AIC's 45th Annual Meeting

Cleaning large-scale architectural elements and sculpture with historic patinas or other decorative surfaces is challenging if these finishes are to be retained while working within the constraints of outdoor sites, large artifact size and tight budgets. Traditional conservation treatments to remove failed coatings, corrosion and other accretions from these materials include the use of solvents, high pressure powerwashing and microabrasion. These have the disadvantages of hazards to health and the environment, and the potential for surface abrasion; it may not be feasible to scale up minimally abrasive chemical or mechanical techniques to use on large architectural artifacts.

Dry ice blasting, a technology that originated in the industrial sector, is becoming more widespread for cleaning applications. It has been shown to facilitate cleaning without the surface abrasion and we have used it successfully in three situations where it was important to retain the decorative finish of architectural surfaces. These were: the aluminum door surround of the first American Airways hangar; aluminum surfaces of Cincinnati's Union Terminal; and the bronze fountain at the National Gallery of Art in Washington, DC.

The Andrew Mellon Memorial Fountain was designed by Sidney Waugh and dedicated in 1952. It comprises three nested bronze basins nested with zodiac sign decorations. After washing, tenacious remaining mineral deposits were cleaned using CO2 blasting, selected because it does not require the containment that microabrasion or laser cleaning do, and was found to be effective at minimizing overcleaning. A combination of adjustments in pressures, feed rates and nozzle sizes was tested during the work to remove the mineral deposits without removing the underlying patina in most cases. Some areas of thick deposits, or those that were tenaciously adhered required higher pressure and more blasting time, which did result in removal of some patina.

American Airways constructed their first hangar at Meacham Field, Fort Worth, in 1933. The Art Deco main entrance is flanked by fluted aluminum columns with an aluminum spandrel panel decorated with the AA logo. Previously, an awning above the doors had been removed, leaving soiling and sealant remnants; previous removal had resulted in gouges in the aluminum. We used dry ice blasting to gently clean away old sealant and soiling. Working on an active airfield, it was important to use a method that did not require containment and that retained the historic dark grey patina. Dry ice blasting was especially effective at removing the silicone sealant without damaging the surrounding patina.

Union Terminal was also built in the Art Deco style and uses aluminum extensively for decorative features throughout. Different alloys and treatments were used to produce particular colors or textures. Deplating was an early process that produced a stabile dark gray finish; it is no longer used, since replaced with anodized finishes. We required a cleaning method that would retain this historic finish. We found that dry ice blasting was a gentle technique for this and did not have the containment issues or risk of driving water into the building that other methods do.