Michael Belman, Abigail Mack, and Shelley Sturman
Alexander Calder’s last and largest mobile, Untitled (1976) has gently rotated in the air currents of the atrium of the East Building at the National Gallery of Art since it’s opening in 1977. At Calder’s request, artist and engineer Paul Matisse fabricated the mobile out of then cutting-edge materials to be both lightweight and durable. Over the course of the mobile’s 29 year history, two separate attempts were made, through the application of thermal spray hardfacing coatings, to combat the persistent divot shaped depressions that formed between the contact points of the aluminum hooks and loops. The mobile was exhibited with its original molybdenum coating for 11 years. In 1989 a new two layer coating of nickel-molybdenum-aluminum plus a titanium dioxide top-coating was applied that lasted another 15 years until 2004, when further treatment was required.
The depressions that formed between 1989 and 2004 were much deeper on some of the aluminum hooks than those that formed during the first decade, possibly because a longer period of time elapsed between treatments, increased movement from air currents in the atrium as the air handling systems were updated, and oil that leaked onto the mobile from its hanging assembly on the roof. Nearly all of the aluminum hooks and loops exhibited depressions with the degree of damage being proportional to the lessening load at each junction. A few of the aluminum hooks that were connected to steel elements were subject to preferential wear.
Through consultation with experts in the field of friction science and the use of an ASTM test for thermal spray wear resistance, a better understanding was obtained of the wear mechanisms occurring on the mobile and a treatment plan was devised using newly developed hardfacing materials. Treatment of the sculpture involved aluminum TiG weld-filling of the worn depressions, adding steel liners to four sensitive aluminum and steel junctions, complete replacement in steel of one structurally compromised aluminum element, and application of a tougher hardfacing coating: tungsten carbide with cobalt applied by High Velocity Oxygen Fuel to select hook and loop contact points. The design and execution of the treatment was performed through close consultation with Paul Matisse. After repainting using the same manufacturer’s paint as originally applied, the mobile was returned to its soaring state.