Historic Preservation - Technical Procedures
- Preservation Briefs: 7 The Preservation Of Historic Glazed Architectural Terra Cotta
- Procedure code:
- National Park Service, Preservation Assistance Division
- Terra Cotta Unit Masonry
- Last Modified:
- Preservation Briefs: 7 The Preservation Of Historic Glazed Architectural Terra Cotta
- Last Modified:
PRESERVATION BRIEFS: 7
THE PRESERVATION OF HISTORIC GLAZED ARCHITECTURAL TERRA COTTA
The link immediately below connects to the NPS Preservation Briefs and latest edition:
de Teel Patterson Tiller
This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:
Superintendent of Documents
P.O. Box 371954
Pittsburgh, PA 15250-7954
Available ONLY in package sets
Briefs 1-14 - $13.00
Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.
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. 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
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
cocta" -- 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. 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
***TYPES OF TERRA COTTA***
Historically there are four types of 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. 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 block, 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
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
***CHARACTERISTICS OF GLAZED ARCHITECTURAL TERRA COTTA AS A
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. 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.
As with most buildings 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). 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
air-borne 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. 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. 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. 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. 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 water-borne
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
Stress-related deterioration of glazed architectural terra cotta
frequently occurs in highrise 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 highrise 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 contradiction of the facade and
moisture expansion of the glazed architectural terra-cotta units
themselves. 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 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 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
***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, although 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
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 procedures become obvious.
Recommended cleaning procedures are covered later in the report.
METHODS OF INSPECTION:
PRIMA FACIE ANALYSIS is the unit by unit, first-hand, 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
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 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 promise in locating deteriorated internal
material in terra cotta. All materials emit 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 that 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
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 electro-magnetic impulses.
These impulses are transmitted onto an oscilloscope where they 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,
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
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 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.
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 architectural 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
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.
The covering of crazed glazing with waterproof coatings is the
subject of an on-going 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 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. On-going 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 by 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 water-proof caulk and the glaze on