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The Preservation and Repair
of Historic Stained and Leaded Glass
Neal A. Vogel and Rolf Achilles
"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.
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 (Fig. 1).
It appears in windows, doors, ceilings, fanlights,
sidelights, light fixtures, and other glazed features found
in historic buildings (Fig. 2). 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 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 and 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 (Fig. 3). 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 (Fig. 4). 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 (Fig. 5).
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 (Fig. 6). 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 (Fig. 7).
Advocates of the Prairie Style, of whom Frank Lloyd Wright is
the best known, rejected Tiffany's naturalistic scenes
and Connick's Gothic imitations. (Fig. 8). 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 that their
leaded glass was "made to order") (Fig. 9).
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 (once again, this is more likely with religious
structures). 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 (Fig.
10).
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" on
page ). 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 three to five shutter speeds up and down
to find the best exposure. 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"
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.
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 weighs 40%
less than lead and its 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 too 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
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
place. practice routine among churches where cost of
restoring large windows can be prohibitive. However,
undertaking place provides only a quick fix. cannot properly
repaired or restored if it bulging sagging far out plane,
over 5% to 10% broken, solder joints are failing.
Unscrupulous glazers introduce great deal stress into by
forcibly flattening and tacking on additional bracing. At
comfortable distance may look fine, but upon close inspection
cracks broken become obvious. subjected this treatment will
deteriorate rapidly, complete, much more costly restoration
likely necessary within few years (while proper repair easily
last two generations 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.
Selected Reading List
The Census of Stained Glass Windows in America. The
Conservation and Restoration of Stained Glass: An Owner's
Guide. Raleigh, NC: Stained Glass Associates, 1988.
Duthie, Arthur Louis. Decorative Glass Processes: Cutting,
Etching, Staining, and Other Traditional Techniques. New
York: The Corning Museum of Glass and Dover Publications,
1982.
Fisher, Charles E., III, ed. The Window Handbook:
Successful Strategies for Rehabilitating Windows in Historic
Buildings. Washington, D.C.: National Park Service and
Georgia Institute of Technology. 1986. Rev. 1990.
Frelinghuysen, Alice Cooney. "A New Renaissance: Stained
Glass in the Aesthetic Period," In Pursuit of Beauty:
Americans and the Aesthetic Movement. New York: The
Metropolitan Museum of Art, 1986.
Heinz, Thomas A. "Use & Repair of Zinc Cames in Art-Glass
Windows." Old House Journal, (September/October 1989),
pp. 35-38.
Koch, Robert. Tiffany: Rebel in Glass. New York: Crown
Publishers, Inc., 1964.
Lee, Lawrence, George Seddon and Francis Stephans. Stained
Glass. New York: Crown Publishers, 1976
Lloyd, John Gilbert. Stained Glass in America.
Jenkintown, PA: Foundation Books, 1963.
Rigan, Otto B. Photographing Stained Glass. Oregon:
Mercury Press, 1983.
Stained Glass Association of America. SGAA Reference &
Technical Manual, Second Edition Lee's Summit, MO: The
Stained Glass Association of America, 1992.
Wilson, H. Weber. Great Glass in American Architecture:
Decorative Windows and Doors Before 1920. New York: E. P.
Dutton, 1986.
Wilson, H. Weber. Your Residential Stained Glass: A Practical
Guide to Repair & Maintenance. Chambersburg, PA:
Architectural Ecology, 1979.
The Window Workbook for Historic Buildings.
Washington, DC: Historic Preservation Education Foundation,
1986.
Acknowledgements
Neal A. Vogel is the Technical Services Coordinator
for Inspired Partnerships in Chicago, Illinois. Rolf
Achilles, an Art Historian, serves on the Technical
Advisory Committee for Inspired Partnerships.
The authors would like to thank the following individuals,
studios and manufacturers who graciously shared information
and provided opportunities to review studio, glass
processing, and on-site works in progress: Botti Studios of
Architectural Arts, Evanston, IL; Bovard Studio, Fairfield,
IA; Chicago Metallic Corporation, Chicago, IL; Chicago Art
Glass and Jewels, Inc., Plymouth, WI; Conrad Schmitt Studios,
Inc., New Berlin, WI; Cummings Studio, North Adams, MA;
Cypress Lawn Memorial Park, Colma, CA; Georgia Trust for
Historic Preservation, Atlanta, GA; Hollander Glass, Stanton,
CA; Jon Lee Art Glass Company, Winono, MN; Shenandoah Studios
of Stained Glass, Inc., Front Royal, VA; Vermont Statehouse,
Montpelier, VT.
Special thanks to Arthur J. Femenella, Association of
Restoration Specialists, Inc., Hoboken, NJ; Richard L.
Hoover, Stained Glass Association of America, Lee's Summit,
MO; and H. Weber Wilson, Olitz-Wilson Antiques, Newport, RI,
for providing editorial assistance.
Michael J. Auer, Preservation Assistance Division,
National Park Service, served as technical editor. Additional
assistance was provided by Anne Grimmer, National Park
Service. The text of this publication is not copyrighted, and
can be reproduced without penalty. Normal procedures for
credit to the authors and the National Park Service are
appreciated.
Cover Photograph: The rose window at St. John Cantius,
Chicago (1906-1908), undergoing restoration (interior view).
Photo: Neal A. Vogel.
End Photograph: Wooden frame of the rose window undergoing
repair (exterior view). Photo: Neal A. Vogel.
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