Historic Churches 2019

16 BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION Within this layer chemical interaction between water and the constituents of the glass take place, most importantly the leaching of some alkaline fluxes used to lower the melting temperature of glass during its manufacture. Medieval glass with a high potash content is especially susceptible to this leaching, the result of which can be pitting or blanket corrosion on the glass surface. This corrosion can be disfiguring as the corrosion products can be opaque. In severe cases the glass can be eaten right through. PAINT LOSS The detail in stained glass is added with the use of vitreous glass paints. These are composed of metal oxide pigments, usually iron and or copper and finely ground glass frit. The paint is applied to the individual pieces of glass and kiln fired. The addition of a flux allows the glass frit to melt at a lower temperature than the glass body, bonding the pigment to the heat-softened glass surface permanently. In the latter half of the 19th century, a boom time for the stained glass industry, kiln technology was improved leading to more rapid firings and paint recipes were experimented with in a bid to improve results and increase productivity. A common side effect of that innovation is that glass paint on Victorian windows can be under-fired or chemically water soluble. Both lead to the loss of painted detail when wet and 150 years later, many Victorian stained glass windows have become virtually illegible. ENVIRONMENTAL PROTECTIVE GLAZING (EPG) Formerly known as isothermal glazing, EPG was developed in Austria and Germany in the second half of the 20th century as a means of protecting medieval stained glass from condensation and thus corrosion, and it has been in use in the UK for the past 50 years or so. Its development and acceptance here owes much to Keith Barley of Barley Studios in York and Sebastian Strobl, formerly of Canterbury Cathedral. It is now widely acknowledged as the best way of preventing wetting of stained glass, and hence halting or greatly reducing corrosion and paint loss. EPG is discussed in greater detail in the article by Robyn Pender on page 20, but in essence it involves the introduction of a second, sealed layer of glass outside of the historic glazing, with the interspace ventilated to the inside of the building through vents placed at the top and bottom of each light. As the interspace is warmed or cooled by variations in the external temperature, the air rises or falls, drawing in fresh air from inside the building, creating a flue effect in the interspace. A more consistent temperature is thus maintained to both sides of the stained glass, preventing or reducing the instances of condensation on it. Condensation which will often occur on the secondary glazing is channelled to the outside of the building through weep holes at the base and across a lead apron laid over the sill. EPG does not require any power to work, nor does it need opening or closing by the parishioners. In fact it doesn’t require anything more than occasional cleaning and maintenance to continue working effectively once fitted. Described like this, EPG sounds too good to be true but it is not without its drawbacks: moving the original stained glass is a significant intervention and, if badly designed, it can have a negative impact on the external appearance of a building. Seen from the outside, stained glass presents a rich, textured surface that shimmers as the observer passes and changes with the time of day, year and the weather. Hiding this under large sheets of flat glass inevitably presents clinically flat reflections that look out of place on historic buildings, while the use of leaded glass in the EPG can cause parallax problems when viewed from inside. Some alterations will also be required to the stained glass itself, whether it is relocated inwards or left in its present position and vented, and the need for secure fixings for the new glazing means drilling into the surrounding historic fabric. The cost of a large EPG scheme often makes a period of environmental monitoring desirable for funding bodies, both before the project to confirm the need for EPG, and after installation to confirm that it is working as intended. CASE STUDY – ST LEONARD’S, MARSTON BIGOT, SOMERSET St Leonard’s is a small, rural church near Frome in Somerset. It sits next to Marston Hall, home of the Earls of Cork and Orrery from the 1640s to the early 20th century, and the current church and its stained glass is a result of their benefaction. The five nave windows (three south and two north) are by Thomas Willement FSA, heraldic glass artist to George IV and later artist in glass to Queen Victoria. The east window contains an important collection of 16th-century glass from France and Germany, arranged into their current configuration by Willement during a major restoration of the 1840s. The deterioration of the groups of windows is typical of their age and orientation within the church. For the purposes of this article the glazing can be grouped thus: Nave south windows: three figurative windows by Willement; Salvator Mundi, St Peter and St John. These south-facing windows were the initial cause for concern for the parish. As a result of deterioration of the lead matrix and its ties, two of the panels had started separating and slowly migrating downwards. The reflection caused by using large sheets of glass for environmental protective glazing would usually pose a significant problem, but this elevation was only visible from farmland. An EPG design – in this case the stained glass remains where it is and is ventilated in situ, and the new glazing plane is to the exterior. SELECTED CLEAR BORDER PIECES REMOVED AT HEAD STAINED GLASS IN ORIGINAL POSITION 4mm TOUGHENED GLASS SELECTED PIECES AT BASE ANGLED INWARD TO VENTILATE INTERSPACE LEAD CONDENSATE TRAY STD 1/2" LEAD SEALED WITH LEAD SAFE SILICONE OUT IN DIVISION APPROX 50mm PARALLAX BAR