Terra Cotta
Overview
The word terra-cotta literally means “cooked earth”. In the broadest sense, terra cotta is a high grade of weathered or aged clay. When mixed with sand or pulverized fired clay it could be fired at high temperatures to produce a product of greater hardness and compactness than was possible with brick or similar masonry units. Simply put, terra cotta is an enriched clay material.
Because of its popularity between the late 19th century and the early 1930’s, glazed architectural terra cotta is the most prevalent masonry building material found in the urban environment today. Terra Cotta was typically a hollow-cast block which was open at the back – like a box. The interior included stiffeners, or webbing, that greatly increased the load=bearing capacity of the unit without greatly increasing its weight. Terra Cotta blocks were often finished with a glaze – a clay wash or an aqueous solution – brushed onto the air-dried block before it was fired. Glazing made it possible to change the color, imitate different finishes, and gave excellent weathering properties to the final product. Glazing provided an unlimited range of fade-resistant and durable colors which are difficult to match with the building materials used today.
Terra Cotta Restoration
Factors of Consideration
CAUSE OF DETERIORATION – Water is the principle cause of damage in glazed architectural terra cotta. Moisture finds a way into the interior of the material through the air as well as from other deficiencies in the handling of water in the whole system. These other deficiencies could be from inadequate methods in the original construction, the deterioration of the metal anchoring system, deterioration of the mortar joints and other adjacent materials, stress-related deterioration, inappropriate repairs, or alteration damage. While there are many different causes of deterioration and the damage that results, the problem created by them is an increase of moisture infiltration into the terra cotta unit. The excessive build-up of moisture is what causes the damage in the first place. It is vitally important to address and resolve the cause of the damage before restoration of the terra cotta itself can be performed.
DAMAGE TYPE – The two main types of damage caused by excessive moisture infiltration is crazing – the formation of small, random cracks in the glazing of the terra cotta, and spalling – the partial loss of the terra cotta material itself.
COLOR & DETAILS – Once we have identified and corrected the issue causing the deterioration of the terra cotta we then carefully match the color of our materials to the existing terra cotta. In instances where the damage has occurred on an intricate, detailed, or ornamental terra cotta component we are able to carefully recreate that detail in our restoration component.
Material Application
When restoring the glazing of the terra cotta we apply a patch material which has been color matched to the base color of the original terra cotta. After this material has cured we sand it to match the profile and texture of the original terra cotta. This is followed by sponging on a custom paint that matches the color and pattern characteristics of the original glazing. Finally, a clear coat is applied to the restored area to seal it and mimic the original surface and weathering properties of the original glazing.
In cases where spalling has occurred and a portion of the terra cotta masonry itself is missing we use a more substantial patch material. This allows us to build up the material in a series of lifts until it matches the original profile. Once the missing area has been filled we finish it with glazing patch material, color matched, and sealed.
Surface Preparation
Before we apply the terra cotta patch material we thoroughly clean the area to remove all build-up of dirt or organic material, and any loose material resulting from damage. We use water with a mild detergent and scrub the area with a natural bristle brush to remove surface dirt. If necessary, a mallet and chisel are carefully used to remove any glazing or terra cotta material that is not still firmly attached.