Making Seamless Repairs to Terra Cotta, Part 1: Restoring in Situ
In this two-part series, I look at a pair of scenarios in which contractors often presume repair is impossible and replacement with cast stone is the only option. Part 1 addresses instances when terra cotta units can be repaired “in situ” rather than removed.
Despite significant damage, this terra cotta panel could be restored in-place.
With its hollow cells and intricate details, terra cotta can be difficult to restore. Adding to the complexity are all the challenges that typically accompany terra cotta restoration. These include incomplete waterproofing, improper installation, poor maintenance, interior structural problems, and corroding steel.
Commonly used restoration techniques and materials can compromise terra cotta features and leave obvious evidence of a repair.
It might seem that replacing an entire unit with cast stone is the only alternative—but it isn’t. With the right materials and tools, it is possible to achieve an almost invisible repair—whether the piece is restored in place (in situ) or removed and restored and then replaced.
Cathedral Stone Products’ pneumatic air chisel, standard and custom pigments, and terra cotta glaze help restoration professionals achieve a generational repair with an authentic finish.
For example, a 19th-century building on Manhattan’s West Side was adorned with intricate terra cotta accents. But the glaze was chipping away and spalling off due to poor waterproofing and a failed weeping system.
Unless the repair addressed these issues, they would persist, shortening the life of the repair and harming the building. The contractor was considering replacing the terra cotta with cast stone. But replacements take time and can be heavy enough to destroy other components. They can also be overkill.
We explained that an effective, long-lasting restoration should start with a sound surface for repairs followed by re-coating of the terra cotta units.
To get to a sound surface, they used a grinder with a diamond cup wheel. This significantly minimized the chipping of the original glaze.
Next, areas of the clay bisque were prepared for M100 patching. This was done with both the grinder and Cathedral Stone Products’ proprietary one-inch Air Hammer, designed solely for the detailed work that is so critical to effective restoration of sensitive historical masonry.
Then, they applied two coats of our Potassium Silicate Coating to all the terra cotta units, creating a molecular bond with the substrate. The final coating was our Terra Cotta Glaze Coating, which provides a durable, non-yellowing semigloss finish without significantly affecting the substrate’s vapor transmission qualities.
The finished unit.
Once the coatings were applied and cured, they repointed with a substrate-specific Pointing Mortar that was compatible (in compression strength, flexural strength, freeze/thaw reactions, and breathability characteristics) with the old terra cotta.
Repairing these terra cotta units in situ achieved three key benefits:
Performance. Formulated specifically for historic preservation, these coatings achieve a generational repair that replicates the purpose of the original glaze, which is to repel water and allow necessary vapor to escape.
Appearance. The use of our pneumatic air chisel allows for the removal of small pieces rather than large sections, preserving the strength of the building and making repairs less obvious. And the Terra Cotta Glaze Coating replicates the look of the original terra cotta.
Structural integrity. Repairing in place puts no additional stress on the surrounding structure.
When the job was complete, both the contractor and the owner were pleased to see that the building’s repaired terra cotta features were indistinguishable from the originals.
In Part 2 of this series, I discuss a second effective method for repairing a terra cotta unit rather than replacing it with cast stone.
