Repairing Terra-Cotta Surfaces
A careful investigation of the conditions is required before recommendations can be made for treatments to repair architectural terra-cotta surfaces.
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The beauty of glazed terra cotta, with its rich palette of earth tones and glassy surface, has been exploited by architects to enliven building façades since very early times. In addition to the tonal variations inherent in fired-clay bodies, color has been introduced for decorative uses with a number of surface treatments. Resilient exterior designs have been developed through the use of glazes and from the application of different clays, known as slip or engobe (if applied as coatings), or encaustic (if inlaid). Although ancient inventions, these techniques became widespread in exterior architectural installations in the 19th and early 20th centuries.
Developments in the structural use of cast iron and steel led to commensurate innovations in the design and installation of ceramic building materials. New mounting systems were created that allowed for the development of previously unknown uses of the colorful and plastic material. Freed from the constraints of structural brick and stone masonry, architectural terra cotta was used on building claddings in robust sculptural façades and unprecedented suspended panelized systems.
Failures that developed from these ingenious installations were often tied to errors in initial design and construction detailing, which is not surprising considering that they represented technologically challenging new usage. The National Terra Cotta Society published substantially different standards for installation between 1914 and 1927 after finding that initial recommendations did not adequately account for expansion, structural support and water migration. Mistaken or missing maintenance practices within the harsh variations of the North American climate frequently contributed to and increased the rate of failures.
Structural failures developed from water penetration, stresses from expansion and contraction, difficulties in firing during fabrication as well as differences in the behavior between the supporting clay bodies and glazes. Major failures leading to structural instability must be addressed by stabilization, repair or unit replacement of damaged or detached units. Testing and analysis leading to an understanding of the causes and remedies for deterioration conditions must be undertaken before implementing treatments. In cases where proper anchoring, waterproofing and expansion allowances can be established, damaged original units may be preserved either in-situ or after removal.
The terra-cotta cornice of El Capitolio in San Juan, Puerto Rico, was in an advanced state of collapse prior to disassembly and investigation by Conservation Solutions, Inc. A combination of treatments, including re-creation or replacement of the support structure, replacement of severely damaged units and stabilization of those units that could be retained using a stainless-steel Cintec anchor system were tested and recommended.
Case Studies in Repair
Conservation Solutions, Inc., performed this type of investigation and recommended repair methods for the stabilization of a terra-cotta cornice on “El Capitolio”, Puerto Rico’s capitol building. Opened in 1929, the building incorporates Classical motifs in terra cotta into the primarily marble structure. The terra-cotta blocks at the cantilevered cornice were observed to be heaved and displaced and there was concern about their structural stability.
Stabilization and partial disassembly of the cornice provided an opportunity to understand the construction methods and view the advanced stages of deterioration. Sulfur in the “cindercrete” backing cement and salts from the marine environment had led to the nearly complete destruction of the supporting ironwork. Open mortar joints, the loss of the structural support system and expansion from the rusting iron had all contributed to the displacement and failure of the terra-cotta units. A combination of treatments, including re-creation or replacement of the support structure, replacement of severely damaged units and stabilization of those units that could be retained using a stainless-steel Cintec anchor system were tested and recommended.
Crazing, spalling and localized surface loss are commonly noted conditions of historical glazed terra cotta. Several causes for these types of losses can be identified, some tied to the structural issues listed above and others more inherent to the materials. Movement between terra-cotta units can lead to stress-loading at contact points that will crack and spall off edges. Salts from backing concrete or mortar may migrate through porous clay bodies to crystallize at or near the surface, developing tensile pressures from subflorescence that also cause spalling of the surface.
The dissimilar thermal expansion rates of the glaze and clay bodies, not always well balanced in the original fabrication, may lead to stresses when subjected to temperature and weather fluctuations over time. Cracks in the glaze skin will allow water to travel to the porous clay body, leading to more serious losses from swelling and freeze-thaw action, as well as increased soiling. Other methods to introduce color through applied or inlaid engobes and slips (semi-vitrified colored clay bodies rather than glazes) can also suffer from differential thermal expansion and vapor transmission rates. Installation detailing, structural deterioration, impact and original design flaws can introduce and contribute to such surface losses as well.
The terra-cotta elements on the façade of the Holy Redeemer College in Washington, DC, suffered a failure when the slip coating separated from the back-up clay body. Biological growths were discovered within edges of the surface failures, encouraged by the entrapped moisture where the slip coating was lifting off the substrate.
Studies of the extensive surface failures at the terra-cotta elements on the façade of the Holy Redeemer College in Washington, DC, performed by Conservation Solutions, Inc., demonstrated the destructive consequences of poor original material manufacturing. Here, too, terra-cotta elements were integrated into a primarily stone structure. Window surrounds, string courses, battlements, parapet cresting, towers and other components of the structure’s façade were created from a buff-colored terra cotta set within the medium-gray granite ashlar.
Substantial losses, fracturing and surface deterioration of the terra-cotta units were noted throughout the inspection of the building’s façade. Although some losses corresponded to problems that are typical of masonry structures – open joints, inadequate maintenance, the use of caulk to overcome poor original waterproofing detailing – much of the surface deterioration could be tied to poor manufacturing techniques.
The clay body used in the backing of the units was found to be both inconsistently mixed and badly formulated, with a high proportion of grog and other inclusions. The material also appeared to have been unevenly vitrified during the firing process, leaving the clay vulnerable to expansion when it absorbed water. The low-gloss finish, likely intended to replicate the appearance of natural limestone, was produced by the use of a clay slip. Its high porosity allowed water to enter into the clay body behind, leading to swelling and losses on the surfaces.
The Minton tile ceiling within the Bethesda Terrace Arcade in New York City’s Central Park showed signs of spalling from the edge due to failing cast-iron backing.
These problems were exacerbated by the migration of salts from backing mortar due to open joints as well as other structural failures typical of architectural terra cotta. Biological growths were discovered within edges of the surface failures, encouraged by the entrapped moisture where the slip coating was lifting off the substrate.
Recommendations for surface restoration involved two separate steps: filling the losses to guard against future water infiltration and to restore surface continuity and the re-creation of an appearance that replicates the original glaze colors and gloss. Surface restoration should occur in concert with cleaning and after careful testing and selection of appropriate methods that avoid introducing additional damages. In cases where soluble salts have been identified, they must be removed during cleaning through controlled and cautious washing, steam cleaning, chemical treatment or poultices. Salts that are not readily soluble in typical “city water” but are soluble in acidic precipitation (magnesium, calcium, other substances) may also be present at loss sites. These generally originate from the backing materials and present problems when there has been a history of significant water infiltration. Mechanical removals including micro-abrasion may be required to remediate these conditions. Biological growths at these sites must also be removed prior to treatment.
Restoration methods for surface loss on terra cotta have been addressed only minimally in the published literature. Recommendations have pointed out difficulties with cementitious and resinous fill and toning materials. Cementitious materials were described as susceptible to failure from poor adhesion with the clay substrate (see Tiller, de Teel Patterson, "The Preservation of Historic Glazed Architectural Terra-Cotta," Preservation Brief 7, US Dept. of the Interior, and resinous fills were noted as tending to discolor (see Stratton, Michael, “The Nature of Terracotta and Faience”, in "Architectural Ceramics, Their History, Manufacture and Conservation," James & James, Ltd., London 1994, page 52). Differences in vapor permeability between resins, essentially impermeable acrylics or epoxies, and more permeable clay bodies have also been a concern.
Restored, re-backed and reinstalled, the tile in the Bethesda Terrace Arcade shows no signs of failure six years later.
Recent observations at the Ross Administrative Building of the United States Holocaust Memorial Museum, also in Washington, DC, appear to reveal the potential difficulties attendant on terra-cotta surface repairs. While further study is required, initial assessments suggest that an overall coating applied during the restoration of the building, probably to repair surface spalling similar to that seen at the Holy Redeemer site, may be acting to seal moisture within the building at the terra-cotta cornice and coping. The trapped water appears to be leading to rusting of the iron supports, jacking and heaving the cornice blocks and causing cracks in the masonry. Further study will be required to confirm these initial observations.
Despite the complexities, successful terra-cotta surface repairs are feasible. In preparation for the restoration of the Minton tile ceiling within the Bethesda Terrace Arcade in New York City, the Central Park Conservancy conducted controlled tests on a number of material combinations. The ceiling, a unique mid-19th-century suspended ceiling composed of nearly 16,000 encaustic tiles hung within an open arcade, had been removed and placed in storage after inspections in the 1980s revealed advanced deterioration of the supporting structure. Substantial efforts went into researching methods for rebacking each panel with new stainless-steel frames to replace the failed cast-iron originals. Methods for restoring the damaged faces of the intricately decorated individual tiles were also explored.
Terra-cotta elements were integrated into a primarily stone structure at Holy Redeemer College. Window surrounds, string courses, battlements, parapet cresting, towers and other components of the structure’s façade were created from a buff-colored terra cotta set within the medium-gray granite ashlar.
An extensive series of accelerated weathering tests followed by in-situ installations were conducted. The tests included combinations of both vapor-permeable masonry restoration materials and impermeable resinous fills and coatings. Each was judged for its effectiveness at retaining dimensional stability, surface cohesion, color and other factors. Several combinations of materials in both categories performed well throughout the tests, and the most successful combinations were then employed to repair tiles on panels that were also remounted with the new backing system. Each was then reinstalled in the arcade. After six years on site, the test repairs using these materials continue to be visually and structurally stable.