4_Record_of_Treatment_Report_111717.pdf

Contract P17PC00226 / NAMA 241684A THJE

Selective Laser Cleaning (Biofilm) Thomas Jefferson Memorial Record of Treatment

Prepared by Justine P. Bello, Architectural Conservator National Park Service – National Mall and Memorial Parks (NPS-NAMA)

DRAFT – November 17, 2017

Image credit: National Park Service by Matthew John

Contract P17PC00226 / NAMA Record of Treatment Selective Laser Cleaning (Biofilm) Thomas Jefferson Memorial DRAFT November 17, 2017

INTRODUCTION

In October 2017, a project was completed to perform test cleaning of the dome roof of the Thomas Jefferson Memorial through laser ablation. Laser ablation successfully cleaned approximately 1000 square feet of the marble dome surface, nearly eliminating any visible sign of pigmented biofilm from the treated areas. The success of this project will help to inform the cleaning protocol for the remainder of this Memorial.

DESCRIPTION

The Thomas Jefferson Memorial is one of the Nation’s most recognizable cultural resources in the National Park system. The Memorial was individually listed in the National Register of Historic Places in 1981 and is also listed as a contributing building in the East and West Potomac Parks National Historic District, 1999. The Thomas Jefferson Memorial is a circular, open air structure with an attached portico, constructed between 1939 and 1943. The main structure is a circular colonnade which features a shallow dome supported by 26 Ionic columns. The portico is supported by an additional 12 columns. The portico supports a triangular pediment. The pitched portico roof connects to the flat colonnade roof system which transitions to an additional flat roof system a few feet above the colonnade.

Finally this upper flat section transitions to the dome in the center. The memorial is constructed of white Imperial Danby marble from Vermont. The interior is constructed of white Georgia Marble with an axed finish; the floor is pink Tennessee marble. The interior frieze is topped by a cornice and a coffered Indiana limestone dome.

PROJECT BACKGROUND

A biofilm is a complex community of microorganisms—bacteria, fungi, and algae—that have soiled the dome of the Thomas Jefferson Memorial. The blackening effect of this biofilm was first noticeable in discrete areas of the Memorial’s white marble in 2006, but became considerably more pronounced only 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.

It was determined that laser ablation would offer 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. Rigorous safety controls would be maintained for the duration of the cleaning process to protect the public and the operators

The intent of the project was to significantly reduce the visually disfiguring biofilm on the Memorial dome, with priority given to the north-facing quadrant. It was not the expectation of the NPS that the entire dome could be cleaned as part of this scope of work; rather, the expectation was that the Contractor should develop and implement a safe and efficient approach to the Work that achieved the cleaning of a select area, prescribed as 1000 square feet (SF).

The solicitation was posted in March, 2017. In June, the project was awarded to Conservation of Sculpture and Objects Studio, LLC (CSOS), a conservation firm based in Forest Park, Illinois, near Chicago.

Selective Laser Cleaning (Biofilm) Thomas Jefferson Memorial DRAFT November 17, 2017

LASER SPECIFICATIONS and EQUIPMENT

The original Statement of Work (SOW) specified that cleaning should be performed through laser ablation. There are various, different lasers currently used in the conservation field for heritage preservation purposes. The two laser models used at this site are the GC-70 laser, and the GC-100 laser. Both lasers were designed by Bartosz Dajnowski and are manufactured in the United States by G.C. Laser Systems, Inc. The lasers operate according to the following general parameters:

GC-70 Laser = 1 Unit (70 watt laser) Wavelength = 1064nm Pulse Duration = Tunable from 10ns – 500ns.

Power supply = 110V Scan Pattern = Circle or oval

GC-100 Laser = 1 Unit (100 watt laser) Wavelength = 1064nm Pulse Duration = Tunable from 100ns.

Power supply = 110V Scan Pattern = Circle or oval

110V power already located on-site was made available to the Contractor for their use. No vacuum extraction unit was coupled with either laser. Small, portable circulating fans were used occasionally to promote a comfortable temperature and air circulation around the lasers.

SAFETY and SITE SECURITY

Safety controls were implemented and maintained for the duration of the project to ensure the Memorial, its visitors, and staff were protected at all times. Dedicated laser curtains were suspended from the scaffold to create an enclosure in which the laser would operate; the beam cannot pass through this site protection. The curtains were relocated as needed depending on the location of each laser unit. A visitor wishing to enter the enclosure was required to verbally announce his or herself before proceeding.

Each laser operator wore wavelength-specific protective eyewear at all times. Multiple extra sets of eyewear were also maintained on-site, available to be worn by visitors. No visitor could enter the enclosure without wearing protective eyewear.

DAILY OPERATIONS

For the majority of the project, two lasers operated concurrently on-site: one GC-70 and one GC-100. On select days, a third laser was in operation as well. The workday generally ran from approximately 7:00 am until approximately 5:00 pm, including start-up and take-down.

Laser ablation may be performed dry or wet—aided by the use of minimal amounts of distilled water that are sprayed on the surface of the stone using small spray bottles. Water enhances the laser cleaning results and minimizes the thermal effect on the surface of the stone. This consumes a very modest amount of water: over the course of the entire project, only approximately 8 gallons of water were used. Use of wet ablation should not be confused with wet cleaning;

the sprayed water is evaporated by laser ablation in a few seconds after application.

Early in the cleaning process, it became clear that the water intended to aid the laser ablation process was actually mobilizing surface soiling and dirt that was not necessarily biofilm.

Selective Laser Cleaning (Biofilm) Thomas Jefferson Memorial DRAFT November 17, 2017

ACCESS

A dedicated scaffold was required to access the dome surfaces for cleaning. The scaffold was designed, furnished and built by Safway Services, LLC, of Laurel, Maryland. Safway was not engaged directly by NPS but rather operated as a direct sub-contractor to the Contractor. This was the only subcontractor engaged by the Contractor.

All scaffold components were hand-carried up a staircase through the Memorial and out the access hatch. Scaffold components were temporarily staged on the west side of the portico; visitor access to the Memorial chamber was not impacted.

The scaffold design, as submitted, is attached as Appendix A. Please note that a number of field modifications were made to this design. Most notably, two of the five bays indicated on the drawing were not built in the field, as three-bay scaffold was sufficiently large enough to access the required ~1000 square feet of surface to be cleaned. No as-built scaffold drawing was required or produced.

PROJECT SCHEDULE and PRODUCTION

Scaffold erection began on August 29, 2017. Work continued through the morning of September 12, 2017. No work occurred on weekends or Federal Holidays (Labor Day: Monday, September 4 and Columbus Day: Monday, October 9).

Initial laser cleaning tests began on the afternoon of September 12, 2017; Wednesday, September 13, constituted the first full day of cleaning. Typical workdays ran from approximately 7:00 am through 5:00 pm, including set-up and take down. Cleaning continued daily through Tuesday, October 10, with modest touch-up performed on Wednesday, October 11. The lasers were demobilized from the site on Thursday, October 12. This comprised a total of 19 full work days, and two (2) partial work-days.

Average production rates over the course of the project can be calculated to approximately 54 SF per day. Depending on the number of lasers in operation on any given day, and the hours worked on any given day, the rates may be further calculated down to a level of approximately 2 SF cleaned per laser, per hour.

EXTENT of CLEANING

The laser was successful at removing nearly all of the disfiguring soiling from stone surfaces within the test area. The laser was less successful at removing biofilm from existing mortar joints. A decision was made by NPS-NAMA not to further pursue cleaning those mortar joints at this time; this question will remain open, and will require resolution in tandem with the full cleaning scope.

Based upon field measurements, approximately 1020 SF of dome stone surfaces were ultimately cleaned. A field map illustrating the extent of cleaning is attached as Appendix B.

LESSONS LEARNED and ADDITIONAL QUESTIONS

Power Although sufficient 110V power was provided to support the use of two or three lasers, the conditions were not at all ideal. Power sources were located at quite some distance from the laser units in operation, meaning that the actual power reaching the unit was reduced over the substantial length of cord. Running any more lasers than what were operated here might overwhelm the available circuits. Alternate and/or additional sources of power should be seriously considered when developing the scope of work for full dome cleaning.

Selective Laser Cleaning (Biofilm) Thomas Jefferson Memorial DRAFT November 17, 2017

Water Cleaning The importance of water to the cleaning process also became clear over the course of the work. The project clearly demonstrated that there is water-soluble soiling on the stone surface: dust, dirt, atmospheric pollutants, etc. This soiling does not need to be removed by laser; it should be removed through water-based cleaning. To increase efficiency and productivity, laser cleaning should be reserved for biofilm removal only, as biofilm cannot be removed through traditional water cleaning methods alone.

A protocol for water cleaning should be developed as part of the scope of work for biofilm cleaning. This will need to take into account the significant logistics associated with getting water to the roof for cleaning. Considerations will need to necessarily include the equipment that can be mobilized; the length(s) of hose needed; questions of water temperature, including heated water versus true steam; questions of water management, run-off, and/or possible containment; and sequencing.

Please note that the Contractor did not use a vacuum extraction unit in tandem with their lasers. A vacuum extraction unit, used with some other laser cleaning systems, can capture material as it is ablated, preventing it from being redeposited on the stone surface. Although this Contractor found that it was useful to lightly rinse the stone surface following cleaning, to mobilize any residual atmospheric soiling and ablated biofilm, it is possible that were an extraction unit used, the post-cleaning condition of the surface might have been different. This should be kept in mind when designing the full cleaning scope.

Scaffold Design and Access to Stone Surfaces The scaffold as it was built for this project had numerous (30+) points of contact with the dome surfaces. Each place where the screw-jack footing and plywood protection came in contact with the stone was an area that could not normally be accessed for cleaning. This required close coordination between the laser Contractor and its scaffold Sub-contractor to “leap frog” over one another, allowing the scaffold to be disassembled—partially, and safely—while allowing the Contractor to complete the laser cleaning at these sites.