The beauty of terracotta, with its rich palette of earth tones, has been widely used by architects and designers to enliven building facades 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.
Terracotta has two components: the interior bisque, and the outer glaze. The hollow bisque is composed of weathered or aged clay which is mixed with sand or pulverized fired clay called “grog” and then glazed and fired in molds at a high temperature. The outer glaze is an aqueous solution of minerals which are brushed or sprayed onto the units. This glaze provides a protective, impervious surface which possesses excellent weathering properties. The glaze can also be colored or textured to provide architectural value. Terracotta was typically “hung” on buildings with carbon steel and wire anchors and then mortared into place.
Terracotta can very difficult to identify which is why it is often mistaken for other materials such as concrete, stone, or granite. Terracotta was once very attractive because of the fact that it could be finished to imitate stone. Another unique quality of terracotta is that it has been used in two separate and very different types of building systems: a traditional load-bearing masonry wall in buildings of modest height and as a cladding material in High Rise construction.
Common Deterioration Problems
Deterioration in terracotta can be put into two categories: stress related, and moisture related. Below is a list of typical types of deterioration from each:
Stress Related Deterioration
-Faulty original craftsmanship
-Introduction of new loads to the unit
-Damage caused by improper repairs or alteration
Moisture Related Deterioration
-Crazing (fine cracks in the glaze)
-Glaze or Bisque Spalling
-Deterioration of Mortar
-Corrosion of Metal Anchoring
It is important to understand the causes of deterioration because no two buildings are the same. The most common cause of deterioration in terra-cotta is usually moisture-related, which can cause a domino effect of damage because once moisture enters the bisque it can be trapped behind the glaze causing further issues as well as causing corrosion of its anchoring system which in turn can apply stress to the terra cotta unit.
Inspecting terra cotta for deterioration is difficult due to the fact that deterioration is insidious, with the outside of the unit not always indicating the more serious problem located within (such as deteriorated anchors). It is critical to understand the composition and behavior of terra cotta, therefore, inspections should always be performed by an expert in terra cotta restoration. Early signs of deterioration can be easy to spot such as: glaze delamination, cracks, chips, displaced pieces, or rust stains. If you see these items, do not turn a blind-eye. Sub-surface exploration should always be performed to better understand the causes of the observed damage.
Maintenance, Repair, & Replacement
Repairs to terracotta should only be performed by a restoration expert with a proven track record in repairing terracotta. It is very important that the repair specialist understands what type of repair materials should be used and how to properly install them.
Terracotta should be gently cleaned to expose true surface conditions, colors and details. Deteriorated mortar joints should be properly repointed because the mortar joints are the first line of defense against moisture intrusion between the units as well as a way for moisture vapor to escape out of the wall system. The compressive strength of replacement mortar should be equal to or less than that of the original mortar and should be compatible with the terracotta.