Roman glass objects have been recovered across the Roman Empire in domestic, industrial and funerary contexts. Glass was used primarily for the production of vessels, although mosaic tiles and window glass were also produced. Roman glass production developed from Hellenistic technical traditions, initially concentrating on the production of intensely coloured cast glass vessels. However, during the 1st century AD the industry underwent rapid technical growth that saw the introduction of glass blowing and the dominance of colourless or ‘aqua’ glasses. Production of raw glass was undertaken in geographically separate locations to the working of glass into finished vessels, and by the end of the 1st century AD large scale manufacturing resulted in the establishment of glass as a commonly available material in the Roman world, and one which also had technically very difficult specialized types of luxury glass, which must have been very expensive.
The Roman Glass was an article of choice for Roman, which it supplied by trade with the Egyptians and Phoenicians. But from the beginning of the empire they made it in the metropolis and outside it (in Gaul and Hispania, according to Pliny and St. Isidore), giving it the same applications as the Egyptians and Phoenicians and perfecting the shapes of its vessels, which are more varied and elegant. Hardly any Roman tomb is discovered that does not contain vials.or glass bottles, already colorless, already greenish, covered with iridescence by the action of moisture and air. These bottles, always in narrow shapes, are often called by tear and ointment collectors, but they were used only to contain oils or perfumes in the tombs, not to deposit tears.
The Romans also perfected the art of producing reliefs of figures in glass cups by adding another layer of enamel or glass of a different color, along with the modeling and chiseling or engraving that had been invented by the Romans. Egyptian, so that the outer surface of such glasses offers all the appearances of a large onyx stone cameo.
They are called fake Murrina glasses today, to mimic the real ones. The most beautiful vessels in this way that are preserved are in the Museum of Naples and the British.
Finally, the Romans decorated the precious glass vessels with the procedures that centuries later were reproduced in Venice and are distinguished by the names reticella (reticulated glasses) and millefiori (yarrow or flowered glasses), which result from the addition of glass rods and filigrees already on the surface for the reticulates, already included in the mass or in the thickness of the vessel walls for the yarrow. Roman factories also produced glass for windows and skylights, although small in size, and made statuettes of this material and also imitations of fine stones with engravings.
The glass was a favorite material for the Romans who were provided with it through trade with Egyptians and Phoenicians. Objects made of Roman glass have been found at numerous sites in the Roman Empire in both domestic, as well as industrial and funeral settings. The glass was mainly used to manufacture containers, although ceramic tiles and window glass were also produced. Roman glass production methodologies were developed from Hellenic technical traditions, initially concentrating on the production of brightly colored glass containers using molds. However, during the 1st century the industry experienced rapid growth that included the development of glassblowing and the preference for colorless or ‘aqua’ tone glasses. Production of raw glass was made in several geographically different areas where took place the worked glass for the finished containers, and the end of the first century the large scale manufacture made the glass became in a material readily available throughout the Roman world, and of which luxury glasses were available that must have been extremely expensive.
From the beginning of the Empire they manufactured in the metropolis and beyond (in Gaul and Spain, in the words of Pliny the Elder and San Isidoro), giving the same applications as the Egyptians and Phoenicians and perfecting the forms of their vessels that they are more varied and elegant. Hardly a Roman burial is discovered that does not contain vials or glass bottles that are already colorless and greenish, covered with iridescence due to the action of humidity and air. These little bottles, always in narrow shapes, are usually called tear drops and ointments by collectors, but they only served to contain oils or perfumes in thegraves, not to deposit tears on them.
The Romans also perfected the art of producing reliefs of figures in glass vessels by the addition of another layer of enamel or glass of a different color, together with the modeling and chiselling or engraving of it which had been invented by the Egyptians so that the The outer surface of such glasses offers all the appearances of a large onyx stone cameo.
Today they are called false murrine vessels, for imitating the true ones. The most precious vessels of this form that are preserved are in the Museum of Naples and in the British.
In short, the Romans decorated the precious glass vessels with the procedures that the Venetians reproduced centuries later and are distinguished by the names of reticella (reticulated glasses) and millefiori (milflores or flower glasses) which result from the addition of bars and glass filigree already on the surface for the reticulated, already included in the mass or in the thickness of the walls of the glass for the flowers. The Roman factories also produced glass for windows and skylights, although small, and made statues of this material and imitations of fine stones with engravings.
Mediterranean glass trade
From Hellenistic times glass production sees considerable technological developments culminating with the introduction of glass blowing in the 1st century BC. Glass objects could now be made in large scale production, with less raw material and faster and as a consequence became more common. From the early Roman times, to Byzantine and early Islamic periods the glass from Northern Europe to Eastern Mediterranean shows an incredible compositional homogeneity in its major elements. Unlike the LBA glass, Roman glass was made from the melting of sand and natron – mineral soda – from the Wadi Natron in Egypt.
With one raw material being common to all glass it should be possible to differentiate between glass made from different sands using composition variation of the trace elements and certain isotope ratios. These compositional analyses aimed to investigate two models for glass production: glass was produced in large scale primary workshops along the Levantine coast and Egypt, mixing natron and sand from the mouth of the Belus river – as mentioned by Pliny – and then traded to local glass working workshops. If natron was traded and mixed with local sands following a strict recipe could result in the homogeneous composition.
Glass making furnaces have been uncovered in Israel at two sites – 17 at Bet Eli’ezer and 3 at Apollonia. These are rectangular in shape measuring about 2m x 4m and matching in size a glass slab found inside a cave at Bet She’arim. The resulting slabs would be broken up into chunks that were traded to glass workshops in the Middle East and across the Mediterranean as evidenced by the contemporaneous shipwrecks carrying such glass chunks.
The situation is not so simple; these workshops are dated from the 6th to the 11th century AD and although similar the compositions do not exactly match those of the earlier Roman period. Pliny, writing in the 1st century AD, describes glass making in the Levantine coast but also in Italy, Spain and Gaul – however the installations that produced the large quantities of primary glass needed to supply the Roman industry have yet to be located. These same authors report a difference in isotopic ratios for oxygen and strontium that differentiates between Middle eastern and Roman glasses. Other authors counter idea of a unique centralised production with primary production reported from Northern Europe and inferred for Italy.
Evidence for large scale production has only come the Eastern Mediterranean and from latter dates and assumes a continuing glass making tradition. That there was a large scale and centralised production even this early is evidenced by the Ouest Embiez I shipwreck – 3rd century – carrying 8 tons of raw glass. This however does not exclude the smaller scale local production reported elsewhere. The Julia Felix, also sunk during the 3rd century AD, was carrying as part of its cargo glass cullet presumably for recycling. Trace element analysis of colourless glass showed these were made using sands from different sources and giving some support to the de-centralised production hypothesis.
By the 9th century the raw materials for glass shift again to quartz pebbles and plant ash and the ‘forest glass’ of medieval Europe. Natron was no longer in use and the low lime composition of the Bet’eliezer glass would be suggestive of a decline in access to the material from at least the 6th century onwards. This could in turn be suggestive of a shift from a trade both in natron and raw glass to tighter control over natron and increase of centralised production.
Growth of the Roman glass industry
Despite the growth of glass working in the Hellenistic World and the growing place of glass in material culture, at the beginning of the 1st century AD there was still no Latin word for it in the Roman World. However, glass was being produced in Roman contexts using primarily Hellenistic techniques and styles (see glass, history) by the late Republican period. The majority of manufacturing techniques were time-consuming, and the initial product was a thick-walled vessel which required considerable finishing. This, combined with the cost of importing natron for the production of raw glass, contributed to the limited use of glass and its position as an expensive and high-status material.
The glass industry was therefore a relatively minor craft during the Republican period; although, during the early decades of the 1st century AD the quantity and diversity of glass vessels available increased dramatically. This was a direct result of the massive growth of the Roman influence at the end of the Republican period, the Pax Romana that followed the decades of civil war, and the stabilisation of the state that occurred under Augustus’ rule. Still, Roman glasswares were already making their way from Western Asia (i.e. the Parthian Empire) to the Kushan Empire in Afghanistan and India and as far Han Empire of China. The first Roman glass found in China came from an early 1st-century BC tomb at Guangzhou, ostensibly via the South China Sea.
In addition to this a major new technique in glass production had been introduced during the 1st century AD. Glassblowing allowed glass workers to produce vessels with considerably thinner walls, decreasing the amount of glass needed for each vessel. Glass blowing was also considerably quicker than other techniques, and vessels required considerably less finishing, representing a further saving in time, raw material and equipment. Although earlier techniques dominated during the early Augustan and Julio-Claudian periods, by the middle to late 1st century AD earlier techniques had been largely abandoned in favour of blowing.
As a result of these factors, the cost of production was reduced and glass became available for a wider section of society in a growing variety of forms. By the mid-1st century AD this meant that glass vessels had moved from a valuable, high-status commodity, to a material commonly available: “a drinking cup could be bought for a copper coin” (Strabo, Geographica XVI.2). This growth also saw the production of the first glass tesserae for mosaics, and the first window glass, as furnace technology improved allowing molten glass to be produced for the first time. At the same time, the expansion of the empire also brought an influx of people and an expansion of cultural influences that resulted in the adoption of eastern decorative styles. The changes that took place in the Roman glass industry during this period can therefore be seen as a result of three primary influences: historical events, technical innovation and contemporary fashions. They are also linked to the fashions and technologies developed in the ceramic trade, from which a number of forms and techniques were drawn.
Glass making reached its peak at the beginning of the 2nd century AD, with glass objects in domestic contexts of every kind. The primary production techniques of blowing, and to a lesser extent casting, remained in use for the rest of the Roman period, with changes in vessel types but little change in technology. From the 2nd century onwards styles became increasingly regionalised, and evidence indicates that bottles and closed vessels such as unguentaria moved as a by-product of the trade in their contents, and many appear to have matched the Roman scale of liquid measurement.
The use of coloured glass as a decorative addition to pale and colourless glasses also increased, and metal vessels continued to influence the shape of glass vessels. After the conversion of Constantine, glass works began to move more quickly from depicting Pagan religious imagery towards Christian religious imagery. The movement of the capital to Constantinople rejuvenated the Eastern glass industry, and the presence of the Roman military in the western provinces did much to prevent any downturn there. By the mid-4th century mould-blowing was in use only sporadically.
Roman glass production relied on the application of heat to fuse two primary ingredients: silica and soda. Technical studies of archaeological glasses divide the ingredients of glass as formers, fluxes, stabilisers, as well as possible opacifiers or colourants.
Former: The major component of the glass is silica, which during the Roman period was sand (quartz), which contains some alumina (typically 2.5%) and nearly 8% lime. Alumina contents vary, peaking around 3% in glasses from the western Empire, and remaining notably lower in glasses from the Middle East.
Flux: This ingredient was used to lower the melting point of the silica to form glass. Analysis of Roman glass has shown that soda (sodium carbonate) was used exclusively in glass production. During this period, the primary source of soda was natron, a naturally occurring salt found in dry lake beds. The main source of natron during the Roman period was Wadi El Natrun, Egypt, although there may have been a source in Italy.
Stabiliser: Glasses formed of silica and soda are naturally soluble, and require the addition of a stabiliser such as lime or magnesia. Lime was the primary stabiliser in use during the Roman period, entering the glass through calcareous particles in the beach sand, rather than as a separate component.
Roman glass has also been shown to contain around 1% to 2% chlorine, in contrast to later glasses. This is thought to have originated either in the addition of salt (NaCl) to reduce the melting temperature and viscosity of the glass, or as a contaminant in the natron.
Archaeological evidence for glass making during the Roman period is scarce, but by drawing comparisons with the later Islamic and Byzantine periods, it is clear that glass making was a significant industry. By the end of the Roman period glass was being produced in large quantities contained in tanks situated inside highly specialised furnaces, as the 8-tonne glass slab recovered from Bet She’arim illustrates. These workshops could produce many tonnes of raw glass in a single furnace firing, and although this firing might have taken weeks, a single primary workshop could potentially supply multiple secondary glass working sites. It is therefore thought that raw glass production was centred around a relatively small number of workshops, where glass was produced on a large scale and then broken into chunks. There is only limited evidence for local glass making, and only in context of window glass. The development of this large-scale industry is not fully understood, but Pliny’s Natural History (36, 194), in addition to evidence for the first use of molten glass in the mid-1st century AD, indicates that furnace technologies experienced marked development during the early-to-mid-1st century AD, in tandem with the expansion of glass production.
The siting of glass-making workshops was governed by three primary factors: the availability of fuel which was needed in large quantities, sources of sand which represented the major constituent of the glass, and natron to act as a flux. Roman glass relied on natron from Wadi El Natrun, and as a result it is thought that glass-making workshops during the Roman period may have been confined to near-coastal regions of the eastern Mediterranean. This facilitated the trade in the raw colourless or naturally coloured glass which they produced, which reached glass-working sites across the Roman empire.
The scarcity of archaeological evidence for Roman glass-making facilities has resulted in the use of chemical compositions as evidence for production models, as the division of production indicates that any variation is related to differences in raw glass making. However, the Roman reliance on natron from Wadi El Natrun as a flux, has resulted in a largely homogenous composition in the majority of Roman glasses. Despite the publication of major analyses, comparisons of chemical analyses produced by different analytical methods have only recently been attempted, and although there is some variation in Roman glass compositions, meaningful compositional groups have been difficult to establish for this period.
The Roman writers Statius and Martial both indicate that recycling broken glass was an important part of the glass industry, and this seems to be supported by the fact that only rarely are glass fragments of any size recovered from domestic sites of this period. In the western empire there is evidence that recycling of broken glass was frequent and extensive(cullet), and that quantities of broken glassware were concentrated at local sites prior to melting back into raw glass. Compositionally, repeated recycling is visible via elevated levels of those metals used as colourants.
Melting does not appear to have taken place in crucibles; rather, cooking pots appear to have been used for small scale operations. For larger work, large tanks or tank-like ceramic containers were utilised. In the largest cases, large furnaces were built to surround these tanks.
In comparison to glass making, there is evidence for glass working in many locations across the empire. Unlike the making process, the working of glass required significantly lower temperatures and substantially less fuel. As a result of this and the expansion of the Empire, glass working sites developed in Rome, Campania and the Po Valley by the end of the 1st century BC, producing the new blown vessels alongside cast vessels. Italy is known to have been a centre for the working and export of brightly coloured vessels at this time, with production peaking during the mid-1st century AD.
By the early-to-mid-1st century AD, the growth of the Empire saw the establishment of glass working sites at locations along trade routes, with Cologne and other Rhineland centres becoming important glass working sites from the Imperial period, and Syrian glass being exported as far as Italy. During this period vessel forms varied between workshops, with areas such as the Rhineland and northern France producing distinctive forms which are not seen further south. Growth in the industry continued into the 3rd century AD, when sites at the Colonia Claudia Agrippinensis appear to have experienced significant expansion, and by the 3rd and early 4th centuries producers north of the Alps were exporting down to the north of Italy and the transalpine regions.
Glass working sites such as those at Aquileia also had an important role in the spread of glassworking traditions and the trade in materials that used hollow glasswares as containers. However, by the 4th and 5th centuries Italian glass workshops predominate.
The earliest Roman glass follows Hellenistic traditions and uses strongly coloured and ‘mosaic’ patterned glass. During the late Republican period new highly coloured striped wares with a fusion of dozens of monochrome and lace-work strips were introduced. During this period there is some evidence that styles of glass varied geographically, with the translucent coloured fine wares of the early 1st century notably ‘western’ in origin, whilst the later colourless fine wares are more ‘international’. These objects also represent the first with a distinctly Roman style unrelated to the Hellenistic casting traditions on which they are based, and are characterised by novel rich colours. ‘Emerald’ green, dark or cobalt blue, a deep blue-green and Persian or ‘peacock’ blue are most commonly associated with this period, and other colours are very rare. Of these, Emerald green and peacock blue were new colours introduced by the Romano-Italian industry and almost exclusively associated with the production of fine wares.
However, during the last thirty years of the 1st century AD there was a marked change in style, with strong colours disappearing rapidly, replaced by ‘aqua’ and true colourless glasses. Colourless and ‘aqua’ glasses had been in use for vessels and some mosaic designs prior to this, but start to dominate the blown glass market at this time. The use of strong colours in cast glass died out during this period, with colourless or ‘aqua’ glasses dominating the last class of cast vessels to be produced in quantity, as mould and free-blowing took over during the 1st century AD.
From around 70 AD colourless glass becomes the predominant material for fine wares, and the cheaper glasses move towards pale shades of blue, green, and yellow. Debate continues whether this change in fashion indicates a change in attitude that placed glass as individual material of merit no longer required to imitate precious stones, ceramics, or metal, or whether the shift to colourless glass indicated an attempt to mimic highly prized rock crystal. Pliny’s Natural History states that “the most highly valued glass is colourless and transparent, as closely as possible resembling rock crystal” (36, 192), which is thought to support this last position, as is evidence for the persistence of casting as a production technique, which produced the thickly walled vessels necessary to take the pressure of extensive cutting and polishing associated with crystal working.
Vessel production techniques
Core and rod formed vessels
Artisans used a mass of mud and straw fixed around a metal rod to form a core, and built up a vessel by either dipping the core in liquified glass, or by trailing liquid glass over the core. The core was removed after the glass had cooled, and handles, rims and bases were then added. These vessels are characterised by relatively thick walls, bright colours and zigzagging patterns of contrasting colours, and were limited in size to small unguent or scent containers. This early technique continued in popularity during the 1st century BC, despite the earlier introduction of slumped and cast vessels.
This technique is related to the origin of glass as a substitute for gemstones. By borrowing techniques for stone and carved gems, artisans were able to produce a variety of small containers from blocks of raw glass or thick moulded blanks, including cameo glass in two or more colours, and cage cups (still thought by most scholars to have been decorated by cutting, despite some debate).
Glass blowing: free and mould blown vessels
These techniques, which were to dominate the Roman glass working industry after the late 1st century AD, are discussed in detail on the glass blowing page. Mould-blown glass appears in the second quarter of the 1st century AD.
Other production techniques
A number of other techniques were in use during the Roman period:
Cage cup production
Cameo glass production
Cast glass patterns
The glass sheets used for slumping could be produced of plain or multicoloured glass, or even formed of ‘mosaic’ pieces. The production of these objects later developed into the modern caneworking and millefiori techniques, but is noticeably different. Six primary patterns of ‘mosaic’ glass have been identified:
Floral (millefiori) and spiral patterns: This was produced by binding rods of coloured glass together and heating and fusing them into a single piece. These were then cut in cross-section, and the resulting discs could be fused together to create complex patterns. Alternately, two strips of contrasting-coloured glass could be fused together, and then wound round a glass rod whilst still hot to produce a spiral pattern. Cross-sections of this were also cut, and could be fused together to form a plate or fused to plain glass.
Marbled and dappled patterns: Some of these patterns are clearly formed through the distortion of the original pattern during the slumping of the glass plate during melting. However, by using spiral and circular patterns of alternating colours producers were also able to deliberately imitate the appearance of natural stones such as sardonyx. This occurs most often on pillar-moulded bowls, which are one of the commonest glass finds on 1st century sites.
Lace patterns: Strips of coloured glass were twisted with a contrasting coloured thread of glass before being fused together. This was a popular method in the early period, but appears to have gone out of fashion by the mid-1st century AD.
Striped patterns: Lengths of monochrome and lacework glass were fused together to create vivid striped designs, a technique that developed from the lace pattern technique during the last decades of the 1st century AD.
The production of multicoloured vessels declined after the mid-1st century, but remained in use for some time after.
Gold sandwich glass or gold glass was a technique for fixing a layer of gold leaf with a design between two fused layers of glass, developed in Hellenistic glass and revived in the 3rd century. There are a very fewer larger designs, but the great majority of the around 500 survivals are roundels that are the cut-off bottoms of wine cups or glasses used to mark and decorate graves in the Catacombs of Rome by pressing them into the mortar. The great majority are 4th century, extending into the 5th century. Most are Christian, but many pagan and a few Jewish; their iconography has been much studied, although artistically they are relatively unsophisticated. In contrast, a much smaller group of 3rd century portrait levels are superbly executed, with pigment painted on top of the gold. The same technique began to be used for gold tesserae for mosaics in the mid-1st century in Rome, and by the 5th century these had become the standard background for religious mosaics.
Other decorative techniques
A number of other techniques were in use during the Roman period, including enamelling and engraving.
Tesserase and window glass
Shards of broken glass or glass rods were being used in mosaics from the Augustan period onwards, but by the beginning of the 1st century small glass tiles, known as tesserae, were being produced specifically for use in mosaics. These were usually in shades of yellow, blue or green, and were predominantly used in mosaics laid under fountains or as highlights.
Around the same time the first window panes are thought to have been produced. The earliest panes were rough cast into a wooden frame on top of a layer of sand or stone, but from the late 3rd century onwards window glass was made by the muff process, where a blown cylinder was cut laterally and flattened out to produce a sheet.
Chemistry and colours
Iron(II) oxide, (FeO)
‘Aqua’, a pale blue-green colour, is the common natural colour of untreated glass. Many early Roman vessels are this colour.
Iron(III) oxide, (Fe2O3)
Colourless glass was produced in the Roman period by adding either antimony or manganese oxide. This oxidised the iron (II) oxide to iron (III) oxide, which although yellow, is a much weaker colourant, allowing the glass to appear colourless. The use of manganese as a decolourant was a Roman invention first noted in the Imperial period; prior to this, antimony-rich minerals were used. However, antimony acts as a stronger decolourant than manganese, producing a more truly colourless glass; in Italy and northern Europe antimony or a mixture of antimony and manganese continued to be used well into the 3rd century.
Iron-sulfur compounds, 0.2%-1.4% S0.3% Fe
Sulfur is likely to have entered the glass as a contaminant of natron, producing a green tinge. Formation of iron-sulfur compounds produces an amber colour.
Manganese, (such as pyrolusite), Around 3%
Blue and green
The natural ‘aqua’ shade can be intensified with the addition of copper. During the Roman period this was derived from the recovery of oxide scale from scrap copper when heated, to avoid the contaminants present in copper minerals. Copper produced a translucent blue moving towards a darker and denser green.
By adding lead, the green colour produced by copper could be darkened.
Royal blue to navy
Opaque red to brown (Pliny’s Haematinum)
Copper, lead, >10% Cu, 1% – 20% Pb
Under strongly reducing conditions, copper present in the glass will precipitate inside the matrix as cuprous oxide, making the glass appear brown to blood red. Lead encourages precipitation and brilliance. The red is a rare find, but is known to have been in production during the 4th, 5th and later centuries on the continent.
Antimony, (such as stibnite), 1–10%
Antimony reacts with the lime in the glass matrix to precipitate calcium antimonite crystals creating a white with high opacity.
Antimony and lead, (such as bindheimite).
Precipitation of lead pyroantimonate creates an opaque yellow. Yellow rarely appears alone in Roman glass, but was used for the mosaic and polychrome pieces.
These colours formed the basis of all Roman glass, and although some of them required high technical ability and knowledge, a degree of uniformity was achieved.