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When Polished Isn’t Hard: Randal Thurston
When Randal Thurston designed the public artwork for the pedestrian underpass at Yerxa Road in North Cambridge, he wanted an off-white ceramic tile as the background for the black silhouettes of butterflies in the tunnel. His concept also called for painted steel cutouts of birds, branches, and other natural elements for the ramp walls and portals.
The construction contractor provided two tile options for the tunnel wall for Randal to consider. One of the sample tiles had a very glossy surface, the other a matte surface.During the conservation pre-fabrication review, the CAC conservator tested both tiles for washability and graffiti removal. In the test, a variety of materials (motor oil, shoe polish, magic marker, and spray paint) were applied to each surface and dried with a warm air gun. The tiles were then cleaned, using the necessary detergents, solvents, or cleaning agents.
The result: the glossy tile absorbed stains and could not be satisfactorily cleaned, but the applied materials on the matte tile were relatively easy to remove.
Why did this happen?
The shiny surface on the glossy tile was achieved not from a fired glaze but from a surface polish. When a glass or glazed surface is machine polished, the grinding action and grit from polishing compounds open minute holes in the surface, making the surface porous. This micro-porosity allowed the applied paint and staining materials to penetrate deep into the surface. The matte tile did not absorb the paint and staining materials, and could easily be cleaned. Randal chose the matte tile as his substrate.
The best laid plans…
Sometimes even when good materials are specified for public art, things can go wrong. The installation of Randal Thurston’s powder coated steel birds and branches on the walls and portals of the Yerxa Road Underpass Project was completed in the fall of 2006. Powder coating is the process of electrostatically covering a metal surface with powdered paint materials in a resin followed by heat curing, usually resulting in a firm and durable surface.
By early February 2007 the powder coating on the steel was discovered to have failed, allowing rusting of the steel substrate. The CAC conservator was asked to investigate the problem.
We’re not sure yet exactly what happened, but visual inspection suggests there was a technical problem possibly from incomplete surface preparation or lack of a pre-treatment zinc conversion coating.
A collaborative solution
The artist, the fabricator, and the conservator are working together to quickly correct the problem. The painted steel bird and portals elements have already been removed from the site and will be re-painted using an aliphatic urethane paint recommended by the conservator.
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