Preservation Briefs: 12 The Preservation Of Historic Pigmented Structural Glass (Vitrolite And Carrara Glass)

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(Vitrolite and Carrara Glass)


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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 predominantly 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 be 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 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

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.  


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.

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.  The first and second stories of
many 19th century storefronts had disappeared by 1940, hidden by
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.
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, 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


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 20th century

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

Shelf angles (18-gauge brass or stainless steel, 3 inch square with
a 1/2 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.  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.  In either case,
the edge was secured to the substrate with a mastic 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.


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.

As a non-veneer 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.


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


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 anchor shelves. This caused the individual glass panels
to gradually slip away from their original positions and fall.


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.


Glass breakage through impact is virtually impossible to prevent.
The material is, by its nature, vulnerable to loss through
vandalism or accident.


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.


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.


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


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.

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.


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 is 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.


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.


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.


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 Oxyx, 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.

                             END OF SECTION

Last Reviewed 2012-09-10