Astro Pak’s Derouging Process Shows Promise for St. Louis’ Gateway Arch

St. Louis’ Gateway Arch

Astro Pak dry cleaning proved successful in removing surface corrosion and embedded metal oxides in the graffiti. The chemical techniques they used created the appearance of the test patches that were closest to the control patch.

Since its completion in 1965, St. Louis’ Gateway Arch has been a shining icon of the Midwest, commemorating the expansion of the United States westward and the area’s role as a commercial hub to the west and south along the Mississippi. However, its luster has diminished somewhat over the past half century due to air pollution, the attention of millions of tourists, and even the methods used in its construction. On the upper levels of the monument, the stainless steel siding shows signs of weathering and discoloration, including marks on the surface left by the machines and crews who built it. Down at the base, de-icing salt, visitor hands and even graffiti have also left their mark.

A number of researches have been conducted since 2005 to remove the worst effects of the passage of time and to ensure that the arc will continue to glow for many decades to come. In 2018, the Association for Preservation Technology (APT), an interdisciplinary organization committed to promoting best practices and technologies for the preservation and preservation of historical structures, hosted an expert workshop at the monument. Three techniques, using a completely different technology, were tested on stainless steel plates identical to those on the surface of the arch. All three teams were challenged with the same requirements where their proposed methods:

  • Remove visible damage and discoloration from the surface
  • Maintain the integrity of the designs, materials, workmanship and aesthetics
  • Be gentle and appropriate for the surface and conditions
  • Avoid damage or discoloration
  • Be reversible or post-treatable so as not to limit future cleaning options

The three methods tested were: pressurized water, steam and dry ice, laser cleaning and Astro Pak’s combination of NeutraRouge derouging solution, derouging gel and manual polishing.

The pressurized water, steam, and dry ice process was identified as a possible solution for removing surface debris from the top surface, but failed to remove discoloration and surface corrosion. In addition, neither the embedded iron caused by graffiti nor the resulting corrosion was removed.

The laser cleaning process was found to be better at removing surface debris and removing some of the surface corrosion, but was unable to remove embedded metal oxides. However, the laser left visible streaks on the surface. Overall, the conclusion was that as the technology advances, it may become more profitable in the future.

Astro Pak’s method was considered the most successful in achieving the stated goals, including removing the stains from the de-icing salt and returning the surface of the panels to almost the original panel. The official Conservation Management Plan published by the Study Committee quoted: “Astro Pak’s dry cleaning has been shown to be effective in removing surface corrosion and embedded metal oxides in the graffiti. The chemical techniques they used created the appearance of the test fields closest to the control panel. “The study also examined the gloss aspect of the panels processed and identified this parameter as“ most frequently used when the optical surface quality of a product has to be assessed ”. The study found that the panel that treated Astro Pak with NeutraRouge, AP410 and a hand buff was the closest thing to the ‘much lower standard deviation’ control. “

The Astro Pak solution achieved the highest score for effectiveness, smoothness and economy. The association continues its studies to consider the natural aging process as a best practice to preserve not only the monument itself but also the integrity of the symbolism. The lessons learned are likely to be applicable not only to the Gateway Arch, but also to other historic buildings and landmarks with stainless steel exteriors as part of their conservation process.

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