CONTEXT 177 : SEPTEMBER 2023 15 WORKING WITH GLASS the import and export of glass from other countries, including Ireland, which had a flourishing glassmaking industry in the 17th and 18th centuries. In 1746, export of all glass from Ireland was prohibited, with import allowed only from Great Britain. Further duty on Irish glass in 1825 effectively shut down any remaining glasshouses.1 Improved cylinder glass, produced in Britain from the 1830s and widely exported, was never made in Ireland.² The abolition of window and glass taxes in Britain coincided with improvements in cylinder glass technology in the 1850s, which reduced manufacturing costs and allowed for larger sheets of glass to be produced. The availability of larger panes, and therefore larger windows with fewer glazing bars, greatly impacted the appearance of the Victorian buildings, and made large glass structures like conservatories and glasshouses affordable to the emerging middle class. What is glass? Window glass was historically made by firing a measured mixture of sand, flux (plant ash) and lime. Incidental or intentional presence of metallic oxides in the mix can offer glass various hues of colour. Glass must rapidly solidify from its hot fluid state to become clear and prevent formation of visible crystals. As it begins cooling, glass becomes viscous and can be shaped, followed by further slower cooling to reach its final solid state. Broad sheet glass, also known as broad cylinder glass, was the earliest type made in Great Britain and Ireland. It was manufactured by blowing molten glass into a pear shape, about 60 to 75 cm long by 20 to 25 cm in diameter. The ends were removed and the resulting hot cylinder was split with shears and flattened on a sanded iron plate. Broad glass is textured and contains imperfections, picked up from the flattening process. The glass is thin, frequently with a grey tint. Due to the limitations of the cylinder size, panes of broad glass were very small, so they were fixed together by a lattice of lead kames to form small window lights, separated by timber or stone mullions.3 Crown glass, or spun glass, was the preferred choice for window glass from the late-17th until the mid-19th century. The glass quality was often very good, with an almost unmarked fire-finished surface. The crown glass making process was eloquently described by Henry Chance in 1888: A man… with a veil before his face, stands in front of a huge circle of flame, into which he thrusts his piece rapidly, meanwhile revolving his ponty (the iron rod). The action of heat and centrifugal To make crown glass, square panes were cut from a round crown, involving wastage. The numbers give the sizes in inches. Crown glass in production, from Roger Dodsworth’s Glass and Glass Making (1984) 1 George O’Brien (1918) The Economic History of Ireland in the 18th Century, Maunsel and Company, London. ² Andrew Carpenter, Rolf Loeber, Hugh Campbell, Livia Hurley, John Montague, Ellen Rowley (eds) (2015) Art and Architecture of Ireland Volume IV: architecture 1600– 2000, Royal Irish Academy. ³ Christopher Salmond (1995) ‘A Brief History of Glass’, Context 48, December. force combined is soon visible. The nose of the piece expands, the parts around can not resist the tendency… the next moment, before the eyes of the spectator, is whirling a thin transparent circular plate of glass which but a few minutes before was lying in the glass pot. The disc, ranging between 1.4 to 1.8 metres in diameter, was cut into orthogonal panes, with some resulting wastage. The central section of the disc, the bullion, was usually discarded. Crown glass is slightly concave and often has distinctive semi-circular lines. It is thinner, brighter and more reflective than cylinder glass.
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