Last week, the National Park Service announced that they had successfully cleaned with laser ablation a 1,000 square foot section of the Jefferson Memorial’s famous white dome, which had become overrun with what they call biofilm and what most southerners would probably diagnose as “mildew.” Further, NPS thinks the experiment went so well that they will use it to finish the larger cleaning project, comprising 10,682 square feet, next year.
As the MDAH Historic Preservation Division observed on Facebook, “This could be a game changer in cleaning historic masonry” (and can we dare hope it will knock sandblasting and bleach out of the ball game entirely?)
The blackening effect of biofilm, a colony of microscopic organisms that adheres to stone surfaces, was first noticeable in discrete areas of the memorial’s white marble in 2006, and has become more pronounced in recent years. A multi-disciplinary team of conservators, architects and other professionals has been studying the growth on the Jefferson Memorial since 2014 to determine the best treatment options.
Since announcing the search for a successful cleaning method in August 2016, the National Park Service reviewed hundreds of potential products and processes, considering not only their effectiveness on the biofilm, but also how they might impact the memorial’s historic stone and the surrounding natural environment.
Laser ablation offers maximum protection to both the memorial’s cultural and natural resources: by fine-tuning the laser settings to the specific stone and soiling types, the specially trained laser operators can remove the biofilm without damaging the historic marble of the memorial. The use of laser is also an environmentally sound procedure and eliminates the need to use more aggressive chemicals or abrasive cleaning methods.
The laser ablation test was conducted by Conservation of Sculpture and Objects Studio, Inc., a Chicago-based conservation firm.