PRESERVATION BRIEFS:  33

THE PRESERVATION AND REPAIR OF HISTORIC STAINED AND LEADED GLASS


Neal A. Vogel and Rolf Achilles

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service. 
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

               Superintendent of Documents
               P.O. Box 371954
               Pittsburgh, PA  15250-7954

               GPO #024-005-01122-6
               $1.50

Please call the Publication Order Information Desk at 202/512-1800
to verify price and availability.  


"STAINED GLASS" CAN MEAN COLORED, PAINTED OR ENAMELED GLASS, OR
GLASS TINTED WITH TRUE GLASS "STAINS."  IN THIS BRIEF, THE TERM
REFERS TO BOTH COLORED AND PAINTED GLASS.  "LEADED GLASS" REFERS
GENERICALLY TO ALL GLASS ASSEMBLIES HELD IN PLACE BY LEAD, COPPER,
OR ZINC CAMES.  BECAUSE THE CONSTRUCTION, PROTECTION, AND REPAIR
TECHNIQUES OF LEADED GLASS UNITS ARE SIMILAR, WHETHER THE GLASS
ITSELF IS COLORED OR CLEAR, "STAINED GLASS" AND "LEADED GLASS" ARE
USED INTERCHANGEABLY THROUGHOUT THE TEXT.

***INTRODUCTION***

Glass is a highly versatile medium.  In its molten state, it can be
spun, blown, rolled, cast in any shape, and given any color.  Once
cooled, it can be polished, beveled, chipped, etched, engraved, or
painted.  Of all the decorative effects possible with glass,
however, none is more impressive than "stained glass."  Since the
days of ancient Rome, stained glass in windows and other building
elements has shaped and colored light in infinite ways.

Stained and leaded glass can be found throughout America in a
dazzling variety of colors, patterns, and textures.  It appears in
windows, doors, ceilings, fanlights, sidelights, light fixtures,
and other glazed features found in historic buildings.  It appears
in all building types and architectural styles - embellishing the
light in a great cathedral, or adding a touch of decoration to the
smallest row house or bungalow.  A number of notable churches,
large mansions, civic buildings, and other prominent buildings
boast windows or ceilings by LaFarge, Tiffany, Connick, or one of
many other, lesser-known, American masters, but stained or leaded
glass also appears as a prominent feature in great numbers of
modest houses built between the Civil War and the Great Depression.

This Brief gives a short history of stained and leaded glass in
America.  It also surveys basic preservation and documentation
issues facing owners of buildings with leaded glass.  It addresses
common causes of deterioration and presents repair, restoration,
and protection options.  It does not offer detailed advice on
specific work treatments.  Glass is one of the most durable, yet
fragile building materials.  While stained glass windows can last
for centuries, as the great cathedrals of Europe attest, they can
be instantly destroyed by vandals or by careless workmen.  Extreme
care must, therefore, be exercised, even in the most minor work. 
For this reason, virtually all repair or restoration work
undertaken on stained and leaded glass must be done by
professionals, whether the feature is a magnificent stained glass
window or a clear, leaded glass storefront transom.  Before
undertaking any repair work, building owners or project managers
should screen studios carefully, check references, inspect other
projects, and require duplicate documentation of any work so that
full records can be maintained.  Consultants should be employed on
major projects.


***HISTORICAL BACKGROUND***

Glassblowers were among the founders of Jamestown in 1607, and
early glass manufacturing was also attempted in 17th-century Boston
and Philadelphia.  Dutch colonists in the New Netherlands enjoyed
painted oval or circular medallions that bore the family's coat of
arms or illustrated Dutch proverbs.  German colonists in the mid-
Atlantic region also began early glass ventures.  Despite the
availability of good natural ingredients, each of these early
American glassmakers eventually failed due to production and
managerial difficulties.  As a result, colonists imported most of
their glass from England throughout the 17th and 18th centuries.

Social values as well as high costs also restricted the use of
stained and other ornamental glass.  This was particularly true
with regard to churches.  The Puritans, who settled New England,
rejected the religious imagery of the Church of England, and built
simple, unadorned churches with clear glass windows.  Consequently,
not much glass remains from the colonial early national periods. 
Less than 1% of the Nation's stained and leaded glass predates
1700.  Considering the enormous loss of 17th, 18th, and early 19th-
century buildings, ANY window glass surviving from these periods is
very significant.  Every effort should be made to document and
preserve it.

Despite many failed starts, the War of 1812, and British
competition, American glass production increased steadily
throughout the 19th century.  Stained glass was available on a very
limited basis in America during the first quarter of the 19th
century, but American stained glass did not really emerge in its
own right until the 1840s.  The windows at St. Ann and the Holy
Trinity Episcopal Church in Brooklyn, New York, made by John and
William Jay Bolton between 1843 and 1848, are perhaps the most
significant early American stained glass installation.  Other
important early stained glass commissions were the glass ceilings
produced by the J. & G. H. Gibson Company of Philadelphia for the
House and Senate chambers of the United States Capitol in 1859.

America's glass industry boomed during the second half of the 19th
century.  (And although stained and leaded glass is found
nationwide, the manufacturing was based in the Northeast and
Midwest, where good natural ingredients for glass, and coal
reserves for the kilns were available.  Moreover, nearly all of the
nationally renowned studios were based in major metropolitan areas
of the central and northeastern states - near the manufacturers
that supplied their raw materials.)  In response to this growth,
the industry formed self-regulating associations that established
guidelines for business and production.  In 1879 the Window Glass
Association of America was established, and in 1903 The National
Ornamental Glass Manufacturers' Association, precursor of the
Stained Glass Association in America, was formed.

The 60 years from about 1870 to 1930 were the high point for
stained glass in the U.S.  In the early years, American stylistic
demands reflected those current in Europe, including various
historic revivals, and aesthetic and geometric patterns.  American
patterns prevailed thereafter; they tended to be more vivid, brash,
and bold.

After the 1893 Columbia World's Exposition, the Art Nouveau Style
became the rage for windows.  Sinuous nymphs, leggy maidens,
whiplashed curves, lilies, and brambles became standard subjects
until World War I.  Among the leading proponents of the Art Nouveau
Style were glassmakers John LaFarge and Louis Comfort Tiffany. 
Both men experimented independently throughout the 1870s to develop
opalescent glass, which LaFarge was first to incorporate into his
windows.  Tiffany became the better-known, due in part to his
prolific output.  He attracted world-class artists and innovative
glassmakers to his studio, which produced over 25,000 windows. 
Today, "Tiffany" remains a household name.  His favorite and most
popular scenes were naturalistic images of flowers, colorful
peacocks and cockatiels, and landscapes at sunrise and sunset. 
LaFarge, while appreciated in his own day, gradually slid into
relative obscurity, from which he has emerged in recent decades. 
Tiffany and LaFarge are the greatest names in American stained
glass.

In dramatic contrast to the American Art Nouveau style was the Neo-
Gothic movement that became so popular for church and university
architecture across the country.  Charles J. Connick was a leading
designer of medieval-style windows characteristic of the style.

Advocates of the Prairie Style, of whom Frank Lloyd Wright is the
best known, rejected Tiffany's naturalistic scenes AND Connick's
Gothic imitations.  Wright's rectilinear organic abstractions
developed simultaneously with the similar aesthetic of the various
European secessionists.  The creation of this style was aided by
the development of zinc and copper cames in 1893.  These cames -
much stiffer than lead - made it possible to carry out the linear
designs of Prairie School windows with fewer support bars to
interfere with the design.  At first, these windows had only an
elitist following, but they were soon widely accepted and
proliferated during the early 20th century.

By 1900, stained and leaded glass was being mass-produced and was
available to almost everyone.  Leading home journals touted leaded
glass windows for domestic use, and a nationwide building boom
created an unprecedented demand for stained and leaded art glass
windows, door panels, and transoms.  Mail order catalogs from sash
and blind companies appeared, some offering over 100 low-cost, mass
produced designs (although the same catalogs assured buyers than
their leaded glass was "made to order").

The fading popularity of the ornate Victorian styles, combined with
inferior materials used for mass production, and America's entry
into World War I (which reduced the availability of lead),
essentially eliminated the production of quality leaded glass.  The
last mail order catalogs featuring stained glass were published in
the mid-1920s, and tastes changed to the point that the 1926 House
Beautiful Building Annual declared:  "the crude stained glass
windows in many of the Mansard-roof mansions of the 'eighties
(1880s) prove how dreadful glass can be when wrongly used."  The
great age of stained glass was over.  However, leaded glass panels
have survived in uncounted numbers throughout the country, and are
now once again appreciated as major features of historic buildings.


***DATING AND DOCUMENTING HISTORIC LEADED GLASS***

Before deciding on any treatment for historic leaded glass, every
effort should be made to understand - and to record - its history
and composition.  Documentation is strongly encouraged for
significant windows and other elements.  Assigning an accurate
date, maker, and style to a stained glass window often requires
extensive research and professional help.  A documentation and
recording project, however, is worth the effort and expense, as
insurance against accidents, vandalism, fire and other disasters. 
The better the information available, the better the restoration
can be.  The following sources offer some guidelines for dating
leaded windows.

BUILDING CONTEXT:

The history of the building can provide ready clues to the history
of its leaded windows, doors, and other elements.  The construction
date, and dates of major additions and alterations, should be
ascertained.  Later building campaigns may have been a time for
reglazing.  This is especially the case with churches and temples. 
They were often built with openings glazed with clear leaded glass. 
Stained glass was added later as finances allowed.  Conversely, the
windows may be earlier than the building.  They may have been
removed from one structure and installed in another.  Bills,
inventories, and other written documents often give clues to the
date and composition of leaded glass.  Religious congregations,
fraternal lodges, and other organizations may have written
histories that can aid a researcher.

INSCRIPTIONS AND SIGNATURES:

Many studios and artists affixed signature plates to their work -
often at the lower right hand corner.  In the case of Tiffany
windows, the signature evolved through several distinct phases, and
helps date the piece within a few years:  Tiffany Glass Company
(1886-1892), Tiffany Glass & Decorating Company (with address,
1892-1902), Tiffany Studios New York or Louis C. Tiffany (post
1902).  Tiffany Studios, like others, did not always sign pieces
and the absence of an inscription cannot be used to rule out a
particular studio or artist.  Windows may feature dated plaques
commemorating a donor.  However, these do not always indicate the
date of the window, since windows were often installed before a
donor was found.  Nevertheless, these features help establish a
reasonable date range.

COMPOSITION AND OTHER STYLISTIC ELEMENTS:

These elements are more subjective, and call for a fairly broad
knowledge of architecture and art history.  Do the windows fit the
general style of  the building?  The style of the window may point
to a general stylistic period (e.g.,  Arts & Crafts, Art Nouveau,
Prairie School).  The imagery or iconography of the windows may
also reveal their overall historical context and establish a
general time period.

FRAMING AND SURROUND:

Framing elements and the window surround can reveal information
central to dating the window.  Do moldings match other interior
trim?  Has the opening been altered?  Is the window set in an iron
frame (post-1850s), a steel frame (generally post-World War I), a
cast stone frame (seen as early as the 1880s, but popular after
1900), or a terra cotta frame (generally after 1900)?

REINFORCEMENT AND LEADING DETAILS:

Does the window or other element have round bars or flat bars? 
Flat bars began to appear about 1890; round bars, used since the
Middle Ages, remained in use until the 1920s, when flat bars
supplanted them.  Cames can also give dating clues.  Zinc cames,
for example, developed by a Midwestern company in association with
Frank Lloyd Wright, first appeared in 1893.  In general, however,
dating a window by the came alone is difficult.  Over one hundred
varieties of lead came were available in the early 20th century. 
Moreover, came was sometimes produced to look old.  Henderson's
Antique Leading from the 1920s was made "to resemble the old hand
wrought lead" and also carried "easy-fix" clip-on Georgian-style
ornaments.

GLASS:

The glass itself can help in dating a window.  Opalescent glass,
for instance, was patented by John LaFarge in 1880.  Tiffany
patented two variations on LaFarge's technique in the same year. 
(Opalescent glass is translucent, with variegated colors resulting
from internally refracted light.  It features milky colored
streaks)  Pre-1880 glass is usually smooth translucent colored
glass (painted or not); glass with bold, deep colors is typical of
the 1880s and 1890s, along with jewels, drapery glass and rippled
glass.  But such flamboyance faded out with the rest of Victoriana
by about 1905.  However, stained glass styles of the late 19th
century continued to appear in ecclesiastical buildings after they
passed from general fashion.  Leaded beveled plate glass was
popular in residential architecture after 1890, and was used
profusely until the 1920s.

The level of documentation warranted depends upon the significance
of the window, but it is very important to document repair and
restoration projects before, during, and after project work. 
Photographs will normally suffice for most windows (see
"Photographing Stained Glass Windows", which follows).  For highly
significant windows (generally, those which were not mass
produced), rubbings as well as written documentation are
recommended.  The leading patterns in such windows are complex,
particularly in plated windows (which have several layers). 
Rubbings are therefore encouraged for each layer; they are
invaluable if a disaster strikes and reconstruction is required. 
Annotated rubbings of the leadwork should be done with a wax stone
on acid-free vellum.

To document windows properly, inscriptions should be recorded word
for word, including misspellings, peculiarities in type style, and
other details.  Names and inscriptions in or on windows can
indicate ethnic heritage, particularly in churches or civic
structures where windows often reflect styles and themes from the
congregation's or community's origins.  Lastly, any conjectural
information should be clearly noted as such.


***PHOTOGRAPHING STAINED GLASS WINDOWS***

Windows should be photographed with daylight color slide film and
black & White film in both transmitted and reflected light. 
Significant windows should be recorded with a positive color film,
such as Kodachrome, with a low ISO, since it is more stable, and
images should be printed on Resin-Coated paper.  Black & white
images should be printed on fiber-based paper to be considered
archival.  Photographing stained glass from the interior is not
difficult if a few basic pieces of equipment are used and if a few
simple rules are observed.  A strong tripod, shutter cable release,
light meter, and camera with through-the-lens metering will make
the job easier.  The key is to photograph windows in even, moderate
daylight with the interior dimmed (lights off and, if necessary,
with the other windows covered).  Although some stained glass is
dazzling in sunlight, the camera lens and film react differently
from the human eye, which can quickly equalize the high contrast of
light and dark glass.  Film cannot discriminate this intense
contrast, and the result can be a washed-out exposure or "hot
spots."  A light meter should be used to average out variations
within the window, with special consideration for the focal point
or most important feature of the window, such as a face.  Since
there is no precise formula for obtaining a balanced exposure,
shots should be bracketed.  When photographing on sunny days, shoot
away from the sun; shoot eastern windows in the afternoon, western
windows in the morning, southern windows at either time, and
northern windows at midday.  The glass should also be photographed
from the inside with reflected light from a flash (positioned away
from the camera to provide a raking light and to avoid reflected
"hot spots").  Although photographing with a flash will neutralize
the transmitted light and black out the glass, interior photography
is valuable because it reveals the location and condition of the
cames, braces, tie-wires, and other elements.  Shoot the windows as
centered and straight on as possible to minimize distortion and to
keep the window frames from blocking details.  Windows should also
be photographed from the outside if there is no protective glazing
to interfere with the view.  This is particularly important with
opalescent glass, which was often meant to be read from the
exterior as well.  As a final note, to photograph glass
consistently well, it is essential to limit the variables (by using
the same film, camera, and lenses), and to record the camera
settings, to compare with the developed pictures and to adjust
accordingly next time.


***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", which follows). 
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.

STRUCTURAL DETERIORATION:

The greatest and the most common threat to leaded glass is
deterioration of the skeletal structure that hold the glass.  The
structure consists of frame members, and lead or zinc (an
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.  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 institution 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
looses 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 realignment 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 (4.25m)
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.  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 for 1/8" (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.


***CAME TYPES AND PROPERTIES***

LEAD CAME:  

Lead is a soft malleable metal (it can be scratched with a
fingernail).  It naturally produces a protective dark bluish-gray
patina.  In the mid-19th century, improved smelting processes
enabled manufacturers to extract valuable metal impurities from
lead, thereby producing 100% pure lead came.  The industry reasoned
that 100% pure lead came was superior to the less pure variety. 
Although pure lead came is very workable and contributes to
intricate designs, time has proven it to be less durable than
medieval came, which contained trace elements of tin, copper,
silver, and antimony.  Unfortunately, the misconception that pure
lead had greater longevity continued throughout the glory years of
leaded glass use in America.  Most glass conservators use a 100-
year rule of thumb for the life expectancy of 19th century came -
less for came produced during war times.  The demand for lead
ammunition and the resulting scarcity of lead required studios to
stretch the available lead to its limits, thus resulting in weaker
cames.  In the 1970s, "restoration lead" (ASTM B29-84) was
developed based on metallurgic analyses of medieval cames, some of
which have lasted for centuries.  Restoration lead should always be
used when releading historic windows.

ZINC CAME:  

Zinc came is more vulnerable to atmospheric corrosion (particularly
from sulfuric acids) than lead, but has proven to be durable in
America because it weights 40% less than lead and it coefficient of
expansion is 7% lower.  Thus, it is somewhat less susceptible to
fatigue from expansion and contraction.  Moreover, it is ten times
harder than lead, and has three times the tensile strength.  Zinc
came is strong enough to be self-supporting and requires little
bracing to interrupt the window's design.  While zinc came is
perfect for the geometric designs of Prairie School windows, it is
usually to stiff to employ in very curvilinear designs.  Zinc can
also take several finishes, including a copper or black finish. 
(As a result, zinc can be mistaken for copper or brass.)

OTHER CAME:  

Other metals, primarily solid brass and copper, were also
occasionally employed as came.  They are generally found only in
windows between ca. 1890 and ca. 1920.


***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 lights.  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