Stained glass is a remarkable and complex medium, combining as it does an artistic function, often related to Christian imagery, with the practical function of completing the building envelope and providing a barrier to the external environment. In material terms, it comprises several different elements: glass, of course, colouring the light; but also lead strips (known as calmes) holding the glass pieces together and enhancing the design composition; decoration of the glass surface in the form of paint, stain and enamel to create the detail of the artistic imagery; and, in the case of architectural glass, a support structure of wooden or metal bars or framework (ferramenta).
All of these elements come together to create a work of art, and each is important to both the history and the future of a stained glass panel. Glass is a fragile material, prone to fractures through stress or impact, and medieval glass in particular is also susceptible to corrosion through the action of moisture on the glass surface. Lead calmes also weaken over time, and may fracture at solder joints or stretch, allowing panels to bend and buckle. The surface decoration is generally the most fragile element, as although paints and enamels are fired on to the glass surface, they can easily be damaged by environmental moisture, biological growth of moulds or lichen, or over-zealous cleaning.
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Enamels, used particularly in the 17th and 18th centuries, are prone to cracking and flaking off from the glass due to their differing expansion behaviour with changes in temperature. Many windows of the mid- 19th century are now showing catastrophic paint loss, leaving completely blank areas of glass where the painted detail should be, due to the use of incorrectly formulated glass paints and firing processes.
Finally, the history of damages, whether accidental or deliberate iconoclasm, especially for medieval windows, and previous repairs, can leave stained glass in a much confused state, with added mending leads and infill pieces replacing lost glass.
Environmental Control
The preservation of stained glass is primarily achieved by the provision of a stable ambient temperature and, especially, humidity. Moisture is the agent of most environmental damage, causing glass (especially medieval glass, with its potash- rich composition) to corrode as well as damaging glass painting, whereas enamels can be badly affected by changes in temperature. In the architectural setting, the use of environmental protective glazing (a second glazing layer to the exterior with the historic glass mounted and ventilated to the interior – the system known as isothermal glazing – offers the best environment to preserve stained glass.
In a museum setting, whether in storage or on display, as well as providing a stable environment, the materials surrounding stained glass should be carefully chosen to avoid any atmospheric contaminants; MDF or fibreboard in particular, may give off acetic acid gas, which is particularly damaging to lead.
The conservation of stained glass should respect and preserve as much original or historic material as possible, and should follow the guiding principles laid down by the Corpus Vitrearum (available at www.cvma.ac.uk/conservation). In many cases, a gentle cleaning of the glass surface using a soft brush to remove dust and other debris is all that is necessary. Any intervention beyond this should ideally be carried out under the guidance of a PACR-accredited conservator-restorer (go to www.icon.org.uk for further information).
More ingrained material such as soot can be removed using cotton wool swabs moistened with deionised water, taking great care of the painted surface. Where microbial growth such as moulds or lichen is present, treatment with a 50:50 mix of ethanol and deionised water may be necessary to kill spores and remove the material.
Lead Conservation
It is commonly stated that window lead must be replaced every hundred years or so, and indeed this has been common practice in the past; thus it is now very unusual to find a medieval window that retains its original lead network. However, a well-made and well-supported window can last much longer than this before re-leading becomes necessary. The process of dismantling and re-leading poses significant risk to the glass, which should be taken into account when deciding on the conservation approach. In the case of the vast collection of 19th-century glass throughout the UK, much is still in its original lead, which should of course be conserved wherever possible as an integral part of the artwork. With older glass, dismantling should only be undertaken where necessary for the structural stability of the panel, or where significant improvement can be made to the visual appearance.
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If the decision is taken to dismantle a panel, or a part thereof, it may be possible to improve the visual appearance by removing mending leads and bonding the broken edges back together using silicone adhesives or epoxy resins. Re-ordering of the glass and replacement of previous (inappropriate) repairs may also be undertaken in an attempt to regain the original artist’s intention; such significant intervention should always be supported by thorough technical and art-historical research. Improvement to the visual appearance can also be made without dismantling, for example by reducing or removing the leaf of mending leads on the glass surface, leaving only the thin line of the heart of the lead.
Paint Restoration
Loss of the painted detail, even where the original glass survives, can dramatically reduce the legibility of the imagery. Restoration of lost paint or enamel can be achieved using painted backplates attached to the reverse of the original glass; an ethically satisfying solution as the new painted detail is fired (and so permanent) but remains physically separate from the original artwork. However, it requires a major intervention to the lead network to remove and replace the plated piece, and backplates can be prone to technical problems, with failed seals leading to trapped moisture and biological growth, causing damage to the original glass. A simpler alternative, possible if the reverse face of the original glass is unpainted, shows no sign of corrosion, and will not be exposed to weathering, is the use of cold (unfired) paints, applied directly to the reverse face of the glass.
Once conserved, if the window is to be returned to its architectural setting, then installation with environmental protective glazing is strongly recommended to preserve both the original material and the conservation materials used (such as adhesives). Protective glazing ensures that windows no longer have to act as the weather barrier for the building, so that simple cracks, where all the pieces are mechanically held in the lead network, can be left without being bonded. The historic panels are mounted in bronze frames, providing support to the lead network and offering protection against settlement of the surrounding stonework, allowing historic lead to be preserved for longer.
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In a museum environment, provision of a stable environment should be somewhat easier to achieve, although careful consideration should be given to panel mounting and illumination. Bronze framing systems, similar to those used in isothermal glazing installations, may be useful to support the panel and its lead network. Light boxes are often used for illumination, although heat build-up from the lights must be avoided, and a uniformly diffused light can have a rather deadening visual effect on the glass. Where possible, it is preferable to use natural daylight by hanging panels in front of windows or even installing within the window opening, where the use of protective glazing systems offers the additional benefit of easy removal and reinstatement.
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