The history of glass in the UK and Ireland

The texture, nature, patterns of imperfections and subtle hues of historic glass derive from the variety of techniques with which the material has been made since the 13th century.

Cylinder glass being blown in the workshop of Glashütte Lamberts, Germany (Photo: Glashütte Lamberts).

1 Introduction
2 Window tax
3 What is glass?
4 Early double glazing
5 Conserving historic window glass
6 Related articles on Designing Buildings

Introduction

Where historic glass survives in old buildings, it is often an inseparable part of their character and history. Visually, the glass bends the light, transforming the interior and gently distorting the views out from the windows. Historically, the texture of the glass, the nature and patterns of its imperfections and subtle hues of colour tell the story of how and when the glass was made, while the size and quality of the panes can hint at the wealth and social status of the past owners.

Between the 13th and mid-20th centuries, glass manufacture in Great Britain and Ireland was a complicated craft requiring the firing of precise quantities of materials in large furnaces under harsh conditions. Depending on the variations in the glassmaking technique over time, molten liquid glass was blown, spun, pulled, polished, ground and cut to make increasingly larger and more affordable window panes. The processes gradually improved to reduce the imperfections and achieve a consistent colour, texture and thickness, culminating in the fully mechanised manufacture of modern clear float glass on an industrial scale in the 1950s.

Window tax

Until the mid-1800s, glass and window taxes made glass an expensive commodity. In Britain, windows were taxed from 1696 to 1851, and glass was separately taxed from 1745 to 1845. Window tax was levied in Ireland from 1799 to 1822, and glass from 1825 to 1845. Glass tax was calculated by glass weight and window tax by the number of windows in a property.

In addition to the taxes, the Crown controlled 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

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

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 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.

Polished plate glass

Polished plate glass was manufactured from 1773, providing high quality glass, mostly for use in mirrors, coaches, shopfronts and the homes of the very wealthy. The process was very labour intensive and therefore expensive.

Molten glass was cast on to large iron tables, rolled flat, and then placed in an annealing oven to cool. A grinding wheel was used to remove roughness and polish the glass to a very smooth finish. In 1789 the British Cast Plate Glass Company began using a steam-powered machine to grind and polish plate glass, a process permitting large panes. The glass was thick and heavy, to bear the large size, and glass tax made it expensive at the time.

Improved cylinder glass

Improved cylinder glass became available in the early 19th century, following developments in the earlier broad sheet process. Blown cylinders were swung in a trench to achieve sizes of around 4 x 0.6 metres. Larger resulting cylinders were flattened out and cut into sizeable panes which were more affordable and of better quality than crown glass. Improved cylinder glass was used to glaze the Crystal Palace in 1851. Cylinder glass has a slightly rippled surface and long air bubbles align with the ripples to form straight, parallel lines.

Machine drawn cylinder glass

Machine drawn cylinder glass was produced from 1903, when glass manufacture was developed on an industrial scale in the USA, propelled by advances in coal and gas firing technologies. The American Window Glass Company in Pittsburgh, Pennsylvania, developed a method for making very large cylinders 12 metres high. These were mechanically drawn vertically from a circular tank, after which the glass was slowly cooled, cut lengthways, reheated and flattened to form vast sheets of cylinder glass. In the UK machine drawn cylinder glass was manufactured by Pilkington from 1910 to 1933.

Flat drawn sheet glass

Flat drawn sheet glass, emerging in Belgium in 1913, was produced in Kent in the UK from 1919. The glass was drawn vertically in a flat sheet until it cooled sufficiently to allow the glass to be cut. The glass has a wave in one direction only.

Single and twin ground polished plate glass

Single and twin ground polished plate glass was cast and then ground and polished on a conveyor belt to a fine quality without distortion. Single ground was available from 1923 and in 1935 Pilkington developed the twin ground polished plate system. A continuous ribbon of glass about 300 metres long was ground simultaneously on both surfaces in the electrically powered machine, with enormous grinding wheels fed with progressively finer sand.

Float glass

Float glass, which is now ubiquitous, had emerged by 1960. Earlier glassmaking processes relied on interrupted sequences of firing, shaping and cooling of glass. In the 1950s Sir Alistair Pilkington invested large amounts into developing an interrupted process to continuously manufacture large quantities of glass. A continuous ribbon of molten glass was floated onto a bath of molten tin, held in a chemically controlled atmosphere at a high enough temperature for a long enough time for the irregularities to melt out. During the process, the surfaces of the glass became flat and parallel, assuming the dead flat qualities of the molten tin, and resulting in a reflective and unblemished glass.[4] In 1959 Pilkington successfully produced high quality glass in this method, which is used to this day to produce window glass.

Until recently, float glass was produced in different grades, with lowest grade ‘horticultural glass’ containing slight imperfections. It was used for greenhouses and industrial buildings, and as replacement for crown glass in conservation.

Early double glazing

As early as the 19th century, concerns about heat and noise transfer of windows led to developments in double glazing. In 1865, an American, Thomas Stetson, patented a simple system comprising two puttied sheets of glass with a narrow cavity and a timber spacer bar. The system had its limitations, as it did not allow for differential movement between the two glass panes. Double glazing was manufactured in the US on a commercial scale by 1944, but it did not take off in the UK until the 1970s.[5]

Double glazing was initially difficult to retrofit into historic windows because of the deeper glazing rebate required by the double glazing unit. Developments in ‘slimglaze’ technology in recent years have permitted slimmer glazing units to be fitted into older windows, resulting in the loss of historic glass.

Conserving historic window glass

While local authority consent is generally required to replace glass on protected structures and in conservation areas, the removal of historic glass in the remaining older housing stock is continuously contributing to the loss of character in the towns and cities in Ireland and in the UK.

Where traditional glass is present in a window, it should be retained if possible. Replacement of panes due to small cracks in the corners of panes should be avoided unless there is evidence of water ingress. It may be possible to repair larger cracks in very valuable glass with epoxy applied by a specialist. Replacement glass should match the original as closely as possible and may require sustainably sourced salvaged glass.[6]

References

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.
4 L.A.B. Pilkington (1969) ‘Review Lecture: The Float Glass Process’, Proceedings of the Royal Society of London, Royal Society, December 16.
5 https://passivehouseplus.ie/blogs/double-talka-brief-history-ofinsulated-glazing
6 Inform Guide: Maintaining Traditional Plain Glass and Glazing, Historic Environment Scotland (2007)

This article originally appeared in the Institute of Historic Building Conservation’s (IHBC’s) Context 177, published in September 2023. It was written by Katia Papkovskaia, a conservation architect, who is an associate of John McLaughlin Architects, Dublin.

--Institute of Historic Building Conservation