Roman glass

• 1. Introduction

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  Roman glass turns up almost everywhere archaeologists dig: in the ruins of houses, in workshops, and pressed into the mortar of underground tombs. At the start of the 1st century AD, there was not even a Latin word for the material in the Roman world. By the end of that same century, a drinking cup made of glass could be bought for a single copper coin. That transformation happened in a matter of decades, driven by a new technology, a new political order, and a trade network that carried Roman glass as far as the Han Empire of China and the royal tombs of Korea. How did a fragile, expensive luxury become an everyday object? And what did Roman craftsmen actually do to make it?

• 2. From Luxury to Copper Coin

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  Glass working in Rome grew out of Hellenistic traditions, and during the late Republican period most techniques were slow and labor-intensive. The initial product was a thick-walled vessel that required extensive finishing. Add to that the cost of importing natron, a naturally occurring salt needed to make raw glass, and the result was a material affordable only by the wealthy.

  Three forces broke that situation open. The first was political: the Pax Romana that followed years of civil war, and the stabilisation of the empire under Augustus, created conditions where trade could expand rapidly. The second was geographic: as Roman influence spread, so did the network of people and cultural styles feeding the industry. The third was purely technical.

  Glass blowing appeared during the 1st century AD and changed everything about scale and cost. A blown vessel had thinner walls than a cast one, which meant less raw glass per piece. Blowing was faster than earlier shaping methods, and the finished vessel needed far less work after it came out of the furnace. By the middle to late 1st century AD, older casting and core-forming techniques had been largely abandoned in favour of blowing. The Geographica of Strabo records that by this point a glass drinking cup could be bought for a copper coin, a detail that captures just how completely the material's social position had shifted.

• 3. Making Raw Glass

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  Glass begins with two primary ingredients: silica and soda. During the Roman period, silica came from sand, which naturally contains some alumina and lime. Soda came almost exclusively from natron, a salt harvested from dry lake beds. The main source of natron during the Roman period was Wadi El Natrun in Egypt, though there may have been a secondary source in Italy.

  Where raw glass was actually made is a question archaeologists are still working through. The evidence is scarce, but one find gives a sense of the scale involved: an 8-tonne glass slab recovered from Bet She'arim shows that primary workshops could fire enormous quantities of raw glass in a single furnace run, a process that might take weeks. A single primary workshop could potentially supply multiple secondary working sites spread across the empire.

  The siting of those primary workshops was governed by three factors: fuel, sand, and access to natron from Egypt and Judea, from places such as Bet Eli'ezer, Alexandria, and Wadi El Natrun. That dependency on eastern sources may explain why primary glass making during the Roman period appears to have been concentrated near the coastal regions of the eastern Mediterranean.

  Shipping records support this picture, up to a point. An early 3rd-century shipwreck called Ouest Embiez 1, discovered off the southern coast of France, was carrying between 15 and 18 tonnes of raw glass blocks alongside already-manufactured glassware, suggesting export from the eastern provinces. Yet out of roughly 470 Roman wrecks dated between the 1st and 4th centuries AD, only about 2% appear to have transported raw glass. That comparatively low proportion has led some researchers to suggest that primary production sites existed elsewhere in the empire too, possibly near London or in the Hambach Forest supplying workshops at Cologne, though those theories remain disputed.

• 4. Recycling and the Workshop

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  The Roman writers Statius and Martial both describe recycling broken glass as a significant part of the trade, and the archaeological record backs them up. Glass fragments of any size are rarely recovered from domestic sites, which suggests that broken pieces were collected and sent back into production rather than discarded.

  In the western empire, quantities of broken glassware were concentrated at local sites before being melted back into raw glass. In the eastern empire, there is evidence that recycled Roman glass was used to glaze Parthian pottery. Repeated recycling left its mark on the chemistry: elevated levels of the metals used as colourants are visible in the composition of recycled glasses.

  For small operations, cooking pots apparently served as melting vessels. Larger work used tanks or tank-like ceramic containers, and in the largest cases purpose-built furnaces surrounded those tanks. This range, from repurposed cookware to specialised industrial furnaces, reflects the full span of Roman glass working, from the artisan's bench to something approaching factory production.

  Secondary glass working, where raw glass was shaped into finished vessels, required lower temperatures and less fuel than primary production. That difference in requirements meant working sites could be set up much more widely across the empire. By the end of the 1st century BC, glass working had established itself in Rome, Campania, and the Po Valley.

• 5. Colour, Chemistry, and Style

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  The earliest Roman glass follows Hellenistic tradition: strongly coloured, often mosaic-patterned, built from cast glass rather than blown. Emerald green, cobalt blue, deep blue-green, and Persian or peacock blue are most commonly associated with the early period. Emerald green and peacock blue were new colours introduced by the Romano-Italian industry and appear almost exclusively in fine wares.

  Then, during the last thirty years of the 1st century AD, strong colours vanished quickly from the market, replaced by what the sources call aqua, a pale blue-green that is simply the natural colour of untreated glass, and by true colourless glass. Pliny's Natural History states that the most highly valued glass is colourless and transparent, as closely as possible resembling rock crystal. That comparison to rock crystal is telling: scholars debate whether the fashion for colourless glass reflected a new confidence in glass as a material in its own right, or whether it was simply an attempt to imitate an already precious stone. The persistence of thick-walled casting as a technique, which produced vessels suited to the cutting and polishing associated with crystal working, is cited as evidence for the latter view.

  Colourless glass was not simply clear by default. Producing it required adding antimony or manganese oxide to oxidise the iron naturally present in sand, converting it into a form too weak to tint the glass. The use of manganese as a decolourant was a Roman invention, first noted in the Imperial period. Antimony remained in use in Italy and northern Europe well into the 3rd century. The eventual end of antimony use has been linked to the end of Roman occupation of Dacia and its stibnite deposits.

  From around 70 AD onward, colourless glass dominated fine wares, while cheaper vessels moved toward pale shades of blue, green, and yellow. After about 70 AD, the finer coloured work shifted toward decoration layered onto pale or colourless bases rather than vivid solid colours throughout.

• 6. Techniques and Luxury Extremes

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  Core-forming, one of the oldest techniques, involved wrapping a mass of mud and straw around a metal rod, then building up a vessel by dipping or trailing liquid glass over it. After cooling, the core was removed and handles, rims, and bases were attached. The vessels produced this way were small, thick-walled, and brightly coloured with zigzag patterns; they were limited to unguent or scent containers. The technique remained popular into the 1st century BC, even as newer methods arrived.

  Mould-blown glass appears in the second quarter of the 1st century AD. Cold-cut techniques borrowed from gem carving allowed craftsmen to produce cameo glass in two or more colours, as well as cage cups, vessels carved so intricately that an outer lattice appears to float free of the inner cup. Most scholars believe those cage cups were decorated by cutting rather than any other method, though some debate continues.

  Gold glass, or gold sandwich glass, fixed a layer of gold leaf between two fused layers of glass. The technique originated in the Hellenistic period, then was revived in the 3rd century AD. The great majority of the approximately 500 known survivals are roundels, cut from the bottoms of wine cups and pressed into the mortar of graves in the Catacombs of Rome as permanent markers. Most are from the 4th century, extending into the 5th. The majority depict Christian subjects, with a smaller number of pagan and Jewish examples; artistically they are relatively unsophisticated. A much smaller group of 3rd-century portrait pieces, by contrast, are superbly executed, with pigment painted directly on top of the gold leaf.

  The Romano-Germanic Museum in Cologne holds the world's largest collection of Roman glass vessels from the 1st to 4th centuries, with more than 4,000 complete pieces. Among them is the famous Cologne cage cup from the 4th century, alongside examples of the Cologne nubs, drinking vessels decorated with attached drops of contrasting-coloured glass that are typical of the Rhineland workshops.

• 7. Trade Routes and Distant Finds

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  Roman glass moved. Before the industry had even reached its peak, Roman glasswares were already travelling from Western Asia to the Kushan Empire in Afghanistan and India. The first Roman glass found in China came from an early 1st-century BC tomb at Guangzhou, carried there via the South China Sea. A particular set of Roman glass objects found as far as Gyeonggi Province, in the royal tombs of the Silla Kingdom in Korea, is designated a National Treasure.

  The Rhineland became one of the most important glass working centres of the Imperial period. The Colonia Claudia Ara Agrippinensium, known today as Cologne, and other sites along the Rhine developed distinctive vessel forms not seen further south. By the 3rd and early 4th centuries, producers north of the Alps were exporting their work down into northern Italy and the transalpine regions.

  After Constantine's conversion, glass workshops began moving from pagan to Christian religious imagery. The shift of the imperial capital to Constantinople gave new energy to the eastern glass industry. Meanwhile, the Roman military presence in the western provinces helped sustain production there. By the mid-4th century, mould-blowing was used only sporadically, and by the 4th and 5th centuries Italian workshops had come to predominate across the trade. The gold tesserae technique, first used for mosaics in Rome in the mid-1st century, had become the standard background for religious mosaics by the 5th century, carrying a Roman technical invention forward into the art of the Byzantine world.