"Stained glass" is an all-encompassing term used to describe decorative windows. This note is relevant for all decorated glass from door-panels in a domestic interior to major collections of windows within a church or cathedral. Stained glass is made up from a number of elements – small sections of coloured, textured glass, held within a lead network (although other metals such as zinc and copper foil have also been used) and then secured as a panel within a timber, metal or stone and mortar framework.
Stained glass may have been painted with a variety of glassy enamels. These are usually dark reddish brown, but can also be red, blue, green or black. They are made from a dry glass powder incorporating metallic oxides which is then mixed with a variety of binders such as water, gum arabic, clove oil, sugar, or vinegar. These kiln-fired enamels are often painted on the front (interior) of the window, whilst the back (exterior) may have a yellow stain - a compound of silver which is fired onto the reverse side of the glass from the paint.
A waterproofing compound is usually used to seal the glass and the lead. Large windows are commonly made up from smaller two-foot-high sections, securely held together by “saddlebars” and copper wires before being installed in the structural frame where it is to remain.
Each of the elements from paint to fixing method plays a critical role in the long-term survival of a window. A failure of one or more of these may have a "knock-on" effect, and eventually over time, the entire window may fail. There is no rule as to how long a stained glass window should last without being re-leaded. A well-made, well-protected window can last for centuries. Conversely, a poorly-made, badly-exposed window may fail within a few years.
Each sheet of hand-made antique glass or machine-made cathedral glass is unique, and may contain tiny bubbles, thin criss-cross lines, and a variety of textures. These should not be regarded as faults, as they have been chosen for their effect by the original artist.
Look out for cracks, however, as these may suggest that there is other deterioration in the window. Single cracks may be caused by internal stresses, multiple cracks by damage from an impact (vandalism and accident) or from external stress. They are usually a sign that conservation is necessary.
Signs of pitting or discolouration of the surface may suggest corrosion or deterioration of the glass; excess water is often but not always the cause of such surface problems.
Lead is by nature soft, malleable and easily soldered. This means that it can also sag easily and lose its structural role. External signs of failure to look out for are: bulging or bowing of the window, cracking along the face of the lead (particularly near the solder joints) and a white, powdery spotting on the surface.
The original kiln firing of the pigments is not always successful, and poorly-fired paint can be very vulnerable and fragile, looking pale and thin as the surface is gently washed away over time. Over-fired paint looks hard and cracked on the surface, attracts moisture, and eventually blisters and peels off. Inspect and monitor glass regularly for these signs.
Over time, the waterproofing compounds used to seal a structure go hard, crack, and fall out, leaving the lead and glass exposed to water damage, if it is external. At this point the window may be seen to be leaking - letting in water between the lead and the glass.
If the window or panel is sagging and bulging, it may be due to poor installation techniques which have left it without sufficient support from the frame or the ties. If it is an old window, this may be acceptable settlement which is best left alone.
The most useful thing an owner can do is to regularly inspect and monitor the piece or window and note problems such as cracking, bowing, paint deterioration or excessive condensation and leaks. Call in a stained glass conservator before problems become insurmountable.
Regular inspection and maintenance of the building the windows are sited in, particularly roof, wall, rainwater goods and pointing will ensure that knock-on problems from the exterior do not affect the window.
In the case of external windows, be aware that - unlike normal glazing - stained glass is vulnerable to the elements and can suffer stress from excessive wind, heat and movement. Secondary glazing is an option for stained glass, but only when designed and installed by a conservator specialised in this field. Ordinary secondary glazing – as well as being unsightly - will trap moisture and set up a potentially aggressive environment which could exacerbate existing problems.
These windows should not be wet-cleaned on the inner surface as water can do excessive damage to the glass, the paint, the lead, the putty, and the metal components. Even with an internal panel, dusting with a very soft bristle brush is better than using a liquid method. Often a single crack within a glass panel may be a tell-tale sign of other internal stresses, which may be due to lead fatigue or insufficient support.
Regular inspection and monitoring of stained glass is extremely important. Potential problems include:
Determining the actual reason for failure is often extremely complex, and requires a familiarity with the materials, an understanding of the causes of deterioration, and experience in stained glass conservation.
Use the Conservation Register to Find a conservator.
© Icon, the Institute of Conservation 2011.
This article offers general guidance and is not intended to be a substitute for the professional advice of an accredited conservator. The views expressed are those of the author or authors, and do not necessarily represent the views of the Institute of Conservation. The Institute of Conservation would like to acknowledge use of the MGC publication 'Ours for Keeps' in the preparation of this text. The Institute of Conservation and its partners accept no liability for any loss or damage which may arise if this guidance is followed.
The Institute of Conservation would like to acknowledge the support of The Royal Commission for the Exhibition of 1851 in the production of this guidance information. Further information on The Royal Commission for the Exhibition of 1851 and its work is available at www.royalcommission1851.org.uk.