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Roman glass objects have been found across the Roman Empire in domestic, industrial and funerary contexts. Glass was used primarily for the production of vessels, although beads and other jewellery, mosaic tiles and windows were also produced in glass. Vessels deposited as cineraria and grave goods represent one of our primary sources of glass, and their recovery peaks in graves from the first century to the beginning of the second century, and again during the third century. Roman glass production techniques and styles are known to have had influence outside the bound of the Empire, with early Imperial Roman glass objects recovered in India and Africa.

It is important to note that archaeologists draw a distinction between the production of raw, unshaped glass, and the working of this raw glass into finished objects, a semantic division which reflects the structure of the industry during antiquity.

The growth of the Roman glass industry
Despite the growth of glass working in the Hellenistic world (see history of glass) and the growing place of glass in material culture, at the beginning of the first century BCE 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, however during the early decades of the first century CE 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.

In addition to this a major new technique in glass production had been introduced during the first century BCE. 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 first century CE 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 first century CE this meant that glass vessels had moved from a valuable, high-status commodity, to a material commonly available: “a [glass] drinking cup could be bought for a copper coin” (Strabo, Geography 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 the 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 second century CE, 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 second 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 mythological or pagan scenes towards adopting Christian 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 fourth century mould-blowing was in use only sporadically.

Composition
(see also glass) 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, stabalisers, 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 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 the 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 predominant stabiliser in use, and may have bene derived from limestone or crushed shells . It is unknown whether these were added explicitly during the Roman period, as lime from crushed shells is likely to have entered the glass as a component of the sand used to supply the silica.

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.

Glass making
Archaeological evidence for Roman glass making facilities is scarce, and as a result chemical compositions are more frequently used as evidence of production models. However, due to the physical structure of archaeological glasses and the processes they are subjected to during burial, comparisons of chemical analyses produced by different methods are only recently being published. In addition, although there is some notable variation in Roman glass compositions, large data sets and thorough statistical analyses are required to understand these, and statistically significant comparison between data sets is difficult.

So far, analysis suggests that there are twelve distinct groups of glass, all of which are found in the Levant, but only four of which are present in the West. This, in conjunction with the general homogeneity of Roman glass indicates that the Roman reliance on natron as a flux resulted in a centralisation of production of raw glass in Egypt and the eastern Meditteranean. Raw glass production was therefore centred around a relatively small number of workshops, where glass was produced on a large scale and then broken into chunks for distribution.

However, what archaeological evidence is available suggests that the majority of glass making sites were situated near to the coast, not just for ease of access to the beach sands which formed the bulk of the glass, but also as the sea represented the primary trade routes along which natron left Egypt for the Roman Empire. The large scale production of glass required considerable quantities of wood as fuel, which necessitated locating production sites near to sources of wood.

There is limited evidence to suggest that window glass may have been produced locally.

chronologically the report by Pliny (Natural History 36, 194) in addition to evidence for the first use of molten glass in the mid first century CE, indicates that furnace technologies improved during the early to mid first century CE, in tandem with the expansion of glass production.

Recycling
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.

Glass working
In comparison to glass making, there is evidence for glass working in many location 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 first 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 first century CE.

By the early to mid first century CE the growth of the Empire saw the establishment of glass working sites at locations along trade routes, with Cologne and other Rhineland centres becoming an 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 third century CE, when sites at the Colonia Claudia Agrippinensis appear to have experienced significant expansion, and by the third and early fourth 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 fourth and fifth centuries Italian glass workshops predominate.

Chemistry
See also modern glass colours.

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.

Colour and style


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 first 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 first century CE 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 his, 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 first century AD.

From around 70 CE colourless glass becomes the predominate 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 of metal, or whether the shift to colourless glass indicated an attempt to mimic highly prized rock crystal. Pliny, in his Historia Naturalis, 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.

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 first century BCE, despite the earlier introduction of slumped and cast vessels.

Cold-cut vessels
This technique is related to the origin of glass as a substitute for precious stones. By borrowing techniques for stone and gem cutting, artisans were able to produce a variety of small containers from blocks of raw glass or thick moulded blanks.

Glass blowing: free and mould blown vessels
These techniques, which were to dominate the Roman glass working industry after the late first century CE, are discussed in detail on the glass blowing page. Mould-blown glass appears in the second quarter of the first century CE.

Other production techniques
A number of other techniques were in use during the Roman period:

Cage Cup production Cameo glass production Slumping Casting

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 first 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 first century CE.

The production of multicoloured vessels declined after the mid first century, but remained in use for some time after.
 * 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 first century BCE.

Other decorative techniques
A number of other techniques were in use during the Roman period:

Enamelling

Engraving

Tesserase and window glass
Sherds of broken glass or glass rods were being used in mosaics from the Augustan period onwards, but by the beginning of the first century small glass tiles (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 third century onwards window glass was made by the muff process, where a blown cylinder was cut laterally and flattened out to produce a sheet.