Terra cotta

Terra cotta, or terracotta, meaning baked earth, is a fired clay product that is popular in construction for fireproofing or architectural cladding. Clay has been a popular medium for creating decorative objects for centuries. Ceramic veneer is a thin, machine-made, pressed-clay product that is usually glazed, simple in shape, but has a variety of textures and finishes.

History
In the eighteenth century, terra cotta was often used for building construction in England. Nearly a century later, America began producing terra cotta products for architecture. Production began in the 1860s. Plants were usually built close to a clay source which was usually sites where fired clay ware had been manufactured. Nearly missing the Chicago Fires of 1871, the Chicago Terra Cotta Company helped to provide the city with materials for rebuilding. The president of the Chicago Terra Cotta Company, Sanford Loring soon expanded the range of architectural products that were terra cotta. Terra cotta saw a growth in popularity in New York City during the 1870s thanks in large part to architects like George B. Post and F.H. Kimball. In 1879, the Chicago Terra Cotta Company closed its doors, losing many of its clients to Northwestern Terra Cotta Company, which produced terra cotta for commercial buildings. By 1900, Northwestern had become the largest manufacturer of terra cotta in the country.

Manufacturing Process
Hand-formed terra cotta that is made today follows the uses the same techniques as when the material was first produced for commercial use. It is made of clay, water, and previously fired products (also called grog). These are first mixed and then kneaded to prepare them for the molds. In order to prepare molds for the terra cotta, full-scale shop drawings are usually made. For ornamental elements, an artist may be commissioned to make a model for the clay out of wood or plaster. The models are allotted an excess to make up for shrinkage of 7 to 11% during drying and firing. Using the model, a plaster mold is handmade of the individual pieces that are keyed together. Pieces of the mold are then assembled and tied together with straps. Clay is pressed by hand and then sculpted onto the sides of the mold until the thickness is 1 ½ inches. Stiffeners are added at regular intervals to add strength during the drying and firing process. After the firing, the mold is removed and the product is hand finished, repairing any surface defects. Surface textures, such as combing, can be added at this point. The terra cotta is allowed to dry between several days to several weeks, according to the size of the product. A glaze is then sprayed or brushed onto the material and placed in a kiln for firing.

Extruding terra cotta is possible by forcing the clay through a die as a continuous column and then cuting it with a wire into individual elements. The addition of a glaze or slip coat of water and clay is applied to the exterior surface of the molded element, and then the element is fired. Hand pressed terra cotta uses a wetter clay mixture than machine-pressed and more shrinkage should be expected. The West Coast developed a veneer in an attempt to make the terra cotta more earthquake-resistant. Machine-pressed ceramic veneer is made of flat forms that are no more than 1 ½ inches thick and a standard size of four square feet. The use of ceramic veneer for panels for curtain wall applications was also common. Several ceramic veneer elements are placed in a form, separated by rubber strips, and brushed with grout. Reinforced concrete is placed in the form and partially cured, then the panels are removed, and the concrete finishes curing. The panels are then placed onto the building façade structure.

Uses and Installation
Because of terra cotta’s fireproof quality, its popularity grew in the United States during the 1870s and 1880s. Also, its ability to be formed into countless shapes and ornamental designs were quickly recognized. One mold could produce countless terra cotta elements, saving time from the traditional hand-carving techniques that were once used with stone ornamentation. Terra cotta is also lighter weight and considerably less costly than stonework. In the 1880s, terra cotta was a traditional ornamental material, not usually utilized to clad entire buildings. A development of a skeletal system with extensive window openings made the use of cladding with terra cotta possible and popular. Terra cotta panels could be plain or ornamental and provided spandrels, mullions, beltcourses, and cornices.

The National Terra Cotta Society published the first standards for construction and installation using terra cotta in 1914. In the 1910s, the Society began a comprehensive research program with the National Bureau of Standards and created a report of their 1927 standards. With a rise in the use of stock terra cotta, the creation of businesses like the Midland Terra Cotta Company, formed in 1910 near Chicago. This company produced foliate ornaments to imitate the Louis Sullivan designs, which were purchased to enhance the façade of modest buildings. The newly developed skeleton-framed structures were clad in terra cotta panels foreshadowed the start of ceramic veneer. Earlier terra cotta facades were usually glazed in white, ivory, or cream and consisted of neoclassical motifs. These features were continued to recreate the stonework experience without the large price tag.

Soon after the Expositional Internationale des Arts Decoratifs and Industriels Modernes in Paris, Art Deco styled terra cotta appeared. Terra cotta could be seen on individual elements on small buildings to complete cladding of high-rises. Because Art Deco used a variety of colors, a need for technological improvements for a greater variety of colors in terra cotta soon arose. Also, a better understanding of ceramic science would lead to a wider variety of glazes and better quality.

Installing terra cotta usually involved a masonry backup and supports of steel lintels and angles set in a masonry wall. In the 19th century, bricks and mortar were used in the cavities of terra cotta to support the masonry wall behind it, but in the 20th century, metal anchors or wires were used more frequently. The metal anchors were held in the masonry backup with mortar and placed into preformed holes in the webs of the terra cotta. Projecting cornices was used with continuous rods being placed through the webs of adjacent units, which were supported by the J-shaped bolts attached to a metal framework above. Ceramic veneer components were installed in two distinct ways: each element was placed in a mortar bed and adhered to a masonry or concrete backup, or they were held in place with metal anchors that were attached to the masonry backup. During the Great Depression, the terra cotta industry was damaged. New construction was replacing terra cotta with ceramic veneers because the manufacturing was less labor intensive and less costly. Modern façade design no longer required terra cotta’s ornamental capabilities. Glass and metal panels in high-rise curtain wall construction shoved terra cotta panels into the backseat.

Conservation
The amount of repairs and preservation projects that are focused on terra cotta-clad structures has pushed the use of terra cotta back into the spotlight. The material is commonly used in new construction. If the material is properly manufactured, installed, and maintained, terra cotta and ceramic veneers are extremely durable materials. Several terra cotta buildings have survived for over one hundred years with little or no evidence of distress.

Deterioration
Water penetration, with all things related (i.e. wetting and drying or freezing and thawing cycles), are the main cause of failure in terra cotta. Deterioration of terra cotta is visible with signs of cracking, crazing, and loss of the surface glaze, spalling of the body of the terra cotta, and fracturing or displacement of terra cotta components. Manufacturing, instillation, and environmental conditions greatly affect the terra cotta’s performance. Most buildings that are clad by terra cotta are supported by steel structures and anchoring elements. If water were to penetrate the terra cotta, corrosion of the embedded metal and anchorage would ensue. The corrosion of this material will cause expansion to up to 10 times its original size, which will exert pressure and cause cracking, fracturing or the terra cotta, or displacement of adjacent terra cotta elements. Cracking and displacement is commonly caused be a lack of expansion joints in the terra cotta façade. Cracking may also be caused by compressive stresses caused by moisture and temperature related expansion. Horizontal expansion and vertical stresses can cause vertical cracking, which normally occurs in corners and returns in the wall. The glaze of the terra cotta can result in crazing or cracking of the glazed surface. Terra cotta nay suffer from distress in colder climates due to thawing and freezing cycles. If water enters into the terra cotta or masonry components; when the water freezes, it expands causing pressure to be exerted on the terra cotta (which causes cracking or spalling). Another issue commonly found in terra cotta is the growth of molds or algae. These growths can occur on the surface of the terra cotta and its mortar joints and caused by the material be exposed to excessive moisture over a period of time. These growths tend to hold water against the surface and can cause the glaze to deteriorate. Humidity can also encourage the growth of algae under the glaze, causing it to spall. Distress in terra cotta can also be caused by improper manufacturing. Badly manufactured terra cotta may be highly absorptive, vulnerable to water penetration, or may lose glaze through incompatibility of the glaze and the tile itself. Distress may also result from inappropriate repairs, like the application of sealant to the mortar joints, which traps water within the terra cotta. Water entrapment can also cause spalling of the glaze and body.

Evaluating the condition of terra cotta on building façades is usually aided by available historic sources including shop drawings, building construction documents, and records of repairs. Investigations will usually find that details found in situ are very similar to those illustrated in the 1914 and 1927 National Terra Cotta Society publications. The use of this research may limit the number of inspection openings needed to determine the causes of distress. Investigation techniques used in evaluating terra cotta façades can be nonintrusive methods. Using a borescope to evaluate concealed conditions, tapping the terra cotta with a wood or rubber mallet to identify delamination within the components, or using a metal detector to locate hidden metal anchorage to compare distress with the presence of metal elements are the most nonintrusive methods used to investigate the façade. More intrusive methods include removing terra cotta components or portions for direct examination and for laboratory sampling.

Laboratory testing in performed on the terra cotta and the ceramic veneer that have been removed from existing buildings or on new components manufactured for replacement. The purpose of this testing from existing buildings is to evaluate the material’s properties, causes of distress, and to gather information for use in finding replacements or making repairs. New elements may be tested for quality control purposes, which ensure that the new material meets the required specifications and maintain consistency in manufacturing. Testing both the terra cotta and the ceramic veneer may include petrographic examination or compressive strength testing. Laboratory tests are also performed to provide information about the composition of the terra cotta and glaze, the glaze’s adhesion properties, and quantitative data about characteristics of the particular terra cotta or ceramic veneer. Petrographic examination usually consists of a visual inspection of the material, followed by the use of a stereomicroscope to evaluate the composition and density of the body, characteristics of the glaze, and nature of deterioration. These studies can also suggest if the original manufacturing process is at fault for inappropriate firing temperatures. Compressive strength tests are conducted by placing small pieces of terra cotta in a machine that will apply compression until the piece fails. The terra cotta’s compressive strength can be calculated by determining the maximum load that the piece can withstand. Because terra cotta’s absorption properties are closely related to its durability, dry samples should be weighed before and after immersion in cold water for 24 hours and before and after immersion in boiling water for 5 hours. The absorption is calculated as a proportion of the saturated weight to the dry weight, this ratio of absorption is a relative measure of the material’s resistance to freeze-thaw damage. Other lab tests can be useful is determining the thermal coefficients and permeability of the terra cotta body and glaze, adhesion of the glaze, moisture expansion, and firing temperature. To test durability, samples can be aged by exposure in a controlled weathering chamber. This chamber accelerates weathering allowing for testing of various physical characteristics before and after aging. Test results, archival research, and field inspection are used to develop conservation and maintenance procedures.

Conservation Techniques
While cleaning improves the overall appearance of terra cotta, it also removes contaminants or inappropriate coatings and makes repairs more obvious. Abrasive methods for cleaning are unsuitable because they will damage the glaze and possibly the terra cotta. Water cleaning techniques may remove mild to moderate soiling on terra cotta but heavy soiling is best removed with chemicals. These chemicals should be an alkali prewash and a diluted acid afterwash, preferably with organic acids. Techniques like façade gommage and laser cleaning are new and have not been extensively tested on terra cotta in the United States. Terra cotta components that are extremely damaged may have to be replaced. Fractured pieces are capable of being fastened together with a stainless steel pin that is set in epoxy. Portions of fractured pieces can be repaired in a similar manner, only placing the pin through the face to anchor each portion to the backup. The pin should be countersunk and the pin location patch painted to match the terra cotta. Because this repair is obvious to the naked eye, it can become visually intrusive and should be avoided. Nonmoving cracks are pointed with mortar that is equally strong and as hard as the terra cotta body. Spalls can be replaced by a cementitious patch if the spall extends through the glaze and body. The cement mixture should then be covered with a breathable masonry coating that matches the glaze. New coatings are being provided that can match the colors and finishes of the existing terra cotta.

Replacement
If terra cotta components cannot be repaired or are missing, new pieces of terra cotta elements can be manufactured. It is important to take into consideration specific factors that can affect the strength and durability of the finished product. The characteristics of the old and new terra cotta or ceramic veneer can differ greatly with production and location. Successful replacement hinges greatly on carefully prepared specifications, manufacturing, and installation.