The National Park Service has begun the process of removing biofilm that has darkened the dome of the Thomas Jefferson Memorial using laser ablation. The successful test paves the way to ultimately clean the rest of the memorial.
Conducted over a six week period in the summer of 2017, the test treatment cleaned 1,000 square feet of the 10,682 square feet that comprise the dome of the Jefferson Memorial. The test leaves a pie-shaped wedge of clean, white marble The National Park Service tentatively plans to remove the rest of the biofilm concurrently with a project to rehabilitate the Jefferson Memorial roofs in 2018. That project is included in the requested FY18 budget for the National Park Service.
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.