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The Preservation of Historic
Glazed Architectural Terra-Cotta
de Teel Patterson Tiller
Glazed architectural terra-cotta was significant in the development
of important architectural idioms in this country--specifically, the
"Chicago School," the High Rise and the Historic or Beaux Arts styles.
In fact, glazed architectural terra-cotta is one of the most prevalent
masonry building materials found in the urban environment today (Fig.
1). Popular between the late 19th century and the 1930s, glazed architectural
terra-cotta offered a modular, varied and relatively inexpensive approach
to wall and floor construction. It was particularly adaptable to vigorous
and rich ornamental detailing. However, with changing vogues in materials
and architectural styles and rising production costs, glazed architectural
terra-cotta fell into disfavor and disuse by the mid 20th century.
Today, information on the maintenance, rehabilitation and replacement
of glazed architectural terra-cotta is limited, as are sources of new
glazed architectural terra-cotta. This report, then, will discuss some
of the major deterioration problems that commonly occur in historic
glazed architectural terra-cotta, methods of determining the extent
of that deterioration and recommendations for the maintenance, repair
and replacement of the deteriorated historic material.
What is Terra-Cotta?
Generically, the broadest definition of terra-cotta refers to a high
grade of weathered or aged clay which, when mixed with sand or with
pulverized fired clay, can be molded and fired at high temperatures
to a hardness and compactness not obtainable with brick. Simply put,
terra-cotta is an enriched molded clay brick or block. The word terra-cotta
is derived from the Latin word terra-cotta--literally, "cooked earth."
terra-cotta clays vary widely in color according to geography and types,
ranging from red and brown to white.
Terra-cotta was usually hollow cast in blocks which were open to the
back, like boxes, with internal compartment-like stiffeners called webbing
(Fig. 2). Webbing substantially strengthened the load-bearing capacity
of the hollow terra-cotta block without greatly increasing its weight.
Terra-cotta blocks were often finished with a glaze; that is, a slip
glaze (clay wash) or an aqueous solution of metal salts was brushed
or sprayed on the air-dried block before firing.
Glazing changed the color, imitated different finishes, and produced
a relatively impervious surface on the weather face of the final product.
The glaze on the terra-cotta unit possessed excellent weathering properties
when properly maintained. It had rich color and provided a hard surface
that was not easily chipped off. Glazing offered unlimited and fade-resistant
colors to the designer. Even today, few building materials can match
the glazes on terra-cotta for the range and, most importantly, the durability
of colors.
Types of Terra-cotta
Historically there are four types or categories of terra-cotta which
have enjoyed wide use in the history of the American building arts:
1) brownstone, 2) fireproof construction, 3) ceramic veneer, and 4)
glazed architectural.
Brownstone terra-cotta is the variety of this masonry material used
earliest in American buildings (mid to late 19th century). The brownstone
type is a dark red or brown block either glazed (usually a slip glaze)
or unglazed. It was hollow cast and was generally used in conjunction
with other masonry in imitation of sandstone, brick or real brownstone.
It is often found in the architecture of Richard Upjohn, James Renwick,
H. H. Richardson and is associated with the Gothic and Romanesque Revival
movements through such ornamental detailing as moldings, finials and
capitals.
Fireproof construction terra-cotta was extensively developed as a
direct result of the growth of the High Rise building in America. Inexpensive,
lightweight and fireproof, these rough-finished hollow building blocks
were ideally suited to span the I-beam members in floor, wall and ceiling
construction (Fig. 3). Certain varieties are still in production today,
although fireproof construction terra-cotta is no longer widely employed
in the building industry.
Ceramic veneer was developed during the 1930s and is still used extensively
in building construction today. Unlike traditional architectural terra-cotta,
ceramic veneer is not hollow cast, but is as its name implies: a veneer
of glazed ceramic tile which is ribbed on the back in much the same
fashion as bathroom tile. Ceramic veneer is frequently attached to a
grid of metal ties which has been anchored to the building.
Glazed architectural terra-cotta was the most complex development
of terra-cotta as a masonry building material in this country. The hollow
units were hand cast in molds or carved in clay and heavily glazed (often
in imitation of stone) and fired. Sometimes called "architectural ceramics,"
glazed architectural terra-cotta was developed and refined throughout
the first third of the 20th century and has been closely associated
with the architecture of Cass Gilbert, Louis Sullivan, and Daniel H.
Burnham, among others. Significant examples in this country include
the Woolworth Building (1913) in New York City and the Wrigley Building
(1921) in Chicago.
Late 19th and early 20th century advertising promoted the durable,
impervious and adaptable nature of glazed architectural terra-cotta.
It provided for crisp, vigorous modeling of architectural details as
the molds were cast directly from clay prototypes without loss of refinement.
Glazed architectural terra-cotta could accommodate subtle nuances of
modeling, texture and color. Compared to stone, it was easier to handle,
quickly set and more affordable to use. Thought to be fireproof and
waterproof, it was readily adaptable to structures of almost any height.
The cost of molding the clay, glazing and firing the blocks, when compared
to carving stone, represented a considerable savings, especially when
casts were used in a modular fashion--that is, repeated over and over
again. Maintenance of the fired and glazed surface was easy; it never
needed paint and periodic washings restored its original appearance.
With the passage of time, many of the phenomenal claims of the early
proponents of glazed architectural terra-cotta have proven true. There
are many examples throughout this country that attest to the durability
and permanence of this material. Yet present-day deterioration of other
significant glazed architectural terra-cotta resources ultimately belie
those claims. Why? Historically, the lack of foresight or understanding
about the nature and limitations of the material has, in many instances,
allowed serious deterioration problems to occur that are only now becoming
apparent
.
Characteristics of Glazed Architectural Terra-cotta as a Building
Material
Glazed architectural terra-cotta has many material properties similar
to brick or stone. It also has many material properties radically different
from those traditional masonry materials. It is those differences which
must be considered for a better understanding of some of the material
characteristics of glazed architectural terra-cotta when it is used
as a building material.
Difficult to identify: Glazed architectural terra-cotta probably comprises
one of the largest if not the largest constituent material in some of
our urban environments today. However, the infinite varieties of glazing
have hidden this fact from the casual observer. One of the attractive
features of glazed architectural terra-cotta in its time was that it
could be finished (glazed) in exact imitation of stone. In fact, many
building owners and architects alike are often surprised to discover
that what they presumed to be a granite or limestone building is glazed
architectural terra-cotta instead.
Two separate systems: Historically, glazed architectural terra-cotta
has been used in association with two specific and very different types
of building systems: as part of a traditional load-bearing masonry wall
in buildings of modest height, and as a cladding material in High Rise
construction. As cladding, glazed architectural terra-cotta often utilized
an extensive metal anchoring system to attach it or to "hang it" onto
a wall framing system or superstructure (Fig. 4). In the first instance
the anchoring was limited; in the second, the anchoring was often extensive
and complex. Likewise, in the first instance, deterioration has generally
been limited. However, where glazed architectural terra-cotta was used
as cladding, particularly in high rise construction, present-day deterioration
and failure are often severe.
Complexity of deterioration: Deterioration is, by nature of the design,
infinitely complex--particularly when glazed architectural terra-cotta
has been used as a cladding material.
Deterioration creates a "domino"like breakdown of the whole system:
glazed units, mortar, metal anchors, and masonry backfill. In no other
masonry system is material failure potentially so complicated.
Poor original design: The root of deterioration in glazed architectural
terra-cotta systems often lies in a misapplication of the material.
Historically, glazed architectural terra-cotta was viewed as a highly
waterproof system needing neither flashing, weep holes nor drips. This
supposition, however, has proved to be untrue, as serious water-related
failure was evident early in the life of many glazed architectural terra-cotta
clad or detailed buildings.
Common Deterioration
Problems
No one case of deterioration in glazed architectural terra-cotta is
ever identical to another owing to the infinite number of variations
with the material: original manufacture, original installation inconsistencies,
number of component parts, ongoing repairs or the various types and
sources of deterioration. However, certain general statements may be
made on the nature of glazed architectural terra-cotta deterioration.
Material failure can most commonly be attributed to water-related
problems. However, less frequent though no less severe causes may include:
faulty original craftsmanship, which is often cited but hard to determine;
stress-related deterioration; damage caused by later alterations and
additions; or inappropriate repairs.
Water-related deterioration: As with most building conservation and
rehabilitation problems, water is a principal source of deterioration
in glazed architectural terra-cotta. Terra-cotta systems are highly
susceptible to such complex water-related deterioration problems as
glaze crazing, glaze spalling and material loss, missing masonry units
and deteriorated metal anchoring, among others.
Crazing, or the formation of small random cracks in the glaze, is
a common form of water-related deterioration in glazed architectural
terra-cotta. When the new terra-cotta unit first comes from the kiln
after firing, it has shrunken (dried) to its smallest possible size.
With the passage of time, however, it expands as it absorbs moisture
from the air, a process which may continue for many years. The glaze
then goes into tension because it has a lesser capacity for expansion
than the porous tile body; it no longer "fits" the expanding unit onto
which it was originally fired. If the strength of the glaze is exceeded,
it will crack (craze) (Fig. 5). Crazing is a process not unlike the
random hairline cracking on the surface of an old oil painting. Both
may occur as a normal process in the aging of the material. Unless the
cracks visibly extend into the porous tile body beneath the glaze, crazing
should not be regarded as highly serious material failure. It does,
however, tend to increase the water absorption capability of the glazed
architectural terra-cotta unit.
Spalling, the partial loss of the masonry material itself, is, like
crazing, caused by water and is usually a result not only of airborne
water but more commonly of water trapped within the masonry system itself.
Trapped water is often caused by poor water detailing in the original
design, insufficient maintenance, rising damp or a leaking roof. In
most cases, trapped water tends to migrate outward through masonry walls
where it eventually evaporates. In glazed architectural terra-cotta,
the water is impeded in its journey by the relatively impervious glaze
on the surface of the unit which acts as a water barrier. The water
is stopped at the glaze until it builds up sufficient pressure (particularly
in the presence of widely fluctuating temperatures) to pop off sections
of the glaze (glaze spalling) or to cause the wholesale destruction
of portions of the glazed architectural terra-cotta unit itself (material
spalling).
Glaze spalling may appear as small coin-size blisters where the glaze
has ruptured and exposed the porous tile body beneath (Fig. 6). This
may occur as several spots on the surface or, in more advanced cases
of deterioration, it may result in the wholesale disappearance of the
glaze. Spalling of the glaze may also be symptomatic of deterioration
(rusting) of the internal metal anchoring system which holds the terra-cotta
units together and to the larger building structure. The increase in
volume of the metal created by rusting creates increased internal pressures
in the terra-cotta unit which, in turn, may spall the glaze, or in more
extreme cases, cause material spalling.
Material spalling is a particularly severe situation. Not only is
the visual integrity of the detailing impaired, but a large area of
the porous underbody, webbing and metal anchoring is exposed to the
destructive effects of further water entry and deterioration (Fig. 7).
Both glaze and material spalling must be dealt with as soon as possible.
Missing units is a serious situation which particularly plagues architectural
terra-cotta systems. Unlike brick or stone, damaged glazed architectural
terra-cotta is exceedingly difficult to replace. New production is extremely
limited. Missing units create gaps which increase the structural load
on the remaining pieces and also permit water to enter the system. Exposed
or freestanding glazed architectural terra-cotta detailing (balusters,
urns, parapet walls, etc.) are particularly susceptible to extensive
loss of material (Fig. 8). These elements face the most severe vicissitudes
of water and temperature-related deterioration in direct proportion
to the extent of their exposure. The replacement of missing units should
be a high priority work item in the rehabilitation of glazed architectural
terra-cotta.
Deterioration of metal anchoring: Deteriorated anchoring systems are
perhaps the most difficult form of glazed architectural terra-cotta
deterioration to locate or diagnose. Often, the damage must be severe
and irreparable before it is noticed on even the most intense "prima
facie" examination. Water which enters the glazed architectural terra-cotta
system can rust the anchoring system and substantially weaken or completely
disintegrate those elements. Where water has been permitted to enter
the system, some deterioration has more than likely taken place. Partial
deterioration results in staining and material spalling. Total deterioration
and the lack of any anchoring system may result in the loosening of
the units themselves, threatening the architectural or structural integrity
of the building. Recently, falling glazed architectural terra-cotta
units have become a serious safety concern to many building owners and
municipal governments (Fig. 9). Early detection of failing anchoring
systems is exceedingly difficult.
Deterioration of mortar and other adjacent materials: Deteriorated
mortar has always been a key to the survival or failure of any masonry
system. This is particularly true with glazed architectural terra-cotta.
In recognition of the fragile nature of the system, the need for insuring
a relatively dry internal system is important. Sound mortar is the "first
line" of defense in terra-cotta systems. It is a maintenance must. Deteriorated
mortar joints are a singularly culpable source of water and, therefore,
of deterioration. Mortar deterioration may result from improper original
craftsmanship or air- and waterborne pollution. More often, however,
lack of ongoing maintenance is mainly responsible. Deteriorated mortar
should not be overlooked as a major source of glazed architectural terra-cotta
failure.
The deterioration of materials adjoining the glazed architectural
terra-cotta (flashing, capping, roofing, caulking around windows and
doors) bears significant responsibility in its deterioration. When these
adjoining materials fail, largely as a result of lack of maintenance,
water-related deterioration results. For instance, it is not uncommon
to find wholesale terra-cotta spalling in close proximity to a window
or doorway where the caulking has deteriorated.
Stress-related deterioration: Stress-related deterioration of glazed
architectural terra-cotta frequently occurs in high rise buildings.
The evolution of stress relieving details (flexible joints, shelf angles,
etc.) occurred late in the development of American building construction.
Consequently, most early continuously clad High Rise buildings (c.1900-1920s)
had little or no provisions for normal material and building movement
in their original design. The development of large stress-related cracks
or wholesale material deterioration is often caused by unaccommodated
building-frame shortening under load, thermal expansion and contraction
of the facade and moisture expansion of the glazed architectural terra-cotta
units themselves (Fig. 10). Cracks running through many units or stories
or large areas of material deterioration often indicate stress-related
problems. This sort of deterioration, in turn, permits significant water
entry into the terra-cotta system.
Inappropriate repairs: Inappropriate repairs result because using
new terra-cotta for replacement of deteriorated or missing glazed architectural
terra-cotta has generally been impractical. Repairs, therefore, have
traditionally been made in brick or cementitious build ups of numerous
materials such as stucco or fiberglass. Some materials are appropriate
temporary or permanent replacements, while others are not. (These issues
are discussed at a later point in this report.) However, improper anchoring
or bonding of the repair work or visual incompatibility of repairs have
themselves, with the passage of time, become rehabilitation problems:
replacement brick that is pulling free, cement stucco that is cracking
and spalling, or a cement or bituminous repairs that are not visually
compatible with the original material.
Alteration damage: Alteration damage has occurred as a result of the
installation of such building additions as signs, screens, marquees
or bird proofing. These installations often necessitated the boring
of holes or cutting of the glazed architectural terra-cotta to anchor
these additions to the building frame beneath. As the anchoring or caulking
deteriorated, or as these elements were removed in subsequent renovation
work, these holes have become significant sources of water-related damage
to the glazed architectural terra-cotta system.
Deterioration
Inspection and Analysis
Certain deterioration in glazed architectural terra-cotta may be on
the building surface and patently obvious to the casual observer--crazing,
spalling, deterioration of mortar joints. Other deterioration may be
internal or within the masonry system and hard to determine--deterioration
of anchoring, deterioration behind the glaze, crumbling of internal
webbing. Prima facie, "first inspection," examination may indicate surface
deterioration problems while not revealing others. This demonstrates
one of the most frustrating aspects of dealing with deteriorated glazed
architectural terra-cotta: that there are two systems or levels of deterioration,
one which is visible and the other which is not.
Material failure in glazed architectural terra-cotta is necessarily
complex. For this reason, it is generally advised that the examination
and repair of this material should be the responsibility of an experienced
professional. Few restorationists have experience in the inspection,
repair and replacement of glazed architectural terra-cotta. This is
certainly never the province of the amateur or the most well-intentioned
but inexperienced architect or engineer. There are some methods of internal
and external inspection and analysis which are relatively simple to
the trained professional. Other methods, however, are expensive, time
consuming, and only in the experimental stage at this writing. These
all generally preclude the use of anyone but an experienced professional.
Preliminary cleaning: Before a terra-cotta building is analyzed for
deterioration, it is often advisable, but not always necessary, to clean
the surface of the material. This is particularly true when the material
has been exposed to the vicissitudes of heavy urban pollution. While
most building materials are cleaned for "cosmetic" purposes, the cleaning
of glazed architectural terra-cotta for the purpose of inspection and
analysis may be advised. Dirt on glazed architectural terra-cotta often
hides a multitude of problems. It is only with cleaning that these problems
become obvious. Recommended cleaning procedures are covered later in
the report.
Methods of inspection:
Prima facie analysis is the unit by unit, firsthand, external inspection
of the glazed architectural terra-cotta building surface. Special note
of all visible surface deterioration (staining, crazing, spalling, cracking,
etc.) should be made on elevation drawings. Binoculars are often used
where cost, height, or inaccessibility prevent easy inspection. However,
much deterioration may go unnoticed unless scaffolding or window-washing
apparatus is used in a true "hands on" inspection of each unit of the
facade.
Tapping, a somewhat inexact method of detection of internal deterioration
is, nevertheless, the most reliable inspection procedure presently available.
Quite simply, tapping is the striking of each unit with a wooden mallet.
When struck, an undamaged glazed architectural terra-cotta unit gives
a pronounced ring, indicating its sound internal condition. Conversely,
deteriorated units (i.e., units which are failing internally) produce
a flat, hollow sound. Metal hammers are never to be used, as they may
damage the glazed surface of the unit. Extensive experience is the best
teacher with this inspection method.
Infrared scanning is only in the experimental stage at this time,
but its use seems to hold great promise in locating deteriorated internal
material in terra-cotta. All materials emit heat--heat which can be
measured in terms of infrared light. While infrared light cannot be
seen by the human eye, it can be measured by infrared scanning. Infrared
photography, a kind of infrared scanning, has been of particular use
in detecting sources of heat loss in buildings in recent years. Broken
or loose internal terra-cotta pieces have a less firm attachment to
the surrounding firm or attached pieces and, therefore, have different
thermal properties, i.e., temperatures. These temperature differences
become evident on the infrared scan and may serve as a fair indication
of internal material deterioration in terra-cotta.
Sonic testing has been successfully used for some time to detect internal
cracking of concrete members. In the hands of an experienced operator,
there are conditions where it can detect internal failure in glazed
architectural terra-cotta. Sonic testing registers the internal configuration
of materials by penetrating the material with sound waves and reading
the patterns that "bounce back" from the originating source of the sound.
Readings at variance with those from undeteriorated material might indicate
collapsed webbing or pools of water in the interior of the terra-cotta
unit.
Metal detection is a nondestructive and generally useful way of locating
the position of internal metal anchoring. Metal detectors indicate the
presence of metals by electromagnetic impulses. These impulses are transmitted
onto an oscilloscope where they may he seen or they are converted to
sound patterns which may be heard by the operator. Original drawings
are eminently useful in predicting where internal metal anchoring should
be. Metal detectors can confirm that indeed they are still there. Without
original drawings, the contractor or architect can
still locate the metal anchoring, however. No reading where an anchor
would be expected could indicate a missing anchor or one that has seriously
deteriorated. The information produced by metal detection is, at best,
only rough. However, it is the most viable way of locating the internal
metal anchoring without physically removing, thus irreparably damaging,
the glazed architectural terra-cotta units themselves.
Laboratory analysis may be carried out on samples of removed original
material to find glaze absorption, permeability or glaze adhesion, or
to evaluate material for porosity. These tests are useful in determining
the present material characteristics of the historic glazed architectural
terra-cotta and how they may be expected to perform in the future.
Maintenance,
Repair and Replacement
Deterioration in glazed architectural terra-cotta is, by definition,
insidious in that the outward signs of decay do not always indicate
the more serious problems within. It is, therefore, of paramount importance
that the repair and replacement of deteriorated glazed architectural
terra-cotta not be undertaken unless the causes of that deterioration
have been determined and repaired. As mentioned before, one of the primary
agents of deterioration in glazed architectural terra-cotta is water.
Therefore, water-related damage can be repaired only when the sources
of that water have been eliminated. Repointing, caulking and replacement
of missing masonry pieces are also of primary concern. Where detailing
to conduct water in the original design has been insufficient, the installation
of new flashing or weep holes might be considered.
Where stress-related or structural problems have caused the deterioration
of glazed architectural terra-cotta, the services of a structural engineer
should be sought to mitigate these problems. This may include the installation
of relieving joints, shelf angles or flexible joints. In any case, stress-related
and structural deterioration, like water-related deterioration, must
be stopped before effective consolidation or replacement efforts may
begin.
Cleaning: The successful cleaning of glazed architectural terra-cotta
removes excessive soil from the glazed surface without damaging the
masonry unit itself. Of the many cleaning materials available, the most
widely recommended are water, detergent, and a natural or nylon bristle
brush. More stubborn pollution or fire-related dirt or bird droppings
can be cleaned with steam or weak solutions of muriatic or oxalic acid.
A note of caution: Any acids, when used in strong enough solutions,
may themselves deteriorate mortar and "liberate" salts within the masonry
system, producing a situation called efflorescence. For further information
on this situation, refer to: "Preservation Briefs 1: The Cleaning and
Waterproof Coating of Masonry Buildings," Heritage Conservation and
Recreation Service, Department of the Interior, Washington, D.C.
Commercial cleaning solutions may be appropriate but probably are
not necessary when water and detergent will suffice. There are, however,
certain cleaning techniques for glazed terra-cotta which are definitely
not recommended and which would damage the surface of the material.
These include: all abrasive cleaning measures (especially sandblasting),
the use of strong acids, (particularly fluoride-based acids), high-pressure
water cleaning and the use of metal bristle brushes. All of these techniques
will irreparably harm the glaze in one fashion or another and subsequently
expose the porous tile body to the damaging effects of water.
It is important to remember that glazed architectural terra-cotta
was designed to be cleaned cheaply and easily. This, in fact, was one
of its major assets and was much advertised in the selling of the material
early in this century.
Waterproofing: The covering of crazed glazing (see Fig. 5) with waterproof
coatings is the subject of an ongoing controversy today. The question
involves whether or not the micro-cracks conduct substantial amounts
of water into the porous tile body. Tests indicate that the glaze on
new unexposed terra-cotta is itself not completely waterproof. Some
testing also indicates that most crazing on historic glazed terra-cotta
does not substantially increase the flow of moisture into the porous
tile body when compared to new material. Excessive and serious crazing
is, however, an exception and the coating of those areas on a limited
scale may be wholly appropriate.
In an effort to stem water-related deterioration, architects and building
owners often erroneously attribute water-related damage to glaze crazing
when the source of the deterioration is, in fact, elsewhere: deteriorated
caulking, flashing, etc. The waterproof coating of glazed architectural
terra-cotta walls may cause problems on its own. Outward migration of
water vapor normally occurs through the mortar joints in these systems.
The inadvertent sealing of these joints in the wholesale coating of
the wall may exacerbate an already serious situation. Spalling of the
glaze, mortar, or porous body will, more than likely, result.
Repointing: Repointing of mortar which is severely deteriorated or
improperly or infrequently maintained is one of the most useful preservation
activities that can be performed on historic glazed architectural terra-cotta
buildings. Ongoing and cyclical repointing guarantees the long life
of this material. Repointing should always be carried out with a mortar
which has a compressive strength (measured in p.s.i.) lower than the
adjacent masonry unit. Hard (Portland cement) or coarsely screened mortars
may cause point loading and/or prevent the outward migration of the
water through the mortar joints, both of which ultimately damage the
terra-cotta unit. Repointing with waterproof caulking compounds or similar
waterproof materials should never be undertaken because, like waterproof
coatings, they impede the normal outward migration of moisture through
the masonry joints. Moisture then may build sufficient pressure behind
the waterproof caulk and the glaze on the terra-cotta to cause damage
to the unit itself.
Repair of glaze spalling: Glaze spalling is also a highly culpable
source of water-related deterioration in glazed architectural terra-cotta.
It is important to coat or seal these blistered areas (see Fig. 6) and
to prevent further entry of water into the system by this route. All
loose or friable material should be removed. This may be done easily
by hand; chisels or similar small tools are most effective. The exposed
material is then painted over. At this time, no permanently effective
reglazing materials are available. However, there are several acrylic-based
proprietary products and masonry paints which can be used effectively
to protect these exposed areas, thus preventing the entry of water.
These materials are effective for 5 to 7 years and can be reapplied.
They also can be tinted to approximate closely the original glaze color.
Repair of minor material spalling: Minor material spalling, where
visual or cosmetic considerations are negligible, should be treated
in a manner similar to glaze spalling damage. That is, areas where small
portions of the body and glaze have spalled and which are far removed
from close scrutiny (i.e., detailing on entablatures, upper story windows,
etc.) are best remedied by painting with a masonry paint or an acrylic-based
proprietary product. Units on which material spalling is easily observed
(on the street level, door surrounds, etc.), and on which visual integrity
is a consideration, may be better replaced. Patching is not appropriate.
Stucco-like or cementitious buildups are difficult to form satisfactorily,
safely and compatibly in situ to replace missing pieces of glazed architectural
terra-cotta. Cementitious repairs never satisfactorily bond to the original
material. The differential expansion coefficients of the two materials
(the repair and the original) preclude a safe, effective and long-term
attachment.
Repair of major spalling: Glazed architectural terra-cotta units,
which have spalled severely thereby losing much of their material and
structural integrity in the wall, should be replaced. Partial in situ
repair will not be long lasting and may, in fact, cause complicated
restoration problems at a later date. Appropriate methods of replacement
are discussed at a later point in this report.
Temporary stabilization: Stabilization measures are necessary when
deterioration is so severe as to create a situation where pieces of
glazed architectural terra-cotta may fall from the building. This is
a particular concern with greatly exposed detailing: cornices, balconies,
balustrades, urns, columns, buttresses, etc. Restoration work on these
pieces is expensive and often must be carried on over a period of time.
Unstable terra-cotta pieces are often removed or destroyed in lieu of
such measures. This is particularly true in areas of heavy traffic-related
vibrations or in earthquake zones. There are, however, less severe measures
which may be employed on a temporary basis. Substantial success has
been achieved in securing unstable glazed architectural terra-cotta
pieces with metal strapping and nylon net (Fig. 11). While these measures
should not be seen as permanent preservation solutions, they do offer
temporary alternatives to the wanton destruction of significant glazed
architectural terra-cotta detailing in the name of public safety and
local code compliance.
Repair of addition and structural damage: Holes, sign anchors, slots
for channel steel, or structural cracking in the surface of glazed architectural
terra-cotta cladding should be permanently sealed with a material that
will expand with the normal dynamics of the surrounding material, yet
effectively keep water out of the system. Any one of a number of commercially
available waterproof caulking compounds would be appropriate for this
work. Holes and static (nonmoving) cracks may be caulked with butyl
sealants or acrylic latex caulks. For dynamic (moving or active) cracks,
the polysulfide caulks are most often used, although others may be safely
employed. It is, however, important to remember that these waterproof
caulking compounds are not viable repointing materials and should not
be used as such.
Temporary replacement: Temporary replacement measures should be implemented
when missing units are scheduled to be replaced but work cannot be undertaken
immediately. Lengthy delivery time, prorating of work or seasonal considerations
may postpone replacement work. Severe deterioration should at least
be ameliorated until work can begin. Temporary repointing, removal and
saving of undamaged units to be reset later, or the temporary installation
of brick infill to retard further deterioration might be considered.
Removing earlier repairs: Removing earlier repairs may be necessary
when the work has either deteriorated or has become visually incompatible.
Cementitious stucco, caulkings with black bituminous compounds or brick
repair work may become structurally or visually unstable or incompatible
and should be removed and properly rehabilitated.
Replacement of glazed architectural terra-cotta: Replacement of severely
spalled, damaged, or missing glazed architectural terra-cotta elements
is always difficult. Certainly, in-kind replacement is advisable, but
it has a number of drawbacks. Stone, fiberglass, and precast concrete
are also viable choices, but like in-kind replacement, also have their
inherent problems.
Several notes on replacement: When replacing glazed architectural
terra-cotta, all of the original deteriorated material should be completely
removed. Half bricks or similar cosmetic replacement techniques are
not advised.
-- When possible and where applicable, replacement units should be
anchored in a manner similar to the original. Both structural and visual
compatibility are major considerations when choosing replacement materials.
-- Removing and reanchoring damaged glazed architectural terra-cotta
is an extremely difficult if not impossible task. The complexity of
the interlocking system of masonry units, backfill, and metal anchoring
system precludes the removal of the glazed architectural terra-cotta
unit without destroying it. Reanchoring deteriorated units is likewise
impossible. Therefore, if the terra-cotta in question is loose, severely
deteriorated, or its structural integrity in serious question, it is
best removed and replaced.
In-kind replacement is possible today, but only on a limited basis.
Most new glazed architectural terra-cotta is machine made, not hand
made as the original. Thus, the porous tile body of the new material
tends to be more uniform but less dense and often not as durable. The
glaze on the new glazed architectural terra-cotta tends to be thinner
than that on the older material and subsequently more brittle. Machine
processing has also produced a glaze that is uniform in color as opposed
to historic glazes which were slightly mottled and, therefore, richer.
Visual compatibility is an important consideration when replacing in-kind.
Only a fairly limited inventory of in-kind pieces is presently available
for replacement such as plain ashlar blocks and the simpler details
such as cappings and sills. When deterioration severely damages the
more ornate pieces (urns, cartouche work, balusters, etc.) either expensive
hand casting or alternative materials must be sought. There is a tendency
today to replace damaged ornamental work with simpler, cheaper and more
readily available units. This decision cannot, however, be supported,
as the removal of this work inevitably diminishes the character and
integrity of the building. Another major consideration in choosing in-kind
replacement is the question of delivery time, which is often quite lengthy.
If new glazed architectural terra-cotta is chosen as a replacement material,
the architect or building owner should plan far in advance.
Stone may be a suitable replacement material for damaged glazed architectural
terra-cotta. Its durability makes it highly appropriate, although the
increase in weight over the original hollow units may be of some concern.
The fact that historic glazed architectural terra-cotta was glazed in
imitation of stone, however, may make the choice of stone as a replacement
material a fortuitous one. Metal anchoring may be accommodated easily
in the carving. Cost, however, is the major drawback in stone replacement,
particularly where rich detailing must be carved to match the original.
Fiberglass replacement is a viable alternative, particularly when
rich and elaborate ornamentation has to be duplicated. Casting from
original intact pieces can produce numerous sharp copies of entablatures,
moldings, balusters, voussoirs, etc. Anchoring is easily included in
casting.
Significant drawbacks in using fiberglass replacement are color compatibility,
fire code violations, and poor weathering and aging processes. The appropriate
coloring of fiberglass is exceedingly difficult in many instances. Painting
is often unsatisfactory, as it discolors at a rate different than that
of the historic glazed original. While fiberglass casting is lighter
than the original units and, therefore, of great interest in the rehabilitation
of buildings in areas of high seismic activity, many fire code requirements
cannot be met with the use of this material.
Precast concrete units show great promise in replacing glazed architectural
terra-cotta at this writing. Precast concrete units can, like fiberglass,
replicate nuances of detail in a modular fashion: they can also be cast
hollow, use lightweight aggregate and be made to accommodate metal anchoring
when necessary. Concrete can he colored or tinted to match the original
material with excellent results. It is cost effective and once production
is in process, precast concrete call be produced quickly and easily.
Experience shows that it is advisable to use a clear masonry coating
on the weather face of the precast concrete units to guarantee the visual
compatibility of the new unit, to prevent moisture absorption, to obtain
the proper reflectivity in imitation of the original glaze and to prevent
weathering of the unit itself. Precast concrete replacement units are
presently enjoying great use in replicating historic glazed architectural
terra-cotta and show promise for future rehabilitation programs.
Once the replacement material is selected (new glazed architectural
terra-cotta. stone, precast concrete, or fiberglass), it must be reanchored
into the masonry system. Original metal anchoring came in numerous designs,
materials and coatings ranging from bituminous-coated iron to bronze.
While most of these anchors are no longer available, they may be easily
replicated in large quantities either in the original material when
appropriate or out of more durable and available metals such as stainless
steel.
Since the masonry backfill is already in place in the historic building,
the new replacement unit with anchoring may simply be fitted into the
existing backfill by boring a hole or slot for anchor and bedding the
anchor and the unit itself in mortar. When replacing historic glazed
architectural terra-cotta which originally employed metal anchoring,
it is important to replace that anchoring when replacing the unit. Serious
problems may result if anchoring is omitted in restoration, when it
was used originally. It is erroneous to assume that mortar alone will
be sufficient to hold these replacement pieces in place.
Summary
Today, many of this country's buildings are constructed of glazed
architectural terra-cotta. However, many of these are in a state of
serious deterioration and decay. Glazed architectural terra-cotta was,
in many ways, the "wonder" material of the American building industry
in the late 19th century and during the first decades of the 20th century.
New technology and methods of rehabilitation now hold promise for the
restoration and rehabilitation of these invaluable and significant resources.
Restoration/rehabilitation work on glazed architectural terra-cotta
is demanding and will not tolerate halfway measures. Today's preservation
work should equal the spirit, attention to detail, pride in workmanship
and care which characterized the craftsmanship associated with this
widely used, historic masonry material.
Suggested Further Readings
"Recipes for Baked Earth." Progressive Architecture (November, 1977).
McIntyre, W.A. Investigations into the Durability of Architectural
Terra Cotta. Special Report 12. London: Department of Scientific and
Industrial Research, Building Research Station, 1929.
Prudon, Theodore H.M. "Architectural Terra-cotta: Analyzing the Deterioration
Problems and Restoration Approaches." Technology and Conservation, Vol.
3 (Fall, 1978), pp. 30-38.
Prudon, Theodore H.M. Terra Cotta as a Building Material. A Bibliography.
Ottawa, Ontario: Association for Preservation Technology, 1976.
The illustrations for this brief not specifically credited are from
the files of the Technical Preservation Services Division.
This Preservation Brief was written by de Teel Patterson Tiller, Architectural
Historian, Technical Preservation Services Division. Information for
this publication was based in part upon interviews and consultation
with Theodore H.M. Prudon, The Ehrenkrantz Group, P.C., New York, New
York. Additional comments and information were provided by Si A. Bortz,
Illinois Institute of Technology Research Institute, Chicago, Illinois,
and Jerry G. Stockbridge, Wiss, Janney, Elstner, and Associates, Northbrook,
Illinois.
Washington, D.C. June, 1979
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