Technical Procedures Disclaimer
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This document includes general information on the characteristics and common uses of terra cotta and identifies typical problems associated with the material along with common causes of its deterioration.
Reference: National Park Service Preservation Brief 7 “The Preservation of Historic Glazed Architectural Terra-Cotta”
Characteristics of Terra cotta:
- Fired clay
- Typically hollow, formed by pressing clay into a mold, by hollowing out portions of a solid, or by extruding it
- Usually low-fired
- Typically a reddish, unglazed ceramic material that may also be a hard-fired glazed or unglazed ceramic material
- Durable (dependent upon the degree of firing)
- Can be molded into virtually any shape
- Available in a variety of designs, colors and finishes
Types of Terra cotta:
- Brownstone terra cotta:
- Hollow cast block - typically dark red or brown
- Often used to imitate sandstone, brick or actual brownstone
- Popular during the mid- to late 19th century
- Fireproof construction terra cotta:
- Not widely used today
- Used to span areas between metal I-beams in wall, floor and ceiling construction
- Ceramic veneer terra cotta:
- Extensively used today
- Not hollow cast, but ribbed on the back for attaching to metal ties anchored into building
- Glazed architectural terra cotta:
- Hollow units cast in molds
- Easier to handle and more affordable than stone
Typical historical uses for terra cotta included:
- Unglazed units used for structural purposes
- Glazed units for building exteriors
Typical current uses for terra cotta include:
- Used in both commercial and residential applications
- Rainscreen cladding with waterproof membrane behind
Problems and Deterioration
Problems may be classified into two broad categories:
- Natural or inherent problems based on the characteristics of the material and the conditions of the exposure
- Vandalism and human-induced problems
Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.
Natural and Inherent Problems
- Crazing: Hairline cracking of the glaze surface. Crazing is normal and typically not a problem unless the crazing goes through the glaze and into the clay body.
- Spalling: The breaking off or peeling away of the outer surface or layers of the clay unit. It is typically caused by the build-up of pressure from moisture trapped under the surface and subjected to cycles of freezing and thawing. This pressure can cause small pieces of the terra cotta glaze or body to "pop-off".
- Fracturing: Often caused by the corrosion of the iron anchoring system. Upon exposure to moisture, the anchors. The expansion of the iron causes the terra cotta to fracture.
- Installation of broken or damaged units: Will result in an increased rate of decay
- Use of too coarsely-graded mortar: Stress points may be distributed unevenly
- Lack of flashing, weep holes and drips designed into system Historically, terra cotta was believed to be waterproof, and therefore, did not require these water-shedding devices
- Metal anchors or supports are unprotected: They are more susceptible to corrosion, causing them to fail and damage the unit
- Inadequate number of anchors used: Can place additional stress on the anchors and the unit
- Insufficient quality of anchors used: May result in early failure of the anchors and the unit
- Inadequate provisions for movement between units such as shelf angles and flexible joints
- Coating exterior terra cotta walls with an impermeable material will seal masonry joints that are the natural way for moisture to escape
- Using joint sealants instead of mortar for repointing will seal the natural way for moisture to move out of the wall resulting in damage to the terra cotta block