(Drawn up by the Comity Technique of the Corpus Vitrearum 1989)

Under the aegis of the Corpus Vitrearum, the international scholarly organisation dedicated to the study of ancient stained and painted glass, the Comité Technique is concerned with the problems of conservation and restoration of ancient glass painting. (Since 1982 the Comité Technique of the Corpus Vitrearum has also been associated with the Comité International Pour Le Vitrail of ICOMOS in order to be responsive to more broadly based issues for monuments preservation in various countries). Following decades of experience by experts working in this field, it is possible to lay down some guidelines for the preservation of stained and painted glass.

1. Methodological Precepts

Research and conservation are basically indivisible activities. The prerequisites for conservation should not only include technical study, but also art historical understanding; the historical development of artistic aspects, and of available materials and techniques (types of coloured glass, lead profiles, working methods, manner of painting, special technical characteristics and so on) are only meaningful in relation to each other, and they are equally relevant to the principles of restoration and conservation. Especially important is the history of the glass, because a knowledge of former restorations can be particularly informative about the nature of any damage it has suffered. The Corpus Vitrearum guidelines include such research, and the maintenance of files concerning the documentation of ancient stained and painted glass, as a basis for study.
The basic assumption is made that the restoration and conservation of monumental stained and painted glass requires the same attention and painstaking care as that of other works of art, such as paintings on panel or canvas, polychrome sculpture etc.. This means that only specially trained restorers and experienced craftsmen should be authorised to conserve stained glass, and they should co-ordinate their working procedures. Members of the Comité Technique of the Corpus Vitrearum (and of the Comité International Pour La Vitrail) are available for expert professional advice. In several countries in the past it has proved invaluable to call together a committee of international experts from the Comité Technique to discuss very complicated and difficult problems.

2. Conservation

According to article 4 of the Charter of Venice, the conservation of art works must involve permanent maintenance. In the case of monumental stained glass this means the periodical inspection of:

  • The architectural structure of the window-frame (tracery and mullions, iron armature etc.)

  • The outer protective wire mesh

  • The protective glazing (see also paragraph 4 and supplement)

  • The stability of the stained glass (cracks, bulges and breaks in the lead matrix etc.)

  • Damage caused by corrosion, condensation and changes in temperature (weathering layers on the inside and outside, diminution of transparency by darkening or devitrification, loss of paint, etc.)

We recommend that damaging environmental influences be kept under constant surveillance, by monitoring:

  • The micro-climate surrounding the glass, including temperature and moisture at the inside and outside

  • Air pollution (data for SO2 etc.), which in the recent past has enormously accelerated the process of corrosion.

3. Condition Report

Only an expert well acquainted with the material and with significant experience in the conservation of stained and painted glass is able to view the extent of deterioration, and determine a program of conservation and restoration based on the following studies:

3.1. Examination of the stained glass in situ
Rough estimate of the extent of deterioration, and of the risks involved in the removal and transportation of the glass panels (only skilled experts under the supervision of a restorer should be assigned these tasks, since the removal and transportation of glass frequently causes so much damage)

3.2. Examination and documentation of the state of preservation of the glass once it has been removed are the basis for restoration and conservation.

3.3. Additional scientific study of the extent of deterioration and technical analyses (such as analyses of glass, lead, paint and weathering crust, scrutiny of the mechanism of corrosion and of any peculiar phenomena like devitrification etc.)

In view of the considerable accumulated experience of experts in the field, such examinations should only be carried out by scientific laboratories, research institutes or individual experts associated with the Corpus Vitrearum.

4. Conservation and Restoration

The primary goal is the preservation of the work of art. Any restoration that goes beyond that is an anomaly (Charter of Venice, article 9). The most effective measure for the conservation of medieval stained and painted glass, according to informed international opinion, has proved to be the installation of outer protective glazing (see also the 1987 guidelines of the Comité Technique on page 5 and 6).

The following conservation and restoration measures are involved along with the installation of outer protective glazing:

4.1. Consolidation of the window frames and supporting architecture, structural restoration and protection of the mullions, the tracery and so on by a stone mason.

4.2. Repair and conservation of the framing and structural iron elements in the window (saddle bars, armature)

4.3. Cleaning of the stained glass.
Careful removal of dirt and of the weathering crust which affect the transparency of the glass always improves the aesthetic appearance. However, since any cleaning also has a direct impact on the work of art, the following principles must be observed:

  • Execution by an experienced restorer, who is familiar with the material and cognisant with its vulnerability.

  • Minimal intervention: Cleaning is primarily viewed as a conservation measure; restoration of transparency is secondary.

  • The selection of cleaning methods and agents should be based on the kind of deterioration observed, which is often very complex; as a rule of thumb, in the hands of a cautious restorer the strongest agent is less dangerous than a more harmless medium used by an inexperienced restorer. Risk to the glass and to the painted layers covered by dirt and weathering crusts is multiplied with the use of chemicals.

  • The application of drastic methods of cleaning (such as airbrasive) is not normally admissible, though it may be considered in extraordinary circumstances.

4.4. Securing the paint layers
At the moment the treatment of loose paint, whether it is the traceline or matt that is about to fall off, is an unsolved problem in the conservation of stained glass. Accordingly, one should restrict the application of only partly tried and tested methods and fixing agents to exceptional cases, so that risks are taken only in these instances where without some intervention one would have to reckon with a direct loss of part of the work.

4.5. Structural consolidation and conservation of stained glass

  • Repairing cracks in the glass. Synthetic resins, which are used in modern technical conservation, enable cracked glasses to be mended edge to edge, a great improvement on the use of mending leads or "Dutchmen" (lead strips over the cracks).

  • Fitting each panel with rigid frames that follow the outer contour, to provide stability.

  • Repair of the saddle bars together with the flexible ties that attach them to the leads.

  • Consolidation of the lead matrix. Medieval or old lead matrices have to be conserved in every case as an important part of the art work. Newer leads are a different case (especially if they are the poor type generally used in former restorations). If new leads are needed, they should follow the profile and contours of the original, when these can be established.

  • Puttying
    The use of putty in the space between lead and glass is not necessary when the stained and painted glass is to be protected by outer protective glazing. Its use should therefore be confined to the filling of any bigger cracks and gaps. Brushing with linseed-oil putty, a popular and traditional practice in the trade, is to be entirely avoided, since layers of resin varnish left on the surface of the ancient glass by this method have caused serious and irreparable damage to the paint, as seen in cases of earlier restorations.

5. Additions

The principles agreed for the restoration of paintings, as in article 9 of the Charter of Venice are applicable to supplying missing parts in stained and painted glass panels.
A reliable method for marking the newly added pieces of glass is to scratch the year of the restoration on the surface (in such a way that it is only visible from close up).

6. Documentation

Written files, photographs and charts must record the results, both from technical examinations of the state of preservation, and from scientific analyses used to study materials and techniques, as well as the phenomena of weathering crusts. The conservation procedures used must also be documented.
Due to the intimate connection between the art historical and technical aspects of conservation procedures, it is practical to combine or co-ordinate the technical data needed for conservation with the art historical documentation associated with the Corpus Vitrearum.
If the documentation for use in restoration is not compiled under the aegis of the Corpus Vitrearum, one copy of the files should be sent to the National Committee responsible for the Corpus Vitrearum (the original remains with the conservation authority).
For recording the state of preservation, the clear and expedient system of charting symbols used by the Corpus Vitrearum. should be adopted. For the documentation of the process of conservation and restoration standardised forms are being prepared.

7. Organisation and Teamwork

The complexity of the tasks of conservation and restoration of monumental stained and painted glass requires the co-ordinated teamwork of qualified and experienced restorers and craftsmen, as well as that of art historians and scientists, who are able to formulate and work out a detailed conservation program based on the concepts in paragraphs 2. 6 above.
Cost estimates for such conservation and restoration are only feasible if they are based on thorough prior examination (paragraph 3) and adequate experience (paragraph 4); estimates not so based cannot be reliable and are therefore not acceptable.


Conservation of Stained and Painted Glass:
Instructions for Protective Glazing

Since the end of World War II, "isothermal glazing" (that is double-glazing with ventilated space) has proved to be the most effective method to ensure the protection and conservation of stained and pained glass. In the present state of research and experience in the conservation of endangered stained glass, isothermal glazing guarantees an improvement in its environment, without involving any direct intervention with the work of art itself.

The effects of isothermal glazing are as follows:

  1. protection against mechanical damage (projectiles, windforce, hail, sonic booms etc.)

  2. protection against the weather: rain, snow, dew; also damage from destructive pollutants in the air (SO2 etc.)

  3. freeing of the glass panels from their ordinary function as the only separation between interior and exterior climates

  4. prevention of condensation on the particularly vulnerable, painted inner side of the stained glass

However, double-glazing can only provide effective protection when its installation guarantees that:

  1. the outer glazing is wind and rain-proof

  2. the outer glazing resists mechanical damage (e.g. by the use of bullet-proof glass or of metal screens in cases of plain window glass)

  3. the dimension of the inner space guarantees enough free circulation of air (sufficient distance between glass panel and outer glazing with necessary openings on top and bottom, and both sides if necessary)

  4. 4Due to the fact that the actual state of our knowledge does not allow a determination of the exact dimension of the space required for optimal ventilation, it is advisable to plan the construction in such a way that corrections can be made at any time.

The present state of experiments in several countries may be summarised:

  1. As far as aesthetic considerations for protective glazing are concerned, various solutions have been found that allow the integration of the protective-glazing into the architecture (such as: geometric patterns; lead-lines following simplified contours based on the leading of panels behind; non-reflecting glass; or large panes with a textured surfaces etc.). Even the problems presented by huge windows, with complicated ferramenta and tracery lighter seem solvable. Viewed from the interior there are minimal aesthetic problems since both slits admitting light and the new frames for the panels are easy to hide; any covers for those, however, should not hinder the circulation of the air.

  2. In the light of research and experiments the majority of countries prefer inside-ventilated protective glazing. Research continues on this topic.

  3. The design and execution of protective glazing enjoys a variety of possibilities. Great importance is attached to the fact that the construction should be kept simple, easy to execute and to handle, and as cheap as possible. The best types of construction respect to the utmost the fabric of the monument itself (such as masonry, iron-work etc.).

  4. Every material used (especially metals) must be carefully considered for its compatibility with its surrounding.

We have learned from experience that conditions created by the presence of protective glazing have to be checked on a regular basis and that its effectiveness can only be guaranteed when it is inspected by experts; regular inspections by qualified restoration workshops should be written into maintenance contracts. It goes without saying that adequate documentation must accompany each inspection.

There is a need for exact measurements to be taken in conjunction with these inspections on an ongoing basis, in order to collect reliable long-term data about the effectiveness of various protective glazing systems. These measurements must include: the temperature, relative humidity of the atmosphere, and airstream speed in the ventilation space. Efforts are needed to optimise and standardise the instruments and methods used in taking such measurements. To this end it would be advisable to use a set of measuring instruments put together by experts which could be made available on an international level. As a first step towards comparing the effectiveness of protective glazing at different sites, special "chips" of glass developed by the "Fraunhofer-Institut für Silikatforschung'' will be used; these will be tested in as many sites with stained glass as possible, and eventually climatic measurements will also be taken in conjunction with these test chips.



  • Bundesdenkmalamt Hofburg Vienna


  • Hogeschool Antwerpen –Conservation/Restoration Glass, Antwerpen

  • Institut Scientifique du Verre, Charleroi


  • Laboratoire de Recherche des Monuments Historiques Chateau de Champs sur Marne


  • Bayerisches Landesamt fur Denkmalpflege Zentrallabor Monaco

  • Forschungszentrum für mittelalterliche Glasmalerei – Research Center for Medieval Stained Glass, Freiburg i.Br.



  • Fundacio Institut del Vitrall Barcellona


  • Scientific Research Corning Museum of Glass Corning NY

  • The Metropolitan Museum of Art – The Sherman Fairchild Center for Object Conservation


  • Centre Suisse de Recherche et d’Information sur le Vitrail, Au Chateau –Romont



  • Studio Fenice, Bologna

  • Studio Laura Morandotti, Milano

  • Studio Polloni , Firenze