Deterioration of Stained and Leaded Glass

     Three elements of leaded glass units are prone to damage and 
deterioration: the glass itself; the decorative elements (mostly applied 
paint); and the structural system supporting the glass.

Glass Deterioration
     
     Glass is virtually immune to natural deterioration. Most American glass 
is quite stable-due to changes in glass composition made in the mid-19th 
century, particularly the increased silica content and the use of soda lime 
instead of potash as a source of alkali. Rarely, however, glass impurities or 
poor processing can cause problems, such as minor discoloration or tiny 
internal fractures (particularly in opalescent glass). And all glass can be 
darkened by dirt; this can often be removed (see "Cleaning" on page ). However, 
while glass does not normally deteriorate, it is susceptible to scratching or
etching by abrasion or chemicals, and to breakage.

     The greatest cause of breakage or fracture is physical impact. Leaded 
glass in doors, sidelights, and low windows is particularly susceptible to 
breakage from accidents or vandalism. When set in operable doors or windows, 
leaded glass can crack or weaken from excessive force, vibration, and 
eventually even from normal use. Cracks can also result from improperly set 
nails or points that hold the window in the frame, or more rarely, by 
structural movement within the building. Leaded glass that is improperly 
annealed can crack on its own from internal stress. (Annealing is the process 
by which the heated glass is slowly cooled; the process is akin to tempering 
metal.) Glass can also disintegrate from chemical instability or the intense 
heat of a fire. Finally, windows assembled with long, narrow, angular pieces 
of glass are inherently prone to cracking. Often the cause of the cracks can be
determined by the path they travel: cracks from impact typically radiate 
straight from the source. Stress cracks caused by heat or improper annealing 
will travel an irregular path and change direction sharply.

Deterioration of Painted Glass

     Painted glass, typically associated with pictorial scenes and figures 
found in church windows, often presents serious preservation challenges. If 
fired improperly, or if poor quality mixtures were used, painted glass is 
especially vulnerable to weathering and condensation. Some studios were 
notorious for poorly fired paints (particularly those working with opalescent 
glass), while others had outstanding reputations for durable painted glass. 
Paints can be applied cold on the glass or fused in a kiln. Since they are 
produced from ground glass, enamels do not "fade," as often suggested, but 
rather flake off in particles. Several steps in the painting process can 
produce fragile paint that is susceptible to flaking. If applied too thick, the
paint may not fuse properly to the glass, leaving small bubbles on the surface. 
This condition, sometimes called "frying," can also result from poor paint 
mixtures or retouching. Paint failure is more commonly caused by under firing 
(i.e., baking the glass either at too low a temperature or for too little 
time). Unfortunately, in American stained glass, the enamels used to simulate 
flesh tones were typically generated from several layers that were fired at 
too low a temperature. This means the most difficult features to 
replicate-faces, hands and feet-are often the first to flake away (Fig. 11).

Structural Deterioration

     The greatest and the most common threat to leaded glass is deterioration 
of the skeletal structure that holds the glass. The structure consists of frame
members, and lead or zinc (and occasionally brass or copper) came that secures 
individual pieces of glass. Frame members include wood sash and muntins that 
decay, steel t-bars and "saddle bars" that corrode, and terra cotta or stone 
tracery that can fracture and spall (Fig. 12). When frames fail, leaded glass 
sags and cracks due to insufficient bracing; it may even fall out from wind 
pressure or vibration.

     Wood sash are nearly always used for residential windows and are common 
in many institutional windows as well. Left unprotected, wood and glazing 
compounds decay over time from moisture and exposure to sunlight-with or 
without protective storm glazing-allowing glass to fall out.

     Steel frames and saddle bars (braces) corrode when not maintained, which 
accelerates the deterioration of the glazing compound and loosens the glass. 
Moreover, operable steel ventilators and windows are designed to tight 
tolerances. Neglect can lead to problems. Eventually, they either fail to 
close snugly, or corrode completely shut. The leaded glass is then frequently 
reinstalled in aluminum window units, which require wider sections for equal 
strength and typically trim an inch or more off the glass border. Instead of 
relocating glass in aluminum frames, historic steel frames should be repaired. 
Often the corrosion is superficial; frames in this condition need prepping, 
painting with a good zinc-enriched paint, and realigning in the frame.

     Masonry frames typically last a long time with few problems, but removing 
leaded glass panels set in hardened putty or mortar can be nearly impossible; 
as a last resort, glass borders may have to be sacrificed to remove the window.

     Occasionally, leaded glass was designed or fabricated with inadequate 
bracing; this results in bulging or bowing panels; leaded panels should 
generally not exceed 14 linear feet (4.25 m) around the perimeter without 
support. More often, the placement of bracing is adequate, but the tie-wires 
that attach the leaded panels to the primary frame may be broken or 
disconnected at the solder joints.

     Lead and zinc cames are the two most common assembly materials used in 
stained and other "leaded" glass. The strength and durability of the leaded 
panel assembly depends upon the type of came, the quality of the craftsmanship,
and the glazing concept or design, as well as on the metallic composition of 
the cames, their cross-section strength, how well they are joined and soldered, 
and the leading pattern within each panel. Came is prone to natural 
deterioration from weathering and from thermal expansion and contraction, 
which causes metal fatigue.

     The inherent strength of the assembly system is also related to the 
cross-section, profile and internal construction of the came (Fig. 13). Came 
can have a flat, rounded, or "colonial" profile, and aside from a few specialty
and perimeter cames (U-channel), is based on a variation of the letter "H" and 
ranges from _" (3.2mm) wide to 1" (38mm) wide. The cross-section strength of 
came varies depending on the thickness of the heart and flanges. Occasionally,
came with reinforced (double) hearts or a steel core was used for rigidity.
Such came added strength at the expense of flexibility and was typically used 
for rectilinear designs, or for strategically placed reinforcement within a 
curvilinear design.

     How the cames are joined in a leaded panel is crucial to their long-term 
performance. Poor craftsmanship leads to a weak assembly and premature failure,
while panels fabricated with interlocking (weaving) cames and lapped leads add 
strength. Soldered joints often reveal the skill level of the artisan who 
assembled the window, and can give evidence of past repairs. Solder joints 
should be neat and contact the heart of the came-wherein lies its greatest 
strength. Came joints should be examined closely; large globs of solder 
commonly conceal cames that do not meet. (Lead cames typically crack or break 
along the outside edge of the solder joint; stronger zinc cames frequently 
break the solder itself where it bridges junctures.) Weak joints contribute to 
a loose glass housing, and if glass rattles in the cames when the window is 
gently tapped, it is an indicator that repair or restoration is needed.

     Leading patterns designed with inadequate support also contribute to 
structural failure. Panels with a series of adjacent parallel lines tend to 
hinge or "accordion," while lines radiating in concentric circles tend to 
telescope into a bulge. Stronger leading techniques, support bars, or specialty
cames are sometimes required to correct poor original design. Minor sagging and
bulging is to be expected in an old window and may not require immediate 
action. However, when bulges exceed 1" (38mm) out of plane, they cross into a
precarious realm; at that point, glass pieces can crack from severe sagging and
pressure. If the bulged area moves when pressed gently, or if surrounding glass
is breaking, it is time to address the problem before serious failure results.  

Cleaning, Repair, Restoration, and Protection

     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 collapse.

Cleaning

     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 (Fig. 14). 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 chemicals.)

     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.

Repair

     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 product.

     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 
(see "Protective Glazing" on page ).

     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 
damage.

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," 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 (Fig. 15).

     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 (Fig. 16), epoxy 
edge-gluing (Fig. 17), 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 
(Fig. 18). (It is essential to request a copy of all window rubbings if the 
windows are to be completely releaded.)

     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 (Fig. 8). 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.

Protective Glazing and Screens

     The use of protective glazing (also known as secondary or storm glazing) 
is highly controversial. Potential benefits of protective glazing are that it 
can shield windows from wind pressure; increase energy savings; protect against
environmental pollutants and UV light; provide vandalism and security 
protection, and reduce window maintenance. Potential drawbacks are that it 
can promote condensation; cause heat to build up in the air space and thereby 
increase the window's expansion/contraction; eliminate natural ventilation; 
reduce access for maintenance; offer only minimal energy payback for 
intermittently heated buildings (such as churches and temples), and mar the 
appearance. Protective glazing can also be presented as a cheaper alternative 
to full-scale restoration. And all too often protective glazing is installed 
as a routine matter when there is little threat of damage from vandalism or 
other causes. Protective glazing, especially when improperly installed, may 
hasten deterioration of stained glass windows.

     Various types of metal grills or screens are also used. They add security 
and vandalism protection but often impair the appearance of the window 
(inside and out) by creating new shadows or diffusing transmitted light.

     As a general rule, protective layers should not be added. In most cases 
the potential drawbacks outweigh the potential benefits.Under some 
circumstances, however, protective glazing or screens may be necessary. 
(This applies to windows. Domes and ceilings present a special case. See 
"Domes and Ceilings" on page ). A real vandalism or security threat warrants 
protective glazing, such as when the windows can be reached easily or are in 
an isolated location (Fig. 19). Protective glazing is also warranted when 
employed historically on a particular window as original plating (Tiffany 
Studios, for example, often used plate glass to keep dirt and moisture out 
of their multi-plated windows). Unusual circumstances (such as when the 
windows are painted on the outside) may also dictate the use of protective 
glazing. Finally, protective glazing is warranted when a UV filter is needed 
to prevent epoxy glass repairs from breaking down.

     A variety of protective glazing materials are available. They include 
polycarbonates, acrylics, laminated glass, plate glass, and tempered glass. 
The plastic products are very strong, lightweight, and relatively easy to 
install, but tend to scratch, haze, and yellow over time, despite UV 
inhibitors. They also have a high coefficient of expansion and contraction, 
so the frames must be designed to accommodate change induced by temperature 
fluctuations. Poor installations in restrictive frames cause distorted 
reflections from bowing panels. Protective panels of glass are heavier and 
more difficult to install, making them more expensive than plastic. However, 
glass will not bow, scratch, or haze and is usually the best option in 
aesthetic terms; laminated glass provides additional impact resistance.

     A common error in installing protective glazing is to create a new 
window configuration (Fig. 20). Insensitive installations that disregard the 
original tracery destroy the window's aesthetics-and the building's. When 
protective glazing is added, it should be ventilated. If a window is not 
ventilated, heat and condensation may build up in the air space between the 
ornamental glass and the protective glazing. The surface temperature of 
unvented glass has been measured up to twice the outdoor ambient temperature. 
This differential affects the expansion and contraction of the support 
system, particularly lead cames, thereby accelerating metal fatigue. Protective
glazing may also cause condensation on the historic window, depending on the 
window's orientation, indoor/outdoor humidity, and whether or not the building 
is air conditioned.

     When absolutely necessary, protective glazing should be installed in an 
independent frame between _" (16mm) and 1" (25mm) from the leaded glass. This 
allows the protective panel to be removed for periodic maintenance of both the 
historic window and the new feature. The conditions of the air space between 
the two elements should be monitored on a regular basis; the glass should not 
feel hot, and condensation should never collect on the window.

     No ideal formulas have been developed for venting the air space between 
the ornamental glass and the protective glazing, but it is typically vented to 
the outside (unless the building is air conditioned most of the year). 
Generally, a gap of several inches is left at the top and bottom when glass 
is used, or holes are drilled in the protective glazing at the top and bottom 
when polycarbonates and acrylics are used. Small screens or vents should be 
added to keep out birds and insects. Finally, it is important to realize that 
some original plating of glass softened or tinted the transmitted light 
intentionally, as designed by the original window maker; in this case any new 
or replacement plating should simulate this effect to respect the artisan's 
intentions (Fig. 19).

Domes and Ceilings

     Stained glass domes and ceilings were very popular throughout the 
Victorian and Classical Revival periods. They are often principal interior 
features of churches, hotels, restaurants, railway stations, and civic 
buildings (Fig. 21). The loss or unsympathetic alteration of leaded glass 
ceilings and domes is a widespread problem. Poorly planned rehabilitation 
projects sometimes cause the removal or alteration of overhead leaded glass 
in order to comply with fire codes or to achieve perceived energy savings; 
occasionally, they are even concealed above suspended ceilings.

     Moreover, stained glass in the horizontal position readily collects dust 
and dirt over the years and is relatively inaccessible for cleaning. It is also
more likely to "creep" or slump when the reinforcement is inadequate. Most 
importantly, leaded glass cannot be sufficiently weatherproofed in a horizontal
(or arched) position. It must always be protected by skylights or 
"diffusers"-rooftop features that diffuse the natural daylight into the attic 
or light shaft, and protect the leaded glass ceiling or dome from the elements 
(Fig. 22).

     Due to the inferior quality of glazing sealants of the late 19th and early
20th centuries, and to deferred maintenance, glass ceilings have frequently 
been removed or covered with roofing materials. Artificial lighting is then 
required to backlight the ceiling or dome, which robs the stained glass of its 
life-the vibrant effects created by ever-changing natural light. All types of 
artificial lighting can be found from floodlamps to fluorescent tubes. Outside 
sensors are even used to modulate the light level in an attempt to simulate 
changes in daylight. However, daylight is impossible to emulate. Moreover, 
it's free. Artificial lighting requires maintenance, introduces an additional 
fire hazard in the attic, increases the building's electrical load, and is 
supplied only at a cost.

     Stained glass ceilings and domes that have been sealed off from natural 
light should be investigated for restoration. Once natural light is restored 
and the stained glass is cleaned, the lighting effect on an interior can be 
extraordinary. Improved skylight designs and major advances in glazing 
sealants since World War II (particularly silicones) encourage the restoration 
of skylights without the fear of inheriting a maintenance nightmare.

Conclusion

     Much of the Nation's stained glass and leaded glass has recently passed, 
or is quickly approaching, its 100th anniversary-yet much of this glass has not
been cleaned or repaired since the day it was installed. With proper care, the
stained and leaded windows, transoms, and other elements that add so much to 
historic buildings can easily last another hundred years.