This presentation is part of the Mid-Century Modern Structures: Materials and Preservation Symposium, April 14-16, 2015, St. Louis, Missouri.

Study of Stains and Markings on the Gateway Arch in St Louis
by Stephen J. Kelley, AIA, SE and Al O’Bright, architect, NPS COTR

The Gateway Arch is constructed with a 1/4 in. stainless steel exterior skin and steel interior skin and is quite possibly the most fascinating and important stainless steel structure ever constructed. Commemorating westward expansion and designed by Eero Saarinen, the Arch was constructed between 1963 and 1965 and has a basic shape of an inverted catenary curve with the Arch legs equilateral triangular in cross section. Both the height and the span are 630 ft.

In the past two decades streaks, stains and discolorations have become apparent on the monument that were suspected to be corrosion. Visitors and vandals have marked the base of the monument with scratched graffiti that had captured elements of corrosion.

The speakers have been involved with a comprehensive Arch staining study since 2006.The task was to determine the cause of the staining and develop cleaning and treatment strategies.

  • Comprehensive archival research was performed on the Arch and other stainless steel clad structures worldwide that predate or are contemporary with the Gateway Arch. It was found that other monuments have weathered well but that the Arch stands alone from other monuments in its extensive use of shop and field welds.
  • The exterior surfaces of the Arch intrados were visually inspected using telephoto photography, field microscopes, and a hand held XRF device. Higher areas were accessed using personnel lifts.
  • A series of inspection openings were created through the interior carbon steel skin in order to view the space between the two skins. Samples of carbon steel and concrete were removed from the legs of the Arch for testing in the laboratory.
  • Field and shop weld samples were removed from the exterior of the building for metallurgical analysis.
  • Long term monitoring instrumentation was installed within the legs of the Arch. It was found that there is a propensity for condensation on specific spring and fall days.
  • Cleaning mock ups were implemented on the north leg at the base of the monument.
  • Means of access to the upper reaches were studied extensively including scaffolds and cranes before settling on industrial rope access (IRA)
  • Access to upper reaches of the monument exterior were obtained using IRA to remove soiling samples, perform cleaning mockups, and inspect weld and skin integrity.


Speaker Bios

Stephen J. Kelley, FAIA, FAPT, FUSICOMOS, SE, is a registered architect and structural engineer in private practice who has devoted these two skills to the preservation of our built cultural heritage. With 35 years of experience, his projects range from small to immense, simple to sophisticated and cover a wide range of building materials and systems. He has developed and worked closely with state-of-the-art multidisciplinary teams that were designed to meet the demand of each project. Domestic projects of note include the Oklahoma, Nebraska, Kentucky and Illinois State Capitols; the Eisenhower Executive Office Building in Washington DC; and the Tribune Tower and Reliance Buildings in Chicago as well as the Gateway Arch in Saint Louis.

Al O’Bright has served as a historical architect with the National Park Service since 1981 through the Omaha Midwest Regional Office, and is duty stationed in St. Louis. He has been involved with many aspects of historic building preservation including documentation, condition surveys, planning, research, historic structures reports, construction management, and hands-on preservation projects. Projects include the preservation and stabilization of numerous Great Lakes and Great Plains structures within the thirteen-state Midwest region, and occasional special projects outside the borders of the Region and United States. He has performed as NPS project manager on the Arch corrosion investigation since project inception.

National Center for Preservation Technology and Training
645 University Parkway
Natchitoches, LA 71457

Email: ncptt[at]
Phone: (318) 356-7444
Fax: (318) 356-9119