The level of cleaning, repair, or restoration depends on the condition, 
quality, and significance of the glass, and, as always, the available budget. 
Hastily undertaken, overly aggressive, or poorly executed repairs can cause 
more damage than does prolonged deterioration. Repairs should, therefore, only 
be undertaken after carefully evaluating the condition of the glass-and only by 
professionals. Minor cracks, sagging, and oxidation are part of the character 
of historic leaded glass, and require no treatment. More extensive cracks, 
major bulges (generally, more than 1˝" [38mm]), and other signs of advancing 
deterioration call for intervention, but caution must always be exercised. And 
each window must be evaluated separately. In some cases, windows have bulged 
up to 4" (102mm) out of plane without harming the pieces of glass or risking 


     Perhaps the greatest virtue of stained glass is that its appearance is 
constantly transformed by the ever-changing light. But dirt, soot, and grime 
can build up on both sides of the glass from pollution, smoke, and oxidation. 
In churches the traditional burning of incense or candles can eventually 
deposit carbon layers. These deposits can substantially reduce the transmitted 
light and make an originally bright window muted and lifeless. Simply cleaning 
glass will remove harmful deposits, and restore much of its original beauty, 
while providing the opportunity to inspect its condition closely. The type of 
cleaner to use depends on the glass. Water alone should be tried first (soft 
water is preferable); deionized water should be used for especially significant 
glass and museum quality restorations. If water alone is insufficient, the next 
step is to use a non-ionic detergent. Occasionally, windows are covered with a 
yellowed layer of shellac, lacquer, varnish, or very stubborn grime which 
requires alcohol, or solvents to remove. Most unpainted art glass can be 
treated with acetone, ethanol, isopropyl alcohol or mineral spirits to remove 
these coatings if gentler methods have failed. All chemical residue must then 
be removed with a non-ionic detergent, and the glass rinsed with water. (All 
workers should take normal protective measures when working with toxic 

     Painted glass must never be cleaned before the stability of the 
paint is confirmed, and only then with great caution. If the paint is sound, it 
can be cleaned with soft sponges and cloth. If the paint was improperly fired 
or simply applied cold, paint can flake off during cleaning and special 
measures are required such as delicate cleaning with cotton swabs. 
Occasionally, paint is so fragile the owner must simply accept the windows in 
their current state rather than risk losing the original surface.

Acidic, caustic, or abrasive cleaners should never be used. They can damage 
glass. Most common household glass cleaners contain ammonia and should not be 
used either; ammonia can react with the putty or metallic cames.  


     As with all elements in older and historic buildings, maintenance of 
leaded glass units is necessary to prevent more serious problems. It is 
essential to keep the frame maintained regardless of the material. Often, this 
simply entails regular painting and caulking, and periodic replacement of the 
glazing compound. Wood frames should be kept painted and caulked; new sections 
should be spliced into deteriorated ones, and epoxy repairs should be made 
where necessary. Masonry frames must be kept well pointed and caulked to 
prevent moisture from corroding the steel armature and anchors within.

     Windows that leak, are draughty, or rattle in the wind (or when gently 
tapped) indicate that the waterproofing cement ("waterproofing") and sealants 
have deteriorated and maintenance or restoration is needed. Waterproofing is a 
compound rubbed over the window-preferably while flat on a table-and pressed 
under the came flange to form a watertight bond between the leading and the 
glass. Traditionally, waterproofing was made of linseed oil and whiting, and a 
coloring agent. (Hardening agents should not be included in the mixture; 
solvent-based driers should be used sparingly.) The waterproofing allows leaded 
glass in a vertical position (i.e., in windows) to be used as a weatherproof 
barrier. It does not provided adequate protection for leaded glass in a 
horizontal or arched position; leaded glass ceilings and domes must always be 
protected by a secondary skylight or diffusing skylight.

     Sealants (e.g., putties, caulks, and silicones) are used to seal the 
leaded panel against the sash, and to seal any open joints around the window 
frame. Sealants have improved dramatically since the development of silicones 
from World War II technology. Silicones are not without problems, however. Some 
release acetic acid as they cure. Acetic acid can harm lead, and should never 
be used on leaded glass. Instead, "neutral cure" silicones should be used. 
Developed in the early 1970s, "neutral cure" silicones have an expected 
lifespan of 50 years. These high-tech construction sealants are not sold in 
consumer supply stores. The appropriate type of sealant depends on the 
materials to be bonded and on the desired appearance and longevity. When 
windows are to be restored, the contractor should explain what types of 
waterproofing and sealants are to be used, and how long they are expected to 
last. On large projects, a letter from the product manufacturer should be 
obtained that approves and warranties the proposed application of their 

     Leaded panels will generally outlast several generations of waterproofing. 
When the waterproofing has failed, the window should be removed from the 
opening and waterproofed on a bench. Leaded glass cannot be adequately 
waterproofed in place. Removing the windows will provide an opportunity to 
perform maintenance on the window surround and to secure the reinforcement. 
This is far less expensive than totally releading the window, which is 
typically required if maintenance is deferred. When waterproofing or sealants 
break down, many building owners attempt to resolve the problem by installing 
protective glazing, when the window only needs maintenance. Protective glazing 
is not an alternative to maintenance; in fact, it impedes maintenance if not 
installed properly and can accelerate the deterioration of the stained glass.

     A very common-but extremely harmful-practice in the American stained glass 
industry is performing major window repairs in place. The practice is routine 
among churches where the cost of restoring large windows can be prohibitive. 
However, undertaking major repairs in place provides only a quick fix. A window 
cannot be properly repaired or restored in place if it is bulging or sagging 
far out of plane, if over 5% to 10% of the glass is broken, or if solder joints 
are failing. Unscrupulous glazers can introduce a great deal of stress into the 
glass by forcibly flattening the window in place and tacking on additional 
bracing. At a comfortable distance the window may look fine, but upon close 
inspection the stress cracks in the glass and broken solder joints become 
obvious. Windows subjected to this treatment will deteriorate rapidly, and 
complete, much more costly restoration will likely be necessary within a few 
years (while a proper repair can easily last two generations or more).

     Major repairs to windows are sometimes part of a larger preservation 
project. In such cases, the risk of damaging the windows can be very great if 
their removal and reinstallation have not been carefully planned. When major 
building repairs are also to take place, the windows should be removed first 
to prevent damage during other work. Windows should be reinstalled as the 
next-to-last step in the larger project (followed by the painters or others 
working on the finishes surrounding the stained glass).

     And glass should be protected whenever other work is undertaken on 
buildings-whether or not the windows are also to be repaired. External 
scaffolding, for example, erected for repointing or roofing projects may 
offer vandals and thieves easy access to windows and, through them, to 
building interiors. Finally, stained and leaded glass should always be well 
protected whenever chemical cleaners are used on the exterior of the building; 
some products, such as hydrofluoric-acid cleaners, will cause irreversible 

Repairs to Glass

     Minor repairs, such as replacing a few isolated pieces of broken 
glass, can be performed in place as a reasonable stop-gap measure. This work, 
typically called a "drop-in," "stop-in," “float,” or "open-lead" repair, 
entails cutting the came flange around the broken piece of glass at the solder 
joints, folding it back to repair or replace the old glass, and resoldering the 
joints. Repairing a zinc came window is not as easy. Zinc cames are too stiff 
to open up easily, so they must be cut open with a small hack saw and 
dismantled until the broken area is reached.

     The glass is then repaired or replaced and the window is reassembled. New 
cames can be patinated to harmonize with the originals-but only with 
difficulty. Repatination should never be attempted in place, since it is 
impossible to clean off harmful residues trapped under the came.

     Original glass should always be retained, even though it may be damaged. 
Replacement glass that exactly or closely matches the original piece can be 
very difficult to find, and costly to make. An endless variety of glass colors 
and textures were produced, and given the delicate chemistry of glassmaking, 
even samples from the same run can be noticeably different. The traditional 
secrecy that shrouds the glassmaking trade to this very day, as well as 
environmental bans of historically popular ingredients such as lead and cobalt 
for deep blues and greens, further hinders accurate reproductions. Therefore, 
it is nearly always better to use an imperfect original piece of glass than to 
replace it. If the paint is failing on a prominent feature of a window, a 
coverplate of thin, clear glass can be painted and placed over the original. 
(The coverplates must be attached mechanically, rather than laminated, so that 
they can be removed later if necessary.) A reverse image of the fading feature 
should be painted on the backside of the coverplate in order to get the two 
painted images as close together as possible. With repetitive designs, stencils 
can be created to produce multiple duplicates.

     Sometimes replacement is the only option. Fortunately, custom glass houses 
still exist, including the company that originally supplied much of the glass 
for Tiffany commissions. Stained and leaded glass has also experienced a 
resurgence in popularity, and American glassmakers have revived many types of 
historic glass.

     When missing, shattered, and poorly matched glass from later repairs must 
be replaced, the new pieces should be scribed on the edge (under the came) with 
the date to prevent any confusion with original glass in the future.

     Glass cracks will enlarge over time as the contacting edges grind 
against each other whenever the window is subject to vibration, thermal 
expansion and contraction, and other forces such as building movement. 
Therefore, it is important to repair cracks across important features as soon 
as they are detected, and while a clean break remains. Years ago, cracks were 
typically repaired with a "Dutchman" or "false lead" by simply splicing in a 
cover lead flange over a crack. Although this conceals the crack, it creates an 
even larger visual intrusion and provides no bond to the glass. Today there are 
three primary options for repairing broken glass: copper foil, epoxy 
edge-gluing, and silicone edge-gluing. These techniques differ in strength, 
reversibility, and visual effect, and the appropriate repair must be selected 
on a case-by-case basis by a restoration specialist.

     Copper Foiling: Copper foil has the longest history and, unless the glass 
is unstable, is generally the best option when a piece of glass has only one or 
two cracks. Copper foil is a thin tape which is applied along each side of the 
break, trimmed to a minimal width on the faces, and soldered. A copper wire can 
be soldered on where additional strength is required. However, copper foil 
repairs should not be used on unstable glass, since heat is required that can 
cause further damage. Copper foil produces a strong repair, is totally 
reversible and has a negligible aesthetic impact (a 1/16" [1.6mm] wide line).

     Epoxy Edge-Gluing: This technique produces a nearly invisible line and is 
often used on painted glass, particularly focal points of a window such as a 
face, or a portion of sky intended to be one continuous piece. Epoxy can even 
be tinted to match the glass. It is also used for infusing shattered glass or 
microscopic cracks caused by intense heat from a fire. Epoxy produces a very 
strong repair, but will deteriorate in sunlight and requires secondary glazing 
to protect it from UV degradation. Epoxy is the least reversible of the three 
techniques, and usually the most expensive.

     Silicone Edge-Gluing: This repair method has the lowest strength and 
should be used when a flexible joint is desirable-if, for instance, the window 
will be under continuous stress. Silicone repairs are easily reversible, and 
can be removed with a razor blade-when they are done correctly, that is. 
Silicone edge-gluing is not the same as smearing silicone all over the glass. 
This unfortunate practice, seen throughout the country, is useless as a repair 
technique, and usually causes more damage than if the glass were left alone. 
Silicone is almost clear, but it refracts light differently from glass and is, 
thus, easily detectable. Silicone is not affected by temperature, humidity or 
UV light. Silicone repairs are typically the least expensive repair option.

Repairs to Structural Support Systems

     Windows may have detached from the saddle bars and begun to sag, bulge, 
and bow extensively. This point varies from window to window. Generally, 
however, a window sagging or bulging more than 1˝" (38mm) out of plane has 
reached the point where it should be removed from the opening to be flattened 
out. Under these conditions, it is essential to note if the support system or 
leading pattern has failed so it may be corrected before the window is 
reinstalled. The window must be allowed to flatten over a few weeks in a 
horizontal position. This will minimize stress on the solder joints and glass. 
A moderate weight and controlled heat will help coax the window back into its 
original plane. The process requires patience. Once the window has flattened, 
the original support system should be reattached and additional support added 
as necessary. It is crucial to consider the original design so the new support 
bars do not intrude on important window features. Sometimes small thin braces 
or "fins" can be manipulated to follow existing lead lines exactly. These give 
support, but are almost invisible. Flattening windows also provides a good 
opportunity to apply new waterproofing to help prevent further deterioration. 
Today, synthetic compounds are used.

     Windows should only be removed when they need to be flattened, 
waterproofed, reinforced or releaded. Allow plenty of time for careful, 
thorough work. Large projects can take several months, especially if complete 
releading is necessary. Owners, consulting professionals, and construction 
managers must therefore ensure that vacant openings will be weathertight for 
an extended period-whether the opening is covered by plywood, acrylics, or 
polymer film.

     Rebuilding or releading a window is an expensive and involved process. The 
releading process requires that a window be "unbuilt" before it can be 
"rebuilt." The glass pieces must be removed from the cames, the old cement must 
be cleaned from each piece of glass, and all the pieces must be rejoined 
precisely. At every step the process involves the risk of damaging the glass. 
Furthermore, exceptional studios had unique leading techniques, and thus the 
cames should not be replaced casually. Total releading should only be 
undertaken when necessary to avoid or slow the loss of historic fabric.

     Lead and zinc came, however, is intended to be a sacrificial element of a 
glass unit assembly, as mortar is to brick and paint is to wood; came will 
break down long before glass and must be replaced; came lasts 75 to 200 years 
depending on the window's quality, design and environment. A common 
preservation conflict arises in releading historic windows constructed of flat 
came: whether to retain historical accuracy by using new flat came, or to use 
came with a rounded profile for greater strength and durability. The decision 
must be carefully weighed depending on the significance of the window, the 
contribution of the came profile to the overall design, and the severity of the 
deterioration caused by a weak flat came. In most windows, the came profile is 
essentially lost in transmitted light, but occasionally shadow lines are 
important and should be reproduced. Furthermore, it is important to correct 
technical problems that arise from flimsy original came. Occasionally, a 
slightly heftier came may be the best solution to resolve weak panels that have 
not proven the test of time. Under these circumstances, the thicker lead came 
(even if only 1/64" [0.4mm]) will cause a leaded panel to swell slightly, and 
the frame, perimeter leads, or glass may have to be trimmed slightly to fit the 
opening. (Trimming the glass should be the last resort.) This would not be an 
appropriate solution in a museum-quality restoration or for a highly 
significant window.