Environmental protective glazing

Environmental protective glazing (EPG), usually internally ventilated, is generally accepted to be the best approach to conserving vulnerable historic stained glass.

Polycarbonate sheets being installed across the windows of the former Parliament Street Methodist Church in Nottingham (Photo: Jonathan Taylor).

1 Introduction
2 Protection from mechanical damage
3 Protection from environmental damage
4 Design of EPG systems
5 Aesthetic considerations
6 Information
7 Related articles on Designing Buildings

Introduction

For centuries, stained glass has decorated our prestigious public buildings, from churches and cathedrals to royal palaces. As artworks, stained glass windows may tell a story or teach us how to live a good life, they may commemorate important people or events, and they will often fill a building with the beauty of light and colour.

Stained glass windows are not just works of art. They are also a vital part of the building envelope, providing a weather seal that separates the interior from the exterior. The material nature of stained glass means that it is intrinsically fragile; yet if well designed, well supported and protected, it is capable of surviving many centuries. The primary threats to stained glass windows arise from mechanical damage and from environmental conditions.

Protection from mechanical damage

Mechanical damage to stained glass often results from impact by hail, stones or other missiles, whether accidental or malicious. Protection against such damage has traditionally been provided using wire guards or grilles.

Historically, guards have been made from iron or galvanised wire that eventually rust, or sometimes copper wire, which is durable but can stain the surrounding stonework green. The material most commonly used today is a welded stainless steel mesh, which can have quite a slender gauge, strong enough to provide good mechanical protection while still being fine enough for the guards to be reasonably unobtrusive. Guards are often powder coated to a dark colour, usually black or dark grey. Wire guards that are well made to fit within the stonework reveal of each opening, and securely fixed to the stonework with stainless steel clips and screws, offer effective protection that will last for many years. The main disadvantage of wire guards is the shadow lines that can sometimes be seen through the stained glass from the interior, particularly with clear or lightly painted glass.

An alternative to wire guards that has been used in recent decades is clear polycarbonate sheet. This can provide effective protection even against the smallest projectiles and avoids the shadow lines of wire guards. However, polycarbonate has a relatively short lifetime (manufacturers generally guarantee performance for 10 years) before it starts to deteriorate, gradually losing transparency, and eventually becoming discoloured and opaque. This can greatly detract from the external appearance of the building, giving a blind look to the windows. For this reason, the use of polycarbonate is not recommended by the Church of England (ChurchCare), although it can still be permitted in areas of high vandalism.

Protection from environmental damage

Environmental damage may be a less obvious threat to stained glass. After all, one function of the windows is to seal the building from the external environment. However, as part of the building envelope, stained glass windows are particularly exposed to the weather, and the potentially damaging effects of sun, wind, rain and pollution. Atmospheric moisture, in particular, whether rain (on the external surface) or condensation (on the internal surface), is a powerful agent of environmental damage.

The presence of moisture encourages the corrosion of medieval glass, leading to pitting of the surface and loss of transparency, and the loss of fragile painted detail from both medieval and 19th-century stained glass. Moisture also supports biological growth such as moulds and algae, which hold moisture longer on the surface of the glass, accelerating the rate of deterioration.

The installation of internally ventilated environmental protective glazing (EPG) allows air from within a building to pass freely over both sides of the stained glass. The air, being of equal temperature on both sides, keeps the stained glass dry and in as near museum conditions as can be achieved within the architectural setting.

In the UK the use of EPG was pioneered some 50 years ago by Barley Studio founder Keith Barley. He was able to assimilate the best ideas and materials found on windows throughout Europe with the help of a Winston Churchill Travelling Fellowship project.

The use of EPG is generally agreed to be the best approach to environmental protection for stained glass. Indeed, the international Corpus Vitrearum Guidelines for the Conservation and Restoration of Stained Glass (2nd ed, 2004) state that ‘The installation of a protective glazing system is a crucial part of the preventive conservation of architectural stained glass’.

Relieved from their practical function of being a climatic barrier, the protected stained glass windows are displayed as works of art in EPG that significantly retards further deterioration. The installation of EPG brings other benefits, notably that further conservation treatments can be minimised. For example, re-leading (even of weak or buckled panels) can often be avoided and the existing leadwork conserved. This is particularly important for 19th-century stained glass, where the leadwork is likely to be original to the artwork. Non-invasive treatments, such as ‘cold’ painting to replace lost or faded painted detail, can be introduced to enhance the visual appearance.

It is also possible to incorporate additional protection along with the environmental protection, for example by using laminated safety glass as the secondary layer for additional mechanical protection (removing the need for wire guards), or using ultra-violet protective glass to protect both the historic glass and the interior of the building.

Design of EPG systems

Technically, the installation of protective glazing involves the introduction of a second glazing layer outside of the historic glass. Crucially, the installation must allow for ventilation of the interspace between the two glazing layers. Unventilated protection can be damaging, trapping heat, moisture and dirt against the historic glass, especially if the gap between the two layers is minimal.

Over 50 years of international research and monitoring has shown that the best environmental conditions for the historic glass are achieved through internal ventilation of the interspace, although there are different ways to achieve this, depending on the particular installation. It is always necessary to consider both the stained glass and its surrounding fabric (stone, brick or timber frame; supporting metalwork; decorative features) in designing an appropriate and effective installation.

Internal ventilation ensures that damaging condensation no longer forms on the internal surface of the stained glass, and the reduction in daily temperature cycles helps to prevent thermal stresses on the glass. In the most often used internally ventilated system, the original glass is taken out of its existing position within the window and new protective glazing (appropriate clear glass) is set in its place. The stained glass is then framed and reset in front of the new protective glazing, to the interior of the building, allowing for internal ventilation of the interspace.

The alternative, external ventilation, might be preferred in specific cases, where internal ventilation is difficult or impossible to achieve due to the stonework profile or the presence of historic metalwork (ferramenta). In some cases a period of environmental monitoring by a recognised body may be recommended before or after installation.

The Corpus Vitrearum Guidelines notes: ‘Protective systems vary according to needs, and may range from an externally installed and ventilated layer to the internally ventilated isothermal glazing system, which is the most effective method currently available.’

Aesthetic considerations

The installation of environmental protective glazing is a reversible intervention to the architecture of the building. Aesthetically, the major intervention is the new protective glazing which is visible to the exterior (the change in appearance from the interior should be barely noticeable). This aesthetic impact of the change in the external appearance can be mitigated through appropriate choice of the outer protective layer. Options should be evaluated for each project, and include the use of kiln distorted or machine drawn restoration glasses to reduce surface reflections, and the introduction of leadwork to a simplified version of the stained glass design.

With part leaded EPG, the protective glazing gives the appearance of a leaded light from the exterior. Great care must be taken in the design so that the leadlines of the EPG do not cast distracting shadows through the stained glass to the interior. Barley Studio have installed part leaded EPG at St Denys’ Church in York, and this approach has been taken in recent projects at York Minster and Canterbury Cathedral.

An alternative approach is full-pane EPG, an honest approach to protective glazing, with the exterior appearance of sheet glass. Produced using kiln distorted glass, the gentle undulations of the glass surface eliminate any mirrorlike reflection, giving a clear view through the protective glazing to the historic stained glass. The addition of wire guards for mechanical protection has the effect of breaking up the surface reflections still further. The large panes of glass do not require any additional support structure, and from the interior the EPG has little or no effect on the appearance of the historic stained glass.

Barley Studio has recently undertaken trials of various protective glazing options at major sites, including Lichfield and Wakefield Cathedrals, All Saints North Street Church in York and Browne’s Hospital in Stamford. In all of these cases, full-pane EPG using kiln-distorted or restoration glass has been the preferred option.

As a result of Keith Barley’s pioneering work 50 years ago, and recent analysis by Tobit Curteis and Leonie Seliger for Historic England, environmental protective glazing is now generally accepted to be the best approach to conserving vulnerable historic stained glass. Design and installation of EPG should always be site-specific, considering both the glass and the surrounding fabric, to ensure effective performance and aesthetic appearance.

The installation of EPG is recommended by national and international bodies, including Historic England and the Corpus Vitrearum and it has been adopted by leading cathedral glass conservation studios such as Canterbury, York and Lincoln, and many independent studios. Across the UK and beyond, EPG is helping to preserve our fragile stained glass heritage.

Information

Historic England (2020) Stained Glass Windows: managing environmental deterioration at historicengland.org.uk/advice/technicaladvice/buildings/stained-glass-windows
Church of England (ChurchCare) at www.churchofengland.org/resources/churchcare/advice-and-guidancechurch-buildings/stained-glass

This article originally appeared in the Institute of Historic Building Conservation’s (IHBC’s) Context 177, published in September 2023. It was written by Alison Gilchrist, an accredited stained glass conservator at Barley Studio, York.

--Institute of Historic Building Conservation