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GUIDE TO THE REPAIR OF ARCHITECTURAL TERRA COTTA
Once considered unreliable and temporary in nature, repair systems for terra cotta have become a
mainstream component in Historic Restoration work. This evolution can be attributed to the development of effective, high quality specialty materials designed specifically for long-term
compatibility with terra cotta substrates.
With a record of more than 25 years of successful performance on thousands of architectural Terra
Cotta Restoration projects, Edison Coatings offers the most complete, time-proven, compatible terra cotta restoration systems available today. Edison's custom color and formulation capabilities provide
the highest levels of aesthetic match and finish as well.
A completely integrated system, Edison's products include the following specialty materials designed specifically for use in terra cotta restoration:
The following is a brief guide to terra cotta repair system options and preliminary selection criteria. It should be noted that proper investigation and
correction of the causes underlying the observed deterioration is prerequisite to undertaking repairs, and repair plans and sequencing must include
consideration of the significant quantities of liquid moisture often found within compromised terra cotta building wall systems.
STEP 1: DEEP REPAIR
After proper surface preparation,
repairs to spalls greater than 1/8" (3 mm) in depth are best achieved using Custom System 45 TC grade. A two component cementitious system, Custom System 45
provides higher bond strength, lower shrinkage and more efficient stress relief than competitive systems. This allows installation of
large and deep repairs without cracking, special curing regimens or distress to historic substrates. Low coefficient of thermal expansion (<4 x 10-6 in/in/0F) assures long term thermal compatibility
with fired clay substrates, even in areas subject to rapid, wide swings in ambient temperatures.
Figure 1: Replacement finial (right) was cast with custom color-matched Custom System 45 using Restoration Latex RL-2.
The standard liquid component is Restoration Latex RL-1
which allows non-sag application for vertical and overhanging repairs. In cases involving very large and deep areas of loss, Restoration
Latex RL-2 (Superplasticized) may be used with Custom System 45 TC
to form and pour repairs in place or to cast replacement elements. Restoration Latex RL-3 (Marine Grade) may be used for repairs subject to high constant moisture exposure, such as fountains and planters. In cases
requiring exceptional levels of moisture and vapor permeability, Restoration Latex RL-4 (Air
Entraining) may be used. For hot weather work at temperatures up to 1200F, Restoration Latex RL-5 may be used to extend working times.
General Use: Vertical, Horizontal and Overhanging Repairs
Castings and Poured Repairs
Fountains, Planters, High Constant Moisture
Retaining walls, Ruined Masonry, for Highest Permeability Repairs
For Repairs under Hot Weather Conditions up to 1200F (500C)
For Repairs under cool, marginal weather conditions down to 400F (40C)
For deep repairs up to 3 inches in a single lift; allows up to 24 hours carving time
STEP 2: THIN SECTION REPAIR
It is common for terra cotta subjected to bulk moisture infiltration to develop thin glaze spalls, in which a section of terra cotta glaze and bisque surface
delaminate from an otherwise sound terra cotta element. Typically, section thickness is no more than 1/16" (1.5 mm).
After repairing the sources of leakage, unobtrusive repairs in depths up to ¼" (6 mm) are achieved by using Thin-Fill 55 Reprofiling Mortar. There is no minimum depth and the mortar is designed for easy sanding and/or polishing to achieve perfectly smooth surfaces, when required to replicate existing
glazed terra cotta profiles. The product is also easily filed after initial set to produce fluted profiles, when required to match existing terra cotta.
Figure 2: Thin-Section glaze spalls are common in terra cotta subjected to bulk moisture infiltration.
A cementitious mortar with low coefficient of thermal expansion, high bond strength, low Modulus for efficient stress relief and positive moisture and moisture vapor permeability, Thin Fill 55 facilitates achievement of excellent aesthetic finishes for repairs to architectural terra cotta. It is fully compatible with Custom System 45, and may be applied over deeper patches to achieve special surface finishes when required.
Thin Fill 55 is frequently matched to the color of the existing terra cotta glaze, to facilitate subsequent glaze replication.
STEP 3: CRACK REPAIR
A variety of potential causes of cracking in terra cotta mandates that a variety of repair alternatives be made available. Ideally, crack repair details and material selections are specified
by a design professional experienced in the specific properties, assembly details and deterioration mechanisms of terra cotta. Crack width monitoring can assist in determining the
movement capacity required, if any, for the repair system.
The following systems are used in the repair of cracks in terra cotta:
Custom System 45
is often used to repair cracks determined to be stationery, or non-working. The crack is typically grooved out to a 1/4" width x ½" depth, and is then filled with a matching Custom 45 TC repair mortar.
a 2-component flexibilizedacrylate-epoxy paste filler allows less invasive repairs with higher tensile strength and elongation, for
cracks exhibiting some very limited movement. A Dremel tool may be used to slightly open the face of the crack to a 1/16" width and ¼"depth, and the narrow opening is then filled with the color-matched Flexi-Fill 530 paste grade filler. At initial cure stage (typically 30 – 90 minutes) the product
cures to a soft rubber which is easily trimmed with a razor or utility knife.
cement and lime-based injection mortars are also used in certain repair situations. Although injection of cracks in hollow masonry
units is impractical when using epoxy injection resins, cement and lime-based compositions are more compatible and can be used to fill voids
and cracks without distress to the
terra cotta. A variety of grades is available, including Pump-X53 Masonry Grout for filling of
large voids and cracks, Pump-X53i Microinjection Grout for fine cracks down to 1/16" (1.5 mm), Pump-X53iE Expanding Microinjection Grout for filling of cracks where slight expansion of the grout is desired (+2%) and Pump-X53iL Hydraulic Lime Injection Grout where softer, self-healing performance is desired.
Figure 3 (left): Pump-X53 was used to fill voids between terra cotta band courses and rubble masonry backup on this project in New York City.
Figure 4 (right): Injection points are marked on an interior wall surface; Pump-X53i was injected through
the back-up wall into the masonry exterior veneer to fill cracks and voids in the brickwork.
STEP 4: REBONDING MASONRY UNITS
Cleanly fractured terra cotta elements can sometimes be salvaged and repaired rather than replaced. The use of Flexi-Weld 520T
masonry adhesive permits cleanly broken pieces to be quickly and cleanly rebonded, without creating a gap between the broken surfaces.
The two component, 100% solids acrylate-epoxy adhesive can be used under a wide range of temperatures to quickly grab and re-bond all types of broken masonry elements.
Figure 5: Stone column base, before rebonding with Flexi-Weld 520T.
Figure 6 (above): Stone column base, after rebonding with Flexi-Weld 520T.
STEP 5: GLAZE REPLICATION
Once the underlying problems have been addressed, the appropriate repairs have been made and the correct surface profiles restored, specialty
coatings are used to replicate the color, reflectance and density of the original glaze. In monochromatic terra cotta restoration, the task of matching the
original glaze is much simpler than for polychrome applications. Two systems may be used:
Aquathane UA210 Type NCL
is used to achieve the finest finishes, as it allows tight control of gloss level (from Flat to High Gloss) and translucence (from Opaque to Clear).
It is a highly durable waterborne aliphatic polyurethane capable of withstanding decades of weathering with minimal effect. Clear
coats may also be used to provide higher gloss and depth of finish. It is generally combined with Type G Bonding Additive
to assure tenacious adhesion to smooth, non-porous existing glaze surfaces.
Figure 7: More than 30 colors of Aquathane and Elastowall coatings were custom-matched to the
polychrome terra cotta at this elaborate oceanfront museum building in Florida.
Elastowall 351 is an internally plasticized pure acrylic coating which has been used for decades to
both provide an aesthetic matte finish and to aid in exclusion of water from terra cotta exhibiting
small working cracks. It is favored by some Conservators for its high rate of moisture vapor transmission, its ability to bond to less aggressively prepared
surfaces and to a wide variety of existing materials, and its ability to be removed/reversed if so desired at some later date. Luster can be imparted to Elastowall 351 coatings by applying a clear top coat of Aquathane UA210 Type E.
Both products are available in over 900 standard colors, which can be prepared in-house by Edison
Dealers who participate in the Edison Coatings Tint Base program. Custom color matching service is also available from Edison Coatings, Inc.
Figure 8: Terra Cotta gargoyle, before and after repair with Custom System 45 and coating with Elastowall 351.
STEP 6: GLAZE DETAIL REPLICATION
A particular challenge in terra cotta restoration work is the replication of special details frequently found in original glazes. These may include speckles,
smears, mottling or even multiple layers of glaze of different colors.
These special finishes can often be recreated using multiple applications of Aquathane UA210 Type E,employing various faux finishing techniques.
In addition, AquaSpex 220 may be used to provide speckles of specific size, concentration and color. AquaSpex 220 incorporates color-matched flakes in a clear binder, permitting close control
of speckle color, density and size.
Figure 9: Terra Cotta fragment sits atop a precast concrete panel which has been base-coated with light
greyAquathane UA210 NCL and top-coated with AquaSpex 220, incorporating 1500 micron Charcoal Grey flakes. The combination provides a final finish which closely matches the original material.