GUIDELINES OF ANCIENT MONUMENTAL STAINED AND PAINTED GLASS
(Drawn up by the Comity Technique of the Corpus Vitrearum
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
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
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
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
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
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
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.
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
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).
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
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.
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
The effects of isothermal glazing are as follows:
protection against mechanical damage (projectiles,
windforce, hail, sonic booms etc.)
protection against the weather: rain, snow, dew; also
damage from destructive pollutants in the air (SO2 etc.)
freeing of the glass panels from their ordinary function
as the only separation between interior and exterior climates
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:
the outer glazing is wind and rain-proof
the outer glazing resists mechanical damage (e.g. by the
use of bullet-proof glass or of metal screens in cases of plain window
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
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
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.
In the light of research and experiments the majority of
countries prefer inside-ventilated protective glazing. Research continues on
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.).
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
TECHNICAL COMMITTEES FOR STAINED-GLASS WINDOWS
Hogeschool Antwerpen –Conservation/Restoration Glass, Antwerpen
Institut Scientifique du Verre, Charleroi
Bayerisches Landesamt fur Denkmalpflege Zentrallabor Monaco
Forschungszentrum für mittelalterliche Glasmalerei – Research Center
for Medieval Stained Glass, Freiburg i.Br.
ITALIAN LABORATORIES FOLLOWING CVMA DIRECTIVES