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Historic Preservation - Technical Procedures

Spectitle:

Terra Cotta: Characteristics, Uses And Problems

Procedure code:

0421403S

Source:

Developed For Hspg (Nps - Sero)

Division:

Masonry

Section:

Terra Cotta Unit Masonry

Last Modified:

11/13/2012

Details:

Terra Cotta: Characteristics, Uses And Problems



TERRA COTTA:  CHARACTERISTICS, USES AND PROBLEMS


This standard 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.


INTRODUCTION

 

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 mould, by
    hollowing out portions of a solid, or by extruding it.

-    Usually low-fired

-    Typically a reddish, unglazed ceramic material.  It may also
    be a hard-fired glazed or unglazed ceramic material.

-    Durable (dependent upon the degree of firing)

-    Fireproof

-    Strong

-    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
    -  Inexpensive
    -  Lightweight
    -  Fireproof
    -  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
    -  Durable
    -  Impervious
    -  Easier to handle and more affordable than stone


TYPICAL USES

Typical historical uses for terra cotta included:

-    Sculpture

-    Unglazed units used for structural purposes

-    Glazed units for building exteriors

Typical current uses for terra cotta include:

-    Cladding

-    Used in both commercial and residential applications

-    Rainscreen cladding with waterproof membrane behind


PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) 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

1.   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.

2.   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".  

3.   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.


HUMAN-INDUCED PROBLEMS

1.   Installation of broken or damaged units:  Will result in an
    increased rate of decay.

2.   Use of too coarsely-graded mortar:  Stress points may be
    distributed unevenly.

3.   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.

4.   Metal anchors or supports are unprotected:  They are more
    susceptible to corrosion, causing them to fail and damage the
    unit.

5.   Inadequate number of anchors used:  Can place additional
    stress on the anchors and the unit.

6.   Insufficient quality of anchors used:  May result in early
    failure of the anchors and the unit.

7.   Inadequate provisions for movement between units such as shelf
    angles and flexible joints.

8.   Coating exterior terra cotta walls with an impermeable material will seal masonry joints
    that are the natural way for moisture to escape.

9.   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. 

                         END OF SECTION
 


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