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The Preservation of Historic
Pigmented Structural Glass (Vitrolite and Carrara Glass)
U.S. Department of the Interior, National Park
Service Rocky Mountain Regional Office, Cultural Resources Division
The dramatic growth and
popularization of the early 20th century Art Deco,
Streamline, and Moderne architectural styles were fueled, in
part, by technological advances in the building materials
industry. New products, such as stainless steel and plastics,
enlarged the realm of architectural design. The more
traditional materials, on the other hand, quickly developed
fresh, innovative forms and uses. For example, the
architectural glass industry became especially creative,
introducing a series of new glass products known as
structural glass. Used predominately for wall surfacing,
these now familiar products included glass building blocks,
reinforced plate glass, and pigmented structural glass.
Pigmented structural glass, popularly known under such trade
names as Carrara Glass, Sani Onyx (or Rox), and Vitrolite,
revolutionized the business and rapidly became a favorite
building material of the period's architects and
designers.
The versatility of pigmented structural glass contributed to
its popularity. Not only could the material be applied to
both the exterior and interior, the glass could he
sculptured, cut, laminated, curved, colored, textured, and
illuminated. Often applied directly over existing
architecture to remodel older buildings, as well as in new
construction, a veneer of pigmented structural glass had the
ability to define a building's architectural character as new
and up-to-date. Pigmented structural glass also complemented
the period's silvery metal accents and affinity for slick,
shiny surfaces. A successful application of a structural
glass veneer often resulted in a streamlined look
characteristic of the Art Moderne architectural style.
As tastes changed and
production costs rose, however, pigmented structural glass
fell into disfavor and disuse by mid-20th century. With
today's rekindled interest in the Art Deco, Art Moderne, and
Streamline styles, the preservation and replacement of
pigmented structural glass have now become an integral part
of many rehabilitation projects, particularly in relation to
commercial storefronts. This brief, then, was developed in
order to address some of the major deterioration problems
associated with pigmented structural glass and to recommend
methods for maintaining, repairing, and, if necessary,
replacing damaged or missing pieces of pigmented structural
glass.
Early
Manufacture and Use of Pigmented Structural Glass
Although pigmented
structural glass enjoyed widespread popularity from the
beginning of the Great Depression to the outbreak of World
War II, its origins can be traced to the turn of the century.
In 1900, the Marietta Manufacturing Company claimed to be the
first producer of pigmented structural glass, rolling the
first sheet of a "substitute for marble," Sani Onyx.
Penn-American Plate Glass Company quickly joined its ranks,
manufacturing white and black Carrara Glass around 1906.
Penn- American Plate Glass no doubt selected the name
"Carrara" for the white glass's close resemblance to the
white marble of the Carrara quarries of Italy. Shortly
thereafter, Libby-Owens-Ford Glass began production of their
own version called Vitrolite.
Initially, Sani Onyx was
produced for such utilitarian purposes as refrigerator
linings. Manufacturers perceived the glass as a practical,
easily cleaned, and sanitary product. Its uses, however,
expanded rapidly. By the second decade of the 20th century,
consumers viewed pigmented structural glass as an inexpensive
substitute for marble counter tops, table tops, wainscoting,
and restroom partitions. The first large-scale interior
architectural application of pigmented structural glass was
in the Woolworth Building (1912-1913) when Architect Cass
Gilbert sheathed the restrooms with Carrara Glass. Later in
the decade, the decorative possibilities of the glass
received even more attention.
As the century
progressed, architects began to substitute pigmented
structural glass for traditional building materials in new
construction. Large expanses of architectural detailing such
as sleek door surrounds, polished interior lobbies, and
striking commercial storefronts became expected and familiar
features within new, expanding downtown business districts in
the 1920s and 1930s (see fig. 1).
In addition, designers
quickly found pigmented structural glass to be an
increasingly popular modernizing material for older and
out-of-date buildings. As a result, storefronts became a
favorite subject for "modernization." New Deal programs,
including low-rate insured Federal Housing Administration
loans in combination with a "Modernize Main Street"
competition sponsored by the Architectural Record and
Libby-Owens-Ford Glass, stimulated the remodeling fervor. By
1940, pigmented structural glass veneers had become
synonymous with the "modern look." The numerous pigmented
structural glass storefronts surviving today are testimony to
the popularity of these remodelings.
The winners of the 1935
"Modernize Main Street" competition illustrated what many
considered good contemporary design. The judges of the
competition, including Albert Kahn, William Lescaze, and John
Root, awarded architects who incorporated "simplicity,"
"economy," "unbroken horizontal lines," "expressed function,"
and "pure colors contrasting light and shadow" in their
designs. Simplicity of design often translated into
curvilinear recessed entries which protected consumers from
inclement weather--eliminating cumbersome canvas or metal
awnings--and providing additional display window space (see
fig 2). The first and second stories of many 19th century
storefronts had disappeared by 1940, hidden behind simple,
yet striking, modern pigmented structural glass
veneers.
Although the glass was
originally produced only in white, the range of colors from
which architects could choose soon included black, beige, and
ivory. By the 1930s, more exotic colors such as tropic green,
forest green, robin blue, suntan, and jade were offered by
the principal manufacturers in addition to the stock colors
of gray, yellow, and tan. Agate or marbleized treatments in
fanciful imitation of the "real" materials were also
available. The back surface was occasionally silvered to give
a rich mirror finish (see fig. 3). Most of these colors and
finishes were available in standard thicknesses from 11/32
inch to 1-1/4 inches. The glass's smooth exterior was
obtained either by fire polishing during the normal glass
fabrication process or by mechanical polishing when a high
mirror finish was desired. In both cases, the smooth, slick,
reflective surface made the material intensely popular with
architects or designers who sought the "modern look."
Although focusing on
exterior applications, architects also utilized pigmented
structural glass for interior spaces (see fig. 4), replacing
the porous and more expensive marble and offering a highly
polished, uniform visual appearance in keeping with design
trends of the 1920s and 1930s. Other uses of the material
included small, high-style installations in hotels, office
lobbies, bars, and lounges (see figs. 5 and 6).
Historic
Material and Installation Specifications
Early 20th century
advertisers often promoted pigmented structural glass as a
new panacea of the building materials industry. Their claims
were not without substance. Unlike masonry units such as
terra cotta, pigmented structural glass would not warp,
swell, or craze. Nor was the glass highly susceptible to
staining, fading, or burning. Like most glass products, it
was impervious to moisture and could be easily maintained and
usually cleaned with a damp cloth. Adaptable to a wide range
of uses, the glass could be colored and textured to attain
brilliant visual qualities. Perhaps most important, when
compared to marble, the glass was easier to handle, less
expensive to use, and simpler to install. The key to proper
preservation and repair of both interior and exterior
pigmented structural glass is a thorough understanding of the
original material specifications and detailed installation
techniques. Fortunately, these specifications and techniques
remain virtually unchanged from their first early 20th
century application (see fig. 7).
Exterior
Installation
Essentially, the glass
veneer was applied to a dry, smooth, and solid masonry or
plaster-on-masonry substrate using an asphaltic masonry
adhesive. Manufacturers recommended against affixing the
glass directly to wood, either lath or paneling. Glass
thicknesses of 11/32 inch or 7/16 inch were most common for
commercial storefronts.
Shelf angles--18-gauge
brass or stainless steel, 3 inch square with a ´ inch
leg fastened directly to the masonry substrate--were used to
provide additional support. Inserted along the bottom edge of
the panels, they supported every second course of glass and
were thus spaced not more than 3 feet apart. Horizontally,
the angles were spaced approximately one every 18 inches with
at least two used for any piece.
Actual installation
involved applying daubs (2 inches to 3 inches in diameter) of
hot asphalt-based mastic adhesive to the glass and then
attaching the glass directly to the substrate. Manufacturers
of the mastic recommended coverage of about 50 percent of the
glass panels. A full 3 inch width of mastic coverage was
recommended around detail edges or any holes in the panels.
The mastic was applied in a molten state after being melted
in an electric "hot cup." (Hot cups are still manufactured
for this specific purpose and are made to hold enough mastic
for a single daub.)
The next step in the
installation procedure was to push the glass panel onto the
masonry substrate. Every horizontal seam and abutment was
separated by a 1/16 inch thick adhesive cork tape recessed
from the front surface by 1/8 inch. Vertical edges were kept
apart at a uniform 1/32 inch. In either case, the joint
opening was then buttered with a joint cement which was
colored to match the surrounding glass.
Proper detailing at the
edges of the veneer could prolong the life of the pigmented
structural glass. For example, to prevent possible chipping
and cracking of the glass where it met the sidewalk, a
cushion of neoprene or leather was provided and the exposed
surface then caulked (see fig. 8). The side edges of the
glass were detailed in a variety of methods or the glass
simply terminated at the desired location with the ends
ground smooth (see fig. 9). In either case, the edge was
secured to the substrate with a mastic and the joints or void
filled with joint cement or caulking compound. Where the edge
of the glass abutted another material, such as the brickwork
of a neighboring storefront, the glass was held back 1/8 inch
to 1/4 inch from the adjacent material. The gap was usually
filled with pliable caulk to permit expansion and to prevent
moisture migration (see figs. 10 and 11).
Interior
Installation
Construction methods and
materials were quite similar for interior and exterior uses
of pigmented structural glass. Most interior veneers were the
same thickness and approximate dimension of those used for
exteriors. Minor differences did, however, exist. For
example, joints between the pieces of glass could be reduced
to little more than hairline cracks for interior applications
due to the limited thermal expansion of the substrate. On the
other hand, the use of glass as an indoor ceiling material
created unusual installation requirements.
Ceiling slabs 11/32 inch
in thickness were attached to 1 inch x 4 inch wood furring
strips with mastic (a full 4 inch width coverage was
recommended around the edge of the panels). Brass wood screws
and small rosettes, protected with felt exterior covers,
provided additional support (see fig. 12).
As a nonveneer material,
pigmented structural glass was generally used for counter and
table tops and restroom partitions. Counter tops presented
little or no unusual installation problems. Partitions,
however, often involved formidable installation challenges;
for example, enormous glass panels, weighing up to 16.25
pounds per square foot and measuring 1 inch to 1-1/4 inches
in thickness were used. The desired thickness was obtained by
cementing two 7/16 inch slabs together with mastic. To
accommodate this heavy, yet fragile, load, a reinforced
support and connection system was developed which utilized
metal sleeves, iron anchors, and steel straps bolted directly
into the glass panels .
Reasons for Damage
Although deterioration of
pigmented structural glass itself is rare, or unheard of,
failure of the mechanical support system which bonds the
glass modules to the wall is almost always the cause of
failure, cracking, slipping, or loss. Therefore, damage is
usually attributable to one or a combination of the
following:
-
Deterioration of the
Joint Cement
-
Hardening and Failure
of the Mastic Adhesive
-
Impact Due to
Accident/Vandalism
Deterioration of the
Joint Cement
Historically, the cement
joint between glass panels was intended to provide an
integrated, watertight surface. Unfortunately, the
traditional joint cement did not possess a long lifespan.
Cracked or open joints have been the consequence, usually
resulting from improper original application of the cement or
from the normal thermal expansion and contraction cycle
associated with weathering. Cracked or open cement joints
then accelerated deterioration of the masonry substrate
and/or the mastic adhesive bond by allowing water to
penetrate the internal system. Water entering the system
weakened the bond between the mastic and the masonry
substrate or rusted the anchoring shelves. This caused the
individual glass panels to gradually slip away from their
original positions and fall.
Hardening and Failure of
Mastic
Failure due to long-term
hardening of the original mastic adhesive has accounted for a
substantial loss of pigmented structural glass panels. The
petroleum-based mastics normally possessed a 30 to 40 year
lifespan. Once flexibility of the adhesive is lost, the glass
panels become vulnerable to slippage and eventual destruction
(see fig. 13)
Impact Due to
Accident/Vandalism
Glass breakage through
impact is virtually impossible to prevent. The material is,
by its nature, vulnerable to loss through vandalism or
accident (see fig. 14).
Maintenance
and Repair of Pigmented Structural Glass
The maintenance of a dry
masonry substrate, mastic, and metal anchors is essential to
the longevity of a pigmented structural glass veneer. Thus,
repointing cracked or open joints--particularly at ground
level where glass abuts concrete--and caulking of slightly
cracked glass panels is an ongoing concern. Where drainage to
conduct water away from the wall is faulty or insufficient,
the problem should be immediately corrected. For example,
roof flashing, downspouts, and gutters should be repaired or
new systems installed.
Repair of Cement
Joints
Cracked or open cement
joints, particularly in exterior applications, can present a
serious preservation problem because they permit water to
penetrate the internal system of a pigmented structural glass
veneer. Rusting metal anchors or deteriorating mastic
adhesive may be the result. Although the traditional joint
cements are easily colored and may be neatly applied, they
are no longer recommended for the repair of pigmented
structural glass because their longevity is limited.
Present-day silicone compounds, on the other hand, offer
flexibility, relative impermeability to moisture, ease of
installation, and a long lifespan. The proper color match can
be obtained by mixing the compound with tinted polyester
resins.
Patching Glass
Cracks
Any glass panel that can
be repaired should not be replaced. Thus, the decision to
repair or replace damaged historic pigmented structural glass
panels always needs to be made on a case-by-case basis. In
many instances, the damage may be so minor or the likelihood
of finding suitable replacement glass panels so small that
repairing, reanchoring, and/or stabilizing the damaged glass
is the only prudent choice.
A slightly chipped or
cracked pigmented structural glass panel left unrepaired will
inevitably become a source of water infiltration. Careful
patching of those cracks with an appropriately colored,
flexible caulk will deter moisture penetration while still
allowing expansion and contraction with temperature
fluctuations. Although patching is by no means a permanent
solution, it will help to protect the material from further
damage due to the effects of weather (see fig. 15).
Removal of Pigmented
Structural Glass Panels
Removal of existing glass
panels from a wall in order to reapply mastic adhesive that
is failing or to replace broken panels (see also paragraphs
on "Replacement of Damaged/Missing Glass Panels") is an
exacting operation because the mastic used to attach the
glass panels to the wall may have become hard and extremely
difficult to separate from the ribbed backing of the glass.
Fortunately, commercial solvents may be purchased which are
capable of softening the hardened mastic, such as methyl
ethyl ketone, methyl isobutyl ketone, and acetone. These
solvents may be introduced into the cavity behind the glass
with a crook-necked polyethylene laboratory squeeze bottle or
a large syringe without a needle. (Solvents should be stored
in fire-safe metal containers until used and should also be
handled with extreme care so that they do not come into
contact with the skin.) Such methods make it easy to direct
the solvent into the narrow separation between the glass
panel and the wall with a minimum of waste and effort. After
the mastic has softened, two people using a taut piano wire
sawing down from the top can safely and efficiently separate
the glass from the wall.
If time is a concern, a
fast, simple removal method is to carefully pry the panels
off with a broad flat tool such as a nail puller. A small
piece of wood placed between the flat tool and glass will
minimize splintering of the edges. Stubborn pieces can be
removed by squirting the mastic with a solvent (as described
above), then letting it set several minutes. This procedure
softens the mastic, making it more pliable. The piano
wire/sawing method may be useful in removing the topmost
glass panels of a continuous face where no edges occur. The
wire can be effectively worked into the joints and will cut
through the mastic. With care, a high percentage of the glass
panels can be salvaged using this method (see fig.
16).
Another method of
removing glass panels that has proven to be effective if the
solvent-and-wire method cannot be used, involves directing
steam at the face of the panel in order to soften the mastic.
Although this method can be time-consuming, averaging up to
10 minutes per panel, the glass can be successfully removed.
Remaining mastic may then be removed by directing additional
steam on the panel, soaking the panels in hot water to
further soften the mastic--or applying appropriate chemical
solvents--and scraping off the softened mastic.
Reinstallation of Glass
Panels
Due to an accumulation of
soot behind the glass, the surface of the masonry substrate
usually needs to be cleaned before panels or a wall of
pigmented structural glass are reinstalled. After removal of
the glass panels has been completed, the substrate should be
cleaned using a mild detergent and water, then allowing
sufficient time for it to dry. The old glass must also be
thoroughly cleaned of soot, grease, or old mastic that would
impair bonding of the new adhesive. A mild solution of water
and household ammonia will generally clean the surface
adequately. The glass may then be reinstalled following a
system established during removal.
In reinstalling the glass
panels (or new panels to replace any historic glass that has
been broken), it is recommended that the mastic adhesive used
throughout the 1930s and 1940s be used, because it is still
the best bonding material. Although modern silicone compounds
offer workability, adhesion, and flexibility, they tend to be
expensive when used in the necessary quantity. On the other
hand, butyl adhesives do not provide sufficient adhesion on
nonporous materials such as pigmented structural glass.
Polysulfide-based, synthetic rubber sealants do not have the
short set-up time of the traditional hot-melt asphalt mastic
and thus present installation difficulties. Finally, epoxies
do not appear to have the plasticity essential for longevity
of a glass veneer.
Replacement
of Damaged/Missing Glass Panels
Production of pigmented
structural glass in the United States ceased several years
ago, and only in rare cases have inventories been discovered.
Yet, checking all the obvious and not so obvious sources for
replacement may prove to be rewarding. Occasionally, long
established "jobbers" will have a limited supply of pigmented
structural glass. It is not uncommon for glass contractors to
buy entire stocks of glass when companies or supply houses go
out of business and to use this original material to make
repairs on historic buildings.
Locating a source for new
glass similar to the historic pigmented structural glass is
as much of a problem as finding the original glass. Until
about 10 years ago, glass companies near Bavaria in Western
Germany were producing a pigmented structural glass called
"Detopak." At present, these factories appear to be the only
suppliers in the world. The glass is made in small batches,
and the color can vary due to the lack of modern
mechanization in the pigmenting process. For this reason,
American importers generally only deal in white and black
glass.
If a satisfactory
replacement panel cannot be located, one alternative is to
remove a piece of glass from an inconspicuous part of the
building and position it on the more prominent facade. Modern
spandrel glass, a new substitute material described below,
may be considered as a replacement for the less visible
area.
Substitute
Material for Damaged/Missing Glass Panels
If replacement glass
cannot be found to replace broken or missing panels, a
compatible substitute material may be considered if it
conveys the same visual appearance as the historic material,
i.e., color, size, and reflectivity. Two of the historic
producers of pigmented structural glass now manufacture a
similar product known generically as "spandrel glass" and
marketed under the trade names of Spandrelite and Vitrolux.
This heavy plate glass has a ceramic frit or colored ceramic
surface fired to the back of the glass. Stock colors are
available in a range of grays, browns, bronzes, and black.
Custom colors are also available.
A second option simulates
the appearance of pigmented structural glass by spraying
paint, carefully tinted to match the historic glass, onto the
back of plate glass. However, the paint may fade over a long
period of time and thus require periodic
reapplication.
Sheet plastics may also
be used and are available in a range of colors, sizes, and
thicknesses. These materials are more suitable for interior
applications, however, where the negative effects of
ultra-violet light are lessened.
Conclusion
The preservation of
pigmented structural glass remains more a materials issue
than a detailing problem. The glass panels were and are
extremely susceptible to breakage due to accident or
vandalism. In addition, many of the historic installation
materials such as the mastic adhesive and joint cement did
not possess a long lifespan. Periodic maintenance,
inspection, careful repair, and selective replacement--in
like kind--are essential for the longevity of any historic
pigmented structural glass veneer.
Even though the
architectural glass industry has continued to expand its
production of different types of glazing, the imaginative
innovations of Carrara Glass, Sani Onyx, and Vitrolite in the
early part of this century have not been surpassed. New
technology, combined with human artistry, produced exteriors
and interiors alive with color and dimension. Glittering
movie palaces, sparkling restaurants, and streamlined
storefronts as well as the more mundane kitchens, restrooms,
and laboratories exemplified the extensive variety and
potential of pigmented structural glass. Carrara Glass, Sani
Onyx, and Vitrolite were integrally linked to the
architecture and interior design of the 1930s and 1940s and
helped to define what was "modern." Thus, every effort should
be made to preserve this significant historic material in
both the innovative buildings of the Art Deco, Streamline,
and Moderne styles as well as the "modernization" of earlier
structures.
Acknowledgements
This Preservation Brief
is partially adapted from an article entitled "Material
Conservation for the Twentieth Century: The Case for
Structural Glass," written by Douglas A. Yorke, Jr., AIA,
which appeared in the Bulletin for the Association for
Preservation Technology, 13 (1981), and from an unpublished
manuscript by Thomas L. Hensley of the National Park Service.
Preservation Brief 12 was edited by Gregory D. Kendrick,
Historian, under the technical editorship of de Teel
Patterson Tiller, both of the Rocky Mountain Regional Office,
National Park Service. We wish to thank Mr. Yorke for
permission to use his article and photographic material.
Finally, we want to acknowledge Thomas G. Keohan, Field
Representative, Mountains, Plains Regional Office, National
Trust for Historic Preservation for donating photographs and
assistance to this project.
Additional Reading
Gay, Charles Merick and
Parker, Harry. Materials and Methods of Architectural
Construction. New York: John Wiley and Sons, Inc.,
1931.
Glass, Paints, Varnishes
and Brushes: Their History, Manufacture and Use. New York:
Pittsburgh Plate Glass Company, 1923.
Hornbostel, Caleb.
Construction Materials. New York: John Wiley and Sons, Inc.,
1978.
Kidder, Frank E. and
Parker, Harry. The Architect's and Builder's Handbook, 15th
ed., New York: John Wiley and Sons Inc., 1913.
McGrath, Raymond and
Frost, A.C. Glass in Architecture and Decoration. London: The
Architectural Press, 1937.
"Modernize Main Street."
Architectural Record 78 (October 1935): 209-266.
Ramsey, Charles George
and Sleeper, Harold Reeves. Architectural Graphic Standards.
3rd ed., New York: John Wiley and Sons, Inc., 1941.
Richey, H. G. Richey's
Reference Handbook for Builders, Architects and Construction
Engineers. New York: Simmons-Boardman Publishing Corporation,
1951.
The Secretary of the
Interior's Standards for Rehabilitation and Guidelines for
Rehabilitating Historic Buildings (Revised 1983). Washington,
D.C.: Technical Preservation Services Division, U.S.
Department of the Interior.
Standard Building Code.
Birmingham, Alabama: Southern Building Code Congress
International, Inc., 1979.
Sweet's Architectural
Catalogue, 22nd annual ed., New York: F.W. Dodge Corporation,
Sweet's Catalogue Service, 1927--1928; Section B, pp. 1406
1409, (Vitrolite product literature).
Sweet's Indexed Catalogue
of Building Construction. New York: Architectural Record
Company, 1906.
Time Saver Standards, 1st
ed., New York: Architectural Record, 1946.
Yorke, Douglas A. Jr.,
AIA. "Materials Conservation for the Twentieth Century: The
Case for Structural Glass." APT 13 (1981): 19-30.
Washington, D.C. Febuary,
1984
Last Modified:
January 30, 1998
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