made by either blowing a large glass cylinder, splitting it, and flattening it (the "broad" process) or by spinning a disc of molten glass into a flat plate (the "crown" process). Both produced glass that was wavy, uneven, and required hand-grinding for any precision use. Plate glass was cast on iron tables and then ground smooth at enormous expense.

In 1959, Alastair Pilkington at Pilkington Glass in England commercialized the float glass process. Molten glass is poured onto a bath of molten tin, where it spreads to a uniform thickness and cools with both surfaces perfectly flat (one against the tin, the other against the air, both flatness-producing). The process produces glass of optical quality at a fraction of the cost of grinding. Within twenty years, every plate-glass factory in the world had switched. The cheap, perfectly flat glass we now take for granted in skyscrapers, car windshields, and computer monitors is a 1959 invention.

Fiber optics: glass as information carrier

The most recent chapter is fiber optics. Light bouncing inside a glass cylinder by total internal reflection had been a curiosity since the 19th century. The serious push to use glass for telecommunications began in the 1960s, with the central problem being attenuation: ordinary glass loses light too fast for transmission over more than a few meters.

In 1970, Robert Maurer, Donald Keck, and Peter Schultz at Corning developed glass with attenuation low enough (around 20 dB/km, compared to 1000 dB/km for typical optical glass) that long-distance optical communication became viable. Modern fiber has attenuation under 0.2 dB/km, low enough that signals can travel undersea cables hundreds of kilometers between repeaters. Almost all internet traffic today, including this article reaching your screen, spent some part of its journey as light pulses inside a glass thread.

The unsung material

What is striking about glass is how invisible it has remained as a technology. The 20th century gets credited to plastics, semiconductors, and steel. Glass is the substrate that all of those technologies sit on top of: lenses for the lithography that makes chips, fiber for the network that carries the data, screens that display the result. The world is increasingly glass-mediated, and the glassmakers - Corning, Schott, AGC, Pilkington - are largely unknown to the public.

Perhaps that is the mark of a successful infrastructure technology. You stop seeing the glass after a while. The window does its job. The lens does its job. The fiber does its job. The 5,000-year project of turning sand into something transparent is so successful that it has become invisible.