PRESERVATION 

BRIEFS 

The Preservation of Historic Glazed 

Architectural Terra-Cotta 

de Teel Patterson Tiller 

U.S. Department of the Interior National Park Service 

Preservation Assistance Division Technical Preservation Services 

Glazed architectural terra-cotta was significant in the development of important architectural idioms in this countryspecifically. 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 mid20th 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-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 (Fig. 2). Webbing substantially 

strengthened the load-bearing capacity of the hollow terracotta 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 roughfinished 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 archiFor sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
Figure I. Terra-Cotta Detailing. Adaptable to every nuance of color, 

texture. and modeling. glazed architectural terra-cotta was ideally 

suited to satisfy the tastes of an eclectic age. Its popularity was, however. 

short lived; it endured only 30 or 40 years after its introduction as a 

building material late in the 19th century. (Larry Payne . Houston. 

Texas) 

tectural 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 

Figure 2. Webbing. Webbing. or the hollow internal compartment 

construction of glazed architectural terra-cotta blocks. made them inexpensive to produce. easy to handle and light in construction; these 

were significant factors in the popularity of the material in the first 

decades of this century. 

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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 behe 

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 terracotta 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 cladSection Throllith Typical Arch 

Perspective of Typic.l Arch 

Figure 3. Fireproof construction terra-cotta. Perspective anu section 

through fireproof construction terra-cotta and I-beam detailing in industrial floor construction. (Detail. "Sweets" Industrial Catalogue of 

Building Construction, /906) 
Figure 4. Typical Construction Detail of Glazed Architectural TerraCotta Ornament. Construction detailing was often complex. The terracotta units (1) which were laid in mortar were fitted with holes or slots 

to receive the metal anchors (2) (often called "Z" straps or "light 

iron") which were often fitted directly to the building frame. Masonry 

backfill (3)(either brick or poured cement) was laid between the terracotta units, with the building frame encasing the metal anchor. Overhanging or protruding elements were further secured by metal dowels 

or outriggers (4). (Detail, Architectural Terra Cotta, Charles E. 

White, Jr., 1920) 

ding, 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 terracotta clad or detailed buildings. 

Common Deterioration Problems 

No one case of deterioration in glazed architectural terracotta 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 waterrelated 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 . Terracotta systems are highly susceptible to such complex waterrelated 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. 

Figure 5. Crazing. Water and air-borne moisture entering the glazed 

architectural terra-cotta causes expansion of the porous clay body 

which increases its volume. This, in tum, is sufficient 10 upset the "fit" 

of the glaze and 10 make it shatter, commonly called crazing. 

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 (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 

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Figure 6. Glaze Spalling. Blistering of the glaze. like crazing. is the 

result of the increase in water in the porous clay body and the subsequent destruction of the glaze as a result of water migration and 

pressure. Glaze spalling may also be caused by deterioration of metal 

anchors behind the terra-cotta unit. 

Figure 7. Material Spalling. Excessive expansion of the porous tile 

body caused by water and freezing temperatures produces major material spalling. a situation often difficult to repair. 

turn, may spall the glaze, or in more extreme cases, cause 

material spalling. 

Material spalling is a particularily 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. 

Figure 8. Deterioration of Exposed Detailing. Exposed or freestanding terra-cotta detailing (parapets. urns. balusters. etc.) have traditionally been subject to the most severe vicissitudes of deterioration as 

a result of freezing temperatures and water. (Colorado State Historic 

Preservation Office) 

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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. 

Figure 9. Deterioration of Metal Anchoring and Masonry Backfill. 

Trapped water may deteriorate masonry backfill or rust metal anchoring causing overhanging architectural elements to loosen and drop 

from the building. This is particularly true when unmaintained roof 

drainage systems fail and soak the masonry system. Note the exposed 

metal ancHoring. 

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 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 stressrelated cracks or wholesale material deterioration is often 

caused by unaccommodated building-frame shortening under 

load, thermal expansion and contraction of the fa<;ade 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. 

Figure 10. Structural Cracking. Structural cracking, whether static 

(nonmoving) or dynamic (moving or active), should be caulked to 

prevent water entry into the glazed architectural terra-COlla system. 

Note the exposed webbing. 

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 waterrelated 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. 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 windowwashing apparatus is used in a true "hands on" inspection of 

each unit of the fa<;ade. 

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. 

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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 undeterio rated 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 electro-magnetic 

impulses. These impulses are transmitted onto an oscilloscope 

where they may be seen or they are converted to sound patterns which may be heard by the optrrator. 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 terracotta is water. Therefore , water-related damage can be repaired only when the sou{ces 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 

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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 fluoridebased 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 terracotta 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 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: 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 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 archi-
tectural 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 build-ups 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. Anyone 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. 

Figure 11. Temporary Stabilization Measures. Falling glazed architectural terra-calla detailing has become a source of concern, partic· 

ularly in dense urban areas and locations of high seismic activity. 

Nylon nelling and metal strapping, while not seen as permanent pres· 

ervation measures, do offer a temporary alternative to the removal of 

these elements. 

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, back7 
fill , 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 properties. 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 light-weight aggregate and be made to accommodate metal anchoring when 

necessary. Concrete can be colored or tinted to match the 

original material with excellent results. It is cost effective and 

once production is in process, precast concrete can 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 

8 

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 re~acement 

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. Restorationlrehabilitation work on glazed 

architectural terra-cotta is demanding and will not tolerate 

half-way 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." Tech -

nology 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 Prese rvation Technology , 1976. 

The illustrations for this brief not specifically credited are from the files of thc 

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 . Addi· 

tional 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. 

This publication was prepared pursuant to Executive Order 11593, "Protection 

and Enhancement of the Cultural Environment," which directs the Secretary of 

the Interior to "develop and make available to Federal agencies and State and 

local governments information concerning professional methods and techniques 

for preserving. improving, restoring and maintaining historic properties." The 

Brief has been developed under the technical editorship of Lee H. Nelson. AlA, 

Chief, Preservation Assistance Division, National Park Service, U.S. Department of the Interior, Washington, D.C. 20240. Comments on the usefulness of 

this information are welcome and can be sent to Mr. Nelson at the above address. This publication is not copyrighted and can be reproduced without penalty . Normal procedures for credit to the author and the National Park Service are 

appreciated. June 1979.