The most common damages in stained glass


The present text gathers the most frequent deteriorations that can be found in stained glass windows. The objective is to disseminate this knowledge for the use of collection owners, heritage managers and conservator-restorers not specialized in this field. The conservation of stained glass is a little known specialty with materials and mechanisms of alteration far removed from other fields of restoration. Some specialties, while still being specialties, share commonalities with others, but analogies with stained glass are generally unhelpful. Stained glass windows are movable assets but most of them, unlike other assets, are exposed to the elements and are the enclosure of buildings. In almost all cases, their state of preservation must be judged from a great distance since, unfortunately, there are no elevation means available for their diagnosis. In these cases, the experience of the restaurateur is an invaluable factor.

The lack of knowledge of this art often makes it very difficult for the owner or the person responsible for the object to judge the state of conservation of the stained glass and to establish the slightest conservation strategy.

Stained glass is an object composed of very different materials, glass, lead, grisaille and enamel and the frames and/or mortars that hold the stained glass to the building.

Most common alterations:

1. Lead alterations

The lead H-profile forms a more or less complex network that is susceptible to various deteriorations. The majority of actions are usually focused on their consolidation and if not resolved with the necessary promptness, they end up causing irreversible deterioration and even the complete loss of part of the stained glass.

  • Curvatures of the panels: This is a very common deterioration and is usually easier to detect on the exterior side of the glazing by observing it with reflected light. It may be due to settlement of the structure, although it is usually associated with other alterations, lead netting broken at several points, poor reinforcement arrangement. If the structure is solid to the touch, it may not require any intervention. This is a warning sign and a specialist should be consulted.
  • Lead fractures: Depending on several factors, such as the composition of the material and its manufacture, lead fractures may appear. They generally appear in the vicinity of welds. It is a serious deterioration and in the case of being found in a generalized form throughout the lead network it can be the cause of the detachment of part of the panel. These fractures are visible from close proximity, in case they are in places of difficult access they are undetectable.
  • Whitish appearance of lead: This alteration is the decomposition of metallic lead into lead carbonate. It is due to unstable lead alloys, generally modern leads that are not alloyed. It is a particularly toxic material and the preserver must take precautions in its handling. This deterioration is accelerated by the presence of organic acids and is therefore often associated with the use of acetic silicones in the installation of the glazing or protective glazing.

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Common lead network impairments

2. External waste and dirt

This is a complex section and in some cases the cleaning of the stained glass windows is left to personnel without specific training not only in stained glass but also in conservation-restoration. The main and most difficult part is to detect which parts are external residues and which parts are the result of the natural aging of the materials. The purpose of the cleaning is to remove external residues that may affect the conservation of the stained glass and not so much to restore translucency. Cleaning is an irreversible process that can be very dangerous for the integrity of the glass and should always be left in the hands of specially trained technicians.

Among the most common residues we find: accumulation of dust, bird droppings, cobwebs, remains of candle soot, calcareous deposits, rust stains from frames and reinforcements, also stains of residues such as paint residues, varnishes originating from work carried out in the vicinity of the glass.

Cleaning with aqueous methods should be avoided if not performed by a professional.

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Glass is obviously a fragile material, but paradoxically it is extraordinarily durable if kept under favorable conditions. Most glass compositions are also very durable and unalterable.

Chemical deterioration of glass

Chemical deterioration in glass is due to unstable chemical compositions, such as percentages below 60% Si or low Ca content. These are accelerated depending on the aggressiveness of the environment to which they are exposed. The stability of glasses depending on their composition is a very complex issue. Microscopically, the poor stability of the glass is manifested by very different phenomena, whitish crusts, pitting of the surface, iridescence, micro fractures throughout the material, opacity. Hostile preservation conditions would be the case in environments with frequent condensation or very high relative humidity.


Fractures can be caused by many reasons. They may be due to the poor execution of the pieces, to poorly executed stained glass windows in which the weight is not evenly distributed or may be due to external factors, such as the impact of objects, birds, accidental impacts or vandalism. In the case of altered lead nets, they usually cause the fracture of some pieces. In the case of stained glass exposed to sudden temperature changes, it is common to find fractures caused by thermal contrast.

Color changes

Some glass compositions may change color due to the oxidation change of the metal oxides that give color to the glassy mass. The color change from transparent to pale violet is frequent.

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Various cases of fractures

4. Alterations of the paintings

Most stained glass windows have painted glass that is baked in ovens to fix them almost permanently. Depending on their composition, these paints can be grisaille, enamels, silver yellows, all of them with very different properties. Their degradation mechanisms are very complex and can develop until the paint is almost completely lost. In case of whitish coloration, loss of paint in flakes or disappearance of parts of the paint, call a specialist.

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5. Frames and reinforcements

The lead and glass structure is always supported by a perimeter frame, traditionally made of iron, and has handrail reinforcements or circular section rods that fix the soffits to the frame. These elements are more susceptible to corrosion than lead and their oxidation can cause damage to the glass. The most common is the rust staining of the surface near them. In extreme cases it may corrode completely and cease to perform its function. We must be alert to leaks that could accelerate this phenomenon.

Stone and mortars

In the case of the more traditional installation, in churches, the stained glass windows are installed in more or less complex tracery and mullioned windows. These structures are not completely rigid and their slight movements can cause glass fractures and curved panels.

In some cases, the mortars crack and detach, ceasing to perform their holding function.

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6. What to do in case of an accident

In the event of an accidental accident, it will be of great help to act diligently after the accident. Secure the space in which parts of the glass may become detached. Once this is done, collect the detached fragments, no matter how insignificant they may seem. If it is impossible for the conservator to do so, it should be done by the person responsible for the conservation of the object. These fragments can greatly assist in the restoration of the object. Photograph the window as soon as possible to record the accident, these photographs will be useful to the insurance that can cover the expenses generated in the accident. As soon as possible, call in a professional capable of drawing up an estimate, secure the window to prevent damage from proliferating, and start a conservation and restoration project. An analysis of the accident can show the causes and minimize the risks with simple measures.

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7. What points should be observed when checking the state of conservation of the stained glass window?

A routine and periodically scheduled inspection should track all the elements that can give information about the state of conservation of the stained glass. As it is often done from a distance, it is necessary to observe not only the object but also its immediate surroundings. It should be noted.

  • There are no curvatures in the ceiling panels.
  • There are no water leaks.
  • No paint loss on painted parts.
  • There are no fractures in the glass.
  • Mortars or mastics are in place with no cracks or spalling.
  • Lead has no white coloration or fractures.

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