24 CONTEXT 177 : SEPTEMBER 2023 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 Alison Gilchrist is an accredited stained glass conservator at Barley Studio, York. In the EPG system, the original glazing is brought slightly forwards to allow protective glazing to be located in the original glazing plane. Generally, the relocation is almost indistinguishable internally, but externally the new glass will be obvious if no leading is used. A full-pane EPG installation at Lichfield Cathedral (Photo: Barley Studio)
RkJQdWJsaXNoZXIy MjgyMjA=