The 1893 Columbian Exposition in Chicago demonstrated the versatility and ornamental qualities of terra cotta.* It highlighted the great variance in color and shapes possible with terra cotta and began the demand in the United States for terra cotta that lasted through the late 1930s. Terra cotta is prized for its light weight, longevity, aesthetic qualities, and unit construction. At the peak of production, almost every urban area in America was producing architectural terra cotta in some variation. Today, most replacement units are produced by either Gladding McBean or Boston Valley Terra Cotta.
Specific forming techniques including hand press, machine
press, slip casting, and extrusion are used depending on the shape and style of
unit required. In the analysis of terra cotta failure the forming techniques
are less critical than the strength characteristics of the fired clay, the
integrity of the exterior surfaces, and structural support systems.Exterior ornamental terra cotta was marketed as a light weight water proof cladding. If proper construction techniques were employed, and the system was maintained, and the local climates were mild, terra cotta performed as sold. However, terra cotta adorns buildings in severe weather climates, and is installed with structural materials affected by environmental conditions, and located on façade elements inaccessible for routine maintenance.
Once water enters the system there is no weep path allowing
for water egress. Construction means and methods, as well as the cellular unit
design, trap water and contribute to the potential corrosion of steel lintels,
wire ties, steel structural support members, and other miscellaneous metals. Rapid
freezing and thawing cycles, in addition to steel corrosion, can crack terra
cotta units. If the units remain unrepaired, further water intrusion and/or absorption
will occur.
The repair of terra cotta will depend both on the cause and manifestation of the defect. Typical defects include crazing of the glazed finish, shallow surface spalls, deep spalls affecting the bisque, cracked units, inadequate support and / or anchorage, corrosion induced stress fractures, impact damage, mortar degradation, lack of maintenance, and inadequate repairs.
Proper terra cotta repair methods are linked to the cause of defect. Repair techniques are often performed on-site by skilled tradesmen. When damage to the terra cotta unit is severe, full replacement may be required. Defects due to inadequate support or a result of corrosion to supporting steel members is likely to require more invasive repair strategies including removal and replacement of several courses of interlocked terra cotta units.
When replacement units are not required and the scope is
limited to on-site repair, labor costs exceed material costs. Since many
historic terra cotta units were specialty designed and installed for the
structure, a premium price is paid for replacement. New exterior decorative
terra cotta is available only from the sources referenced and with small
quantity orders, the first unit is approximately $5,000 with much of the costs
attributed to making the form and determining the finish color and texture. Subsequent
costs per unit will decrease with the range of decrease dependent upon
quantities required.
The most important component of terra cotta repair is an
understanding the cause of deterioration and the proper repair specifications.
Both are derived after a full condition assessment and evaluation of the
existing conditions.
Sources
·
First two images from Wikipedia; others property of PMA
·
Last of
the Handmade Buildings, Virginia Guest Ferriday, Mark Publishing Co.,
Portland, OR 1984
·
National Park Service, Preservation Brief No7, Preservation of Glazed Terra Cotta
·
APT Pacific NW Chapter 2005 workshop
·
Terra
Cotta, Standard Construction, Revised Ed., National Terra Cotta Society,
1927
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