Glassmaking, like ceramic production, is one of those skills that went into abeyance in post-Roman Britain. The people of Britain seem to have been supremely competent when working in metal, wood, leather, bone, stone, textile, and even manuscript illumination during the early medieval period, but for glass they went elsewhere. High-status carousers at Tintagel and Cadbury used decorated glassware from south-west France and Spain for their alcohol consumption in the 5th and 6th centuries, while glass of Continental and Mediterranean origin was finding its way to high-status settlement sites as far apart as Devon and Strathclyde. Meanwhile, excavated finds from East Anglia show that this fragile material was reaching the region from the Rhineland. Rare and jewel-like millefiori glass was used as a decorative insert into early medieval metalwork – again possibly imported from the near Continent – and grave goods show that high-ranking women wore necklaces in which glass beads were treated as equivalent to semi-precious stones and jewels.
Various excavations have uncovered evidence for insular glassmaking in the 6th to 9th centuries (for example, on Iona and at Whithorn), but when Benedict Biscop set about building his model Roman-style monastery at Wearmouth in AD 673, he had to bring glaziers from Gaul to make glass for the windows, lamps, and vessels used in the church. It is likely that glass salvaged from Roman sites was being recycled here, rather than being manufactured using raw materials. Certainly, this was true at Glastonbury Abbey, where chemical analysis of the glass from late 7th- or early 8th-century furnaces show that it was made from reused Roman glass. Carbon-dated charcoal from the same furnaces suggests that glassworking here was linked to the building of the first stone church by King Ine (AD 688-726) and that, as at Wearmouth, specialist masons and glaziers were brought to Glastonbury from the Continent to build the church and make the windows – indeed, it has been suggested that the same team from Gaul worked at Wearmouth, Glastonbury, and also, perhaps, at York.
Leaping forward to the 14th century, where David Dungworth’s book on glassmaking in England begins (see ‘Further reading’ on p.27), the archaeological and historical evidence shows that domestic manufacture was now well-established in the Weald of Sussex, Surrey, and Kent, as well as on Cannock Chase in Staffordshire, and probably started in the previous century (an early 13th-century deed grants 20 acres of land near Chiddingfold, Surrey, the site of a later glassworks, to Laurence Vitrearius, a name derived from the Latin vitrum, glass). Crucially, the Weald and the Chase were both home to the extensive tracts of managed woodland that was needed as fuel to form molten glass. This required heating a mixture of silica (often in the form of river sand), a flux (to lower the melting point), stabilising elements (to stop it from tarnishing), and metal oxides (to provide colour) to temperatures of 1,100-1,400°C.
It is more than probable that we also owe the establishment of this ‘native’ industry to glassmakers from the Continent settling in Britain. After the Norman Conquest, ties between England and Normandy were strengthened, with aristocratic families owning estates on both sides of the Channel. The large-scale rebuilding of cathedrals, abbeys, and parish churches after the Conquest created a new market for window glass, though much of this continued to be imported via Antwerp from the Lorraine region of France, as tax records and Port Books show.
The English glass industry received a major stimulus in the late 16th century with the arrival of Protestant refugees from the Wars of Religion (1562-1598) in Catholic France and the Low Countries. The Arras-born glass merchant Jean Carré, who came to London as a religious refugee, seized the opportunity created by the disruption of glass supplies from the Continent to expand local production. In 1567, he obtained a royal patent for the manufacture of window glass ‘such as is made in ffrance, Lorayne, and Burgundy [sic]’ – in effect, a monopoly lasting for 21 years.
Carré then brought over glassworkers from France and introduced new furnace technology that helped to reduce the cost and raise the quality of English glass – hence the products of glassmaking at this time are commonly classified as ‘early’ (pre-1567) and ‘late’ (post-1567). These migrant families were based initially in the established centres of glassmaking in the Weald and Cannock Chase, but production had spread to areas with an adequate fuel supply in Somerset, Gloucestershire, Shropshire, Lancashire, and Yorkshire by the start of the 17th century.
Carré’s success contained the seeds of the industry’s own demise, because the huge quantities of fuel needed by the French glassmakers led to the inevitable rise in timber prices and brought them into conflict with the other users of timber, such as those who sold domestic firewood. A Royal Proclamation of 1615 banned the use of wood as fuel, at which point the French glassmakers relocated to new areas, such as Bristol, south Staffordshire, and Newcastle, where they had access to supplies of coal.
Excavating for evidence
The locations of several medieval glassworking sites in the Weald have been known since the 19th century, in part because placenames (such as Glasshouse Lane and Glasshouse Field) have preserved references to past work, and partly because debris from the industry has regularly been ploughed up. Some sites were excavated and published in the 1960s, while more recently the Wealden Glass Industry Project (undertaken by Surrey County Archaeological Unit with funding from Historic England) has identified 49 sites in 12 parishes straddling the Surrey/Sussex border – though they believe there are many more unrecorded sites, as glassmakers would periodically relocate to new sites when they exhausted the supplies of timber within the vicinity of their furnaces. Three sites have recently been excavated by the project, uncovering the remains of kilns, crucibles, and glassworking waste, all of which is currently being analysed to understand what technical innovations were introduced by the migrant workers and what products were being manufactured.
We know from excavations that the earliest furnaces were rectangular, probably barrel-vaulted, and built of sandstone cemented with clay. A central fire trench, about 3m in length, was fed with fuel from both ends. The trench was flanked by low walls on which the crucibles sat containing the raw materials that were heated to create molten glass. The French flues were similar, built of brick, stone, and clay, but with wings at each corner, containing additional chambers in which hot air from the main furnace was channelled to raw materials that were heated to a lower temperature (a process called ‘fritting’) to drive off moisture and volatile components, prior to final firing. The chambers were probably also used for preparing new crucibles before they were introduced to the full heat of the furnace and for annealing finished products by heating and then slowly cooling them to strengthen the crystalline structure of the glass and make it less brittle. The use of one furnace for all of these processes, rather than several different kilns, made more efficient use of fuel and reduced the cost of the end product.
Further cost reductions could be achieved by increasing the amount of molten glass that each furnace could produce. The earliest crucibles had vertical sides and held 25kg to 50kg of glass, while later versions, of more open form, could hold 85kg to 100kg. The crucibles themselves had to be made of material that would remain stable at the melting temperature of glass. They were commonly made of fine white-firing quartz-tempered clay, probably obtained from sources outside the Weald, such as those used for the Surrey whiteware industry around Farnham. Later crucibles used copious amounts of grog, made from the ground-up particles of old crucibles, which could withstand higher temperatures without the quartz in the surface of the crucible reacting with the molten glass.
Finds from the furnace sites show that the primary products were crown glass, along with lamps and beakers of relatively simple form. Crown glass was made by spinning molten glass at the end of an iron rod to form a disc, typically 0.8m in diameter. When detached from the rod, the disc was cut into small rectangular panes that could be formed into larger windows by joining them with lead cames. The final product had concentric ripples, and the thickness of the glass varied from the centre to the edge. The central pane, the thick and opaque ‘bullseye’, was often used to decorative effect in Victorian and Arts and Crafts windows, but it was not so highly regarded by medieval glaziers, and is often found amid the waste glass intended for recycling at early production sites.
A final difference between early and late medieval glass was probably most visible in the colour of the resulting products. Earlier examples are known as ‘forest glass’ because potassium-rich ash derived mainly from bracken was used as a flux, reacting with silica when heated to lower the melting point. Migrant glassmakers brought a new recipe, probably based on tree ash or a wider range of plants (documentary sources show that weeds and the unused stalks and branches from agricultural plants were used). Less flux was needed, and the resulting product (known as HLLA, ‘high lime low alkali’) was more stable and more durable. Both types of glass had a range of naturally occurring colouring agents – iron oxide in sand and manganese oxide in plant ash, for example – and the different proportions of each in the recipe meant that forest glass is pale green in colour while HLLA is a darker and more blueish green.
Innovation and industry
The amount of glass being produced prior to 1615 can be judged from the chemical analysis of medieval glass surviving in cathedral windows, which shows that around half of the panes are of English origin, half imported. Port Books also show a considerable reduction in imported glass from the mid-16th century onwards. The type of glass that remained in demand from overseas was the almost colourless kind associated with Venice, but also produced by Venetians who migrated northwards to set up workshops in Germany, France, and the Low Countries. Eventually, London began producing it in the façon de Venise when the Italian Jacopo Verzelini set up a glassworks within the walls of the abandoned monastery of Crutched Friars (see CA 338) and obtained letters patent in 1574.
The subsequent development of glassmaking in England owes much to the search for high-quality colourless glass, and the early 17th century was a period of experimentation. At the same time, 21-year patents led to the industry being concentrated in a few hands. Edward Zouch patented a coal-fired furnace in 1611 and, when the use of wood fuel was banned in 1615, Zouch’s company, subsequently acquired by Sir Robert Mansell, effectively gained sole control of the English glass industry. Based in Southwark, and then in the former Austin Friars monastery in Old Broad Street, his company manufactured the façon de Venise vessels that have been found at high-status sites in London and in waste dumps in the City. Chemical analysis demonstrates the use of sodium-rich plant ash as a flux, and records show that Mansell imported around 50 tons of barilla a year from Spain and Portugal; made by burning glasswort, this was purer and chemically more consistent than unrefined plant ash (CA 369).
The ending of Mansell’s monopoly coincided with the beginning of the English Civil War, when all forms of ‘vanity’ were discouraged, but records show that Venetian glassmakers continued to come to England during the Commonwealth, and a tax was imposed on subsequent manufacture, so it must have continued and there must still have been a small market for luxury glass. By the time post-Restoration glassmakers began applying for new patents, various attempts were being made to replace plant-based ingredients of variable composition with carefully selected fluxes and stabilisers, mixed in more precisely measured amounts. George Ravenscroft (1632-1683) was one of the pioneers in this field, based in the grounds of the old palace and hospital at the Savoy in London. Robert Hooke (1635-1703), the scientist and pioneer of microscopy, visited the glassworks with Christopher Wren on 29 July 1673 and recorded seeing ‘calcined flints, as white as flour, borax, niter [saltpetre], and tartar’ being used.
Ravenscroft subsequently succeeded in obtaining a patent for a colourless glass made using ground flint as a purer and less variable source of silica than river sand, with lead oxide as a flux. Issuing the patent in 1674, the Attorney General predicted that glasses made from the new formula will ‘equalise if not excel those that are imported from Venice and France’. After initial teething problems, this new product did indeed reach a level of such crystalline perfection that fine tableware made from English flint glass was in demand throughout Europe, leading the Venetian Secretary in London to bemoan the fact that English manufacture was ruining Venetian trade.
None of the patents issued to glassmakers during this period specified the precise recipes, so almost everything we know about production in this formative period has come from the chemical analysis of artefacts recovered from archaeological excavations. This shows that once the production of flint glass had been perfected in the 1690s, the basic recipe continued to be used unchanged for fine tableware into the 19th century, and that traditional HLLA continued to be used for windows, bottles, and utilitarian tableware throughout the same period. The major innovation of the period for HLLA was the increasing use of kelp ash, derived from seaweed (CA 369), as a flux from the start of the 18th century. Window glass made of kelp ash is less strongly coloured – a pale blue-green rather than the dark green of earlier HLLA, because of seaweed’s lower iron-oxide content. But kelp (used in the ratio of two parts ash to one part sand) was expensive, and the bottle industry continued to use fluxes made from terrestrial plants.Over the next 150 years, glass production developed from a relatively small-scale manufacturing process controlled by a small number of families to something more like an industry. Production volumes often varied according to the prevailing rate of taxation, which was levied at different rates according to the weight of the finished glass. In 1764, the finest glass was taxed at 9s 4d per hundredweight, and bottle glass at 2s 4d; by 1828, the rate of duty on the best glass had risen sevenfold, to £3 13s 6d per cwt, and glassmakers were constantly inspected by Excise officers who aimed to prevent basic commodity glass being used for tableware to reduce the tax. An Act of 1795 made it illegal to ‘stir or break up the fire or add fresh coals… [to] wilfully raise any smoke or vapour, whereby the officer may be hindered or obstructed in gaging or examining any pot or pots placed or set in any furnace or annealing arch’. So frequent were the attempts at tax avoidance that eventually the regulations were extended to allow the locking of annealing chambers, with the keys held by Excise officers, who alone had the power to release glassware for sale.
Advent of automation
Glassmakers constantly sought ways to avoid tax. Cut glass represented the peak of fine tableware and was always expensive until the development of pressed glass in the 1830s, which enabled the mass production of tableware that imitated the light-reflecting patterns of cut glass. But the fashion for cut glass during the early 18th century was initially driven by the desire to reduce the weight of the glass and hence the tax. A Commission of Inquiry in 1835 blamed these Excise regulations for stifling innovation, because new types of glass would always be taxed at the highest rate. This meant, for example, that England failed to develop its own lens-making capability, relying on imports from continental Europe for the lenses used in spectacles, telescopes, and microscopes. Glassmaking had, as a consequence, seen only modest growth compared to the massive increases in volume experienced by other types of manufacture during the Industrial Revolution. There were, according to Excise statistics, 87 glass furnaces in England at the end of the 17th century and only 97 in 1835.
With the repeal of Excise taxation and control in 1845, glassmakers were freed to use whatever materials and processes they thought best, unleashing a rapid series of developments, such as the recognition that sodium and potassium were the active flux ingredients in plant ash and the consequent development of synthetic soda, based on common salt, as a cheap and consistent flux. In a break with past preference for clear glass, consumers began to buy glass made with new colouring agents: copper (blue, green, and red), cobalt (violet), chromium (emerald green), iron (amber), and uranium (fluorescent green and yellow). As well as glasses for drinking, the tableware repertoire expanded to include vases, fruit bowls, plates, candlesticks, water jugs and decanters, salt cellars, butter dishes, finger bowls, and much more.
The analysis of archaeological finds from this period shows that glassmakers began to produce two types of bottle: dark-coloured ones for beer and wine using traditional recipes and cheap sand with high levels of iron oxide as a colouring agent, and clear ones for the milk and soft-drinks sector, designed to show off the contents and made from paler sands and synthetic soda. Soda also acted as a decolourant, and so was much used for window glass. Chemical analysis shows the use of arsenic from 1835, added late in the melting process to remove bubbles of oxygen, but this was discontinued in the 1870s, undoubtedly because of its toxicity, and replaced by potassium nitrate.
The manufacture of fine tableware had long relied on the skills of the glass-blower, but the introduction of press-moulding from the 1830s required far less training and skill. This saw a slow but steady expansion in volume, although early machines still required the worker to gather just the right amount of molten glass by hand. It wasn’t until the end of the 19th century that a fully automated machine for making bottles and jars was invented, and this required new furnace technology, using coal gas as the fuel, to deliver large quantities of molten glass to the bottle-forming machines. Automation did not entirely spell the end for the highly skilled and well-paid glassworker though. The Percival Vickers glassworks in Manchester continued to issue two catalogues for different markets: in 1881, one had 800 press-moulded items, and the other nearly 500 items were blown and cut designs. Even so, work that had long been restricted to the sons of a small group of families, many of them naturalised descendants of migrant workers, was now being undertaken by a much wider pool of labourers, no longer required to undergo an extensive apprenticeship, and bringing an end to a remarkable story of continuity stretching back some seven centuries.
David Dungworth, Glassworking in England from the 14th to the 20th Century, Historic England, £70, ISBN 978-1848022850.