The early modern period in England (c. 1500–1800) brought on a revival in local glass production. Medieval glass had been limited to the small-scale production of forest glass for window glass and vessels, predominantly in the Weald.Kenyon, G.H., 1967. The Glass Industry of the Weald Leicester, Leicester University Press The organisation of production evolved from the small-scale family-run glass houses typical of forest glass-making to large monopolies granted by the Crown.Godfrey, E., 1975. The Development of English Glassmaking 1560-1640. Oxford, Clarendon Press. The influx of immigrants from Europe brought changes in furnace technology and raw materials, creating a better quality glass.Crossley, D.W., 1990. Post-Medieval Archaeology in Britain New York, Leicester University PressCrossley, D.W., 1998. "The English Glassmaker and his search for materials in the 16th and 17th centuries." In McCray, P (ed) The Prehistory and History of Glassmaking Technology, Westerville: Ohio, American Ceramic Society, 167-179 Monastic decrees later banned the use of wood fuel which was then replaced by the less expensive alternative of coal.Willmott, H., 2005. A History of English Glassmaking AD 40-1800, Tempus Publishing Limited The development of lead glass in the late 17th century propelled England to the forefront of the glass industry and paved the way for advancements in the Industrial Revolution.Charleston, R. J., 1984. English Glass and the Glass used in England ca. 400-1940. London, Allen and Unwin
Chemical composition Glass has three major components: a network former (silica), a network modifier (flux), and a network stabilizer (predominantly lime).Terence Maloney, F.J., 1967. Glass in the Modern World. Science and Technology Series. London, Aldus books.Dungworth, D., 2003. Scientific Examination of Glass and Glassmaking Materials from Silkstone, Yorkshire. Centre for Archaeology, English Heritage Report 90/2003 In the early 16th and 17th centuries glassmaking (the manufacture of glass from raw materials) and glassworking (the creation of objects from glass) occurred within the same glasshouse.Crossley, D.W., 1967. "Glassmaking in Bagot's Park, Staffordshire, in the Sixteenth Century." Post-Medieval Archaeology 1,67-72 Glass was also recycled at this time in the form of cullet.
In the early modern era, network formers were obtained from fine or coarse sands which were usually located near the area of production or from silica based pebbles.
Network modifiers were used to alter the chemical composition of the network former and reduce the melting temperature of the batch. These fluxes varied depending on the type of glass. Potassium oxide (K2O) based alkalis were used extensively in glass production.
The type of flux selected heavily influenced the quality of the glass produced. In England, beech wood and oak were preferred for forest glass. For soda-lime glass (Na2O), alkalis were often found in the form of marine plants – either local kelp or imported plants from the Mediterranean and the Near East (barilla, polverine, rochetta, Sevonus, natron).
These were often used for the production of 'white' crystallo glass, a colourless glass or façon de venise, colourless glass of the highest quality.Willmott, H., 2002. Early Post-Medieval vessel Glass in England c. 1500-1670 Council for British Archaeology, Research Report 132
Network stabilizers in early modern England continued to be lime sources. Lime occurs as a natural contaminant in most sands, and may also be intentionally added to the melt.Frank, S., 1982., Glass and Archaeology. London: Academic Press
Compositional groups Five glass compositional groups have been identified through analysis of archaeologically recovered glass from this period.Henderson, J., 1998. "Post-Medieval Glass: Production, Characterization, and Value." In McCray, P (ed) The Prehistory and History of Glassmaking Technology, Westerville: Ohio, American Ceramic Society, 33-61 These have been further reduced into two types, ‘green glass’ and ‘white glass’.Mortimer, C., 1995. "Analysis of Post Medieval Glass from Old Broad Street, London, with Reference to Other Contemporary Glasses from London and Italy." In Hook, D.R. and Gaimster, D.R.M. (eds) Trade and Discovery: The Scientific Study of Artefacts from Post-Medieval Europe and Beyond. London: British Museum, 135-144Dungworth, D., 2005. "The Scientific Study of Late 17th Century Glasswork at Silkstone, England." In AIHV and authors Annales du 16º Congrès de Association internationale pour l'histoire du Verre. Bristol, J.W. Arrowsmith, 254-257 The groups include: Potash-lime-silica glass (forest or green glass), typically has an excess of 10% wt oxide K20 High lime low alkali (green glass) usually has <10% Na2O, K2O, 15-20% CaO Soda-lime glass (white glass/‘ordinary glass’) with low MgO, CaO, high K2O Mixed alkali glass (white glass/crystallo) Na2O K2O and CaO levels are too low for this glass to be incorporated in the other categories. Lead glass (white glass/ façon de venise) has on average 25-35% PbO
The following table represents the mean compositional data derived from the analysis of materials at the Old Broad Street furnace in London, dated to the early 17th century. and those recovered from Phase Two (circa 1680-1700 AD) Silkstone, Yorkshire This information was gathered from Dungworth's compilation and analysis The data is represented in wt% oxides and those below the detection limits (0.2% or less) are shown by '-'.
SiteType Na20 MgO Al2O3 SiO2 P205 SO3 Cl K2O CaO TiO2 MnO Fe2O3 SrO PbO Old Broad St Potash 2.5 6.5 1.4 54.7 3.7 0.07 0.19 11.8 14.5 0.30 1.10 0.92 - - s.d. 1.1 1.2 0.3 3.3 0.4 0.05 0.03 1.3 2.4 0.10 0.23 0.23 - - Old Broad St HLLA 3.0 3.7 3.3 57.9 3.2 0.33 0.26 4.6 20.7 0.32 0.52 1.43 - - s.d. 1.6 1.0 1.8 3.9 1.0 0.28 0.18 1.7 2.0 0.07 0.49 0.52 - -Silkstone phase 2HLLA 1.1 5.1 4.5 52.5 3.1 0.3 0.3 8.5 21.4 0.3 0.9 2.1 0.06 <0.3 s.d. 0.2 0.8 0.8 3.8 0.7 0.1 0.1 0.9 1.8 0.1 0.5 0.4 0.02 - Old Broad St Soda-Lime-Silica 12.2 3.9 1.0 68.6 0.34 0.10 0.35 2.4 10.1 0.07 0.49 0.52 - - s.d. 1.0 0.7 0.3 2.4 0.15 0.02 0.05 0.3 1.8 0.08 0.64 0.27 - - Old Broad St Mixed Alkali 6.0 5.3 2.6 63.5 1.7 0.2 0.7 3.6 15.2 n.d. 0.2 0.2 - - s.d. 0.9 0.4 0.4 1.4 0.6 0.1 0.2 0.6 2.2 - 0.2 0.2 - - Silkstone Phase 2 Mixed Alkali 6.9 2.9 1.4 68.3 0.3 0.1 0.5 6.6 10.5 <0.1 1.0 1.0 0.05 <0.3 s.d. 0.4 0.1 0.3 1.9 0.3 0.1 0.1 0.2 1.6 - 0.1 0.2 0.01 Silkstone Phase 2 Lead glass <.05 <0.3 0.5 53.8 <0.2 <0.2 <0.1 13.2 <0.1 <0.1 <0.1 0.1 <0.02 32.0 s.d. - - 0.1 0.8 - - - 0.6 - - - 0.1 - 0.8
Colorants There are numerous factors that may influence colouration during glass production. These include contaminants in raw materials, furnace conditions, and deliberate additives that would provide known colour variations.
Iron existing as a contaminant in sands, produced either a green or brown colour depending upon the oxidation state. Coal fumes provided a carbon contaminant, which could create a dark brown or black colour. Manganese present in wood ash may have contributed to the lighter, translucent green colour. Other trace elements present in alkalis (such as MnO in beech ash) undoubtedly influenced the finished product.
Other metal oxide colorants were known from earlier periods in antiquity.
Early post-medieval glass Medieval glasshouse traditions continued in the Weald, which was becoming deforested by the early 17th century; local glassmaking spread elsewhere, where timber was available to fire furnaces, to Hampshire, Gloucestershire, North Staffordshire and the Scottish Borders. At Bagot's Park, Staffordshire, one such glasshouse has been recovered, which dates from circa 1535; it contained an early melting furnace and a smaller annealing furnace. The melting furnace had two siege benchesThe siege is the floor of a glass-furnace. for the placement of three crucible pots, each with a central flue cut into the floor to create a draught that would allow the furnace to achieve 1200 °C in order to melt the glass. Fritting, and the preheating of crucibles may have occurred in the upper areas of the main furnace. Annealing (glass) and glass blowing probably occurred using a smaller furnace. Cullet heaps of broken glass residue were found on either side, suggesting the use of a flux to reduce melting temperatures. Some crushed white pebbles were recovered in the bottom of pots, and this may reflect the silica source used at this site. The glass recovered from Bagot's Park was badly weathered, yet the ends of broad glass and crown glass suggest that window and vessel glass were produced.
Glass technology The majority of glass at this time was blown or mould blown into a variety of vessel shapes. This was enhanced by decorative styles, including optic decoration and trailing the glass, sometimes with pre-fabricated glass canes, to replicate Venetian traditions. In glass tank furnace we generally used AZS block in bottom section.
Influences from the Continent In 1567, Jean Carré arrived in London from Antwerp and obtained a crown-sanctioned patent for the production of window glass. This patent was awarded to Carré on the condition that prices remained low and that glassmaking and blowing would be taught to native Englishmen to promote the craft. He brought many Venetian craftsmen to his London workshop and opened a second furnace outside the city to produce vessel and green glass.
Later in 1574, Jacob Verzelini, a Venetian who worked for Carré was granted a monopoly over Venetian-style vessel glass. This effectively banned most of the imports from Venice and promoted glass made locally in England. Verzelini's goal was to produce clear crystallo glass as well as decorative glass façon de venise ("in the Venetian mode"), which he achieved by importing barilla from Spain. This effectively helped to lower the price of clear glassware and made it available to a wider range of the gentry and middle class.
Utilitarian green glass production remained on a small scale and was made by numerous glasshouses in different areas for local consumption, in the tradition of forest glass.
Technological changes With the new influx of immigrants from the European Continent in the mid-16th century, technological changes affected the quality of English glass. This was possibly the combined result of experience and the selection/importation of purer raw materials.
Winged furnaces Additionally, glass furnaces constructed from the mid-16th century began to reflect continental styles. This trend, identifiable in the archaeological record, supports the documentary evidence for immigrant glassmakers. Wing-like additions were added to the late 16th–early 17th century furnace remains at two glass producing sites, Hutton and Rosedale in York, as well as at Vann Copse in the Weald.Aberg, F. A. and D.W. Crossley. 1972. "Sixteenth Century Glass-making in Yorkshire". Post-Medieval Archaeology 6, 107-159 The Hutton furnace had two wings added in the northeast and southeast corners of the original rectangular melting furnace. A smaller nearby furnace was abandoned around the same time as the addition of the wings, suggesting that they provided an area for either annealing or pre-heating pots.