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 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.
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. 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.
"The Preservation of Historic Glazed Architectural Terra-Cotta" an Historic Preservation Brief
