Probably the most obvious type of damage to leaded glass is the breaking of pieces of glass. Roughly speaking, glass breakage can be divided into four degrees:

-single crack

-multiple cracks

-flaw

-multiple cracks with a flaw

Especially in the field of monument conservation, not every breakage requires the replacement of the original piece of glass. On the contrary, in the case of simple cracks where the piece of glass does not run the risk of falling out, it is sometimes not necessary to take any measures at all from a conservation point of view.
There are a number of possible procedures when intervening in the case of broken glass:

Replacement of the glass

In some cases it may make sense to duplicate the glass. Individual broken pieces of leaded glass can be removed from the lead mesh and the new piece reinstalled.
Special care must be taken to ensure that the correct type and colour of glass is used. If there is a layer of paint (usually enamel paint) on the original glass, this must be replicated. To make it clear that it is a duplicate, this must be noted on the glass piece. In certain cases it is even useful to make the duplicate in such a way that it stands out stylistically from the rest of the painting and is thus easily recognisable as a replica.

Adhesive lead

In some cases, the use of so-called adhesive lead is useful for gluing over and securing cracks. Since it is visible as a black line in the view through the window, like the rest of the lead net, its applicability remains limited. Adhesive lead can be used in the installed state.

Adhesive resins

It is precisely the reversibility, the long durability, the good adhesion to glass and the high transparency of adhesive resins that make them suitable for restoration.The adhesives can be used very well for bonding broken pieces of glass both when the glass is installed and in the workshop.The epoxy resin Araldite 2020 or HXTAL NYL-1 is often used for this purpose.
Smaller defects of glass pieces that are particularly worth preserving can be filled with the same resin and then painted with a mixture of acrylic resin Paraloid B 72 and colour pigments.This technique makes it possible to repair the glass piece while preserving all the existing shards.

Tiffany technique

Developed at the end of the 19th century, this technique is suitable for reconnecting broken pieces. The technique provides a more stable and slightly less visible bond than adhesive lead but is much more visible compared to bonding with resins.
The affected piece of glass must be removed for use.
As with adhesive resins, the technique allows a lost part of a piece of glass to be completed and connected to the original without having to replace the entire piece.

Bulging and sagging can have a variety of causes. They frequently occur in leaded glazing in doors or openable windows.
In addition to bulging due to direct mechanical effects, damaged lead rods, detached or defective reinforcing bars, missing putty between glass and lead or certain lead line guides also reduce the stability of the lead light and can lead to sagging.

Measures:
After removing the affected panels, bulges can usually be carefully bent back on the workshop table. To prevent future sagging, it is helpful to renew the cement and to re-solder torn-off wind bar clamps and solder joints. If the lead field has too few reinforcing bars for its size and structure, additional ones can be added to stabilise it.

For a long time, lead was neglected in connection with listed stained glass. As a result, leaded stained glass was often completely dismantled and re-leaded for restoration, whereby the original lead network was completely lost. Original lead from the Middle Ages is therefore hardly preserved.

In the meantime, lead strips have been recognised as an integral part of the craft piece and as historical evidence of the respective manufacturing technique. The H-sections that hold the glass pieces together are therefore preserved as best as possible, which is possible in many cases.

Cracked solder joints
Aged or permanently mechanically stressed lead often shows cracks. In many cases, these can be re-soldered after removing the patina, thus achieving a noticeable stabilisation of the lead field.

Lead corrosion
Due to the reaction with oxygen, lead forms a protective layer on the surface within a short time, which prevents further oxidation. As a rule, this layer provides sufficient protection against the decomposition of the lead. However, acids released from adjacent linseed oil putty, wood or plaster can decompose the lead. This produces toxic white lead oxide dust. Since this dust can easily enter the respiratory tract, it is necessary to wear a protective mask.

Despite the extremely good durability of enamel paints, they can be affected by environmental influences or incorrect processing.
For example, layers of paint can lose their stable base due to glass corrosion and begin to peel off. But improper cleaning or restoration measures can also lead to the loss or detachment of stained glass. Furthermore, incorrect baking of enamel paints can prevent them from forming a stable bond with the glass surface.

Measures:
Painting layers that are threatening to peel off acutely can be effectively protected from loss by infiltrating them with the acrylic resin Paraloid B 72. In the longer term, endangered glass paintings must be shielded from harmful environmental factors such as water, exhaust fumes, acid rain, hail or stone throwing, above all by protective glazing.

Especially with medieval leaded glass, the issue of altered glass surfaces is very multi-layered and complex. Over the course of several centuries, leaded glass was exposed to many different external influences that affected the glass surfaces in various ways.
In addition, the chemical composition of the glass can vary greatly depending on the time, place and technique of manufacture, which is why the possibilities for surface changes are so varied.
Due to limited technological possibilities, attempts were made for a long time to keep the melting point of glass low by using a relatively large amount of flux (potash). Therefore, these forest and potash glasses have a lower proportion of silicon oxide, which makes them more susceptible to a chemical change of their surface.

Glass corrosion, pitting and weathered glass
Above all, sulphur dioxide, which is contained among other things in acid rain, can initially lead to corrosion (pitting) on the outside of the glass in certain places. In combination with water, this can lead to alkaline components of the glass - which is covered with very fine cracks - being dissolved out on the surface and deposited on the glass surface as whitish, powdery corrosion products. These water-attracting deposits in turn promote the corrosion process. Last but not least, corrosion of the glass surface leads to detachment of the painting layer.

Measures:
Here, a rough distinction can be made between mechanical and chemical interventions. Scalpels or glass fibre pens are used as mechanical tools, which are often more suitable than chemical methods because they can be used selectively and are easy to dose. Basically, however, the type and intensity of the intervention must be reassessed from case to case.
The field of chemical intervention is controversial because of the risk of major irreversible damage. With regard to the removal of weathered glass, however, there are promising results with ion exchangers.

Browning
In certain medieval glasses, the formation of weathered glass leads to another undesirable effect: the manganese used in the glass - both for colouring and decolouring - comes into direct contact with oxygen and begins to oxidise. If the manganese is heavily browned, hardly any light can pass through the glass.

Measures:
In the past, successful attempts were made to lighten browned windows with the very toxic hydrazine. Hydrazine lowers the oxidation level of the manganese, but is not able to dissolve it from the gel layer of the glass, which is why browning is to be expected again after treatment. The only way to remove the manganese was with formic acid, but this can severely attack the glass and is therefore too crude an intervention. Therefore, after treatment with hydrazine, it was more common to seal the gel layer of the glass with so-called Ormocer and thus prevent renewed oxidation.
Since all these methods are ultimately crude and partly irreversible interventions, they are not used in most cases.

Microorganisms and fungi
For a long time, little research was done on the influence of microorganisms and fungi on the glass surfaces of stained glass, even though the phenomenon has long been known from optical devices such as camera lenses. Fungal metabolites - such as acidic ones - often cause irreparable damage to the surface.
Although fungi cannot feed on the glass itself, they can feed on a wide variety of deposits such as traces of putty or oil as well as certain organic cold paints or cleaning agents.
In addition to certain climatic conditions, such as heat and high humidity, the porous and rough surface of weathered glass and glass corrosion favours the development of fungal growth.

Measures:
In addition to changing the climatic conditions, there have been investigations into making the glass surfaces uninhabitable for fungi with coatings containing copper. Furthermore, organic substances can also be destroyed or removed by means of UV light and disinfectants. Which measure can be applied must be put up for discussion when drawing up the restoration concept.