The obelisk of Pharaoh Thutmose, also nicknamed Cleopatra’s Needle, is New York’s oldest outdoor monument. Matthew C. Reiley, Associate Director of Conservation and Senior Conservator at the Central Park Conservancy, began the talk by providing background information on the history of the obelisk. The stone monument was commissioned around 1450 BCE by Thutmose III to celebrate his 30th year of rule. The red granite was quarried in Aswan and carved with hieroglyphs. It was one of a pair of obelisks that stood at the sun temple in Heliopolis. The monuments were purportedly toppled during a Persian invasion around 525 BCE and were later moved and re-erected in Alexandria by Romans around 12 BCE. In the 19th century, one of the obelisks was given by Egypt to the United States. William Vanderbilt paid for the transportation of the monument to New York City. It traveled across the Atlantic Ocean in the hull of a ship and was raised in Central Park in 1881.
The first condition study and treatment of the monument came in 1885. The study noted damage from freeze-thaw cycles. Salt migration and deposition occurring over time, while the monument was in Egypt, had created microcracks in the substrate. Water infiltrating into these areas and expanding during freezing caused surface loss. Workers removed unstable fragments, and the surface was impregnated in paraffin wax, which over time trapped dirt and pollution. In 1983, the Metropolitan Museum of Art performed a scientific study, which found that the monument was stable and not aging at an accelerated rate.
In 2011, an Egyptian Antiquities official threatened to take back the obelisk, claiming it was not well cared for. This prompted the Central Park Conservancy’s project to document, clean, and stabilize the monument. Photographs were taken and color annotated to document condition issues. Cleaning tests, using aqueous methods, micro-abrasion, and lasers, were performed in order to find a suitable method for removing years of accumulated soiling and atmospheric pollutants that obscured the carvings. Based on these tests and in situ mockups, laser cleaning was chosen because it was controllable, effective, and did not damage the stone.
For the second half of the talk, Bartosz Dajnowski, Vice Director and Objects Conservator at the Conservation of Sculpture & Objects Studio Inc., described the laser cleaning methods. He listed some of the benefits of laser cleaning: no chemicals, no abrasives, no loud noises, and no public hazards or contamination to the surrounding areas. Unlike a laser pointer, the beam of this laser is focused to a point and then it spreads out so that the radiation diffuses past the focal point. The cleaning process is called laser ablation, and it works discriminately as it excites one material so that it separates from the substrate. Since laser cleaning is not a mechanical process, it is safe to use on fragile substrates. Bartosz noted that it is important to use the correct settings because if the laser is used incorrectly, it could damage the substrate, for example by melting bronze or shattering quartz and melting inclusions in stone. He also pointed out that adding water to a surface during laser ablation has a micro-steam cleaning effect as the laser turns water into steam. The water also helps reduce the effects of plasma formed by the laser, minimizing the possibility of phase changes in the iron within the stone.
Bartosz spoke about his prior experience and research with lasers, including work done during his graduate studies at Winterthur. Extensive testing and analysis was done on stone samples before laser cleaning began on the obelisk. Small fragments that had previously fallen off of the obelisk were cleaned and then examined by George Wheeler at the Metropolitan Museum of Art to confirm that safe laser cleaning parameters were being used.
Seven lasers were used to clean the entire stone monument and the four bronze crabs around its base. One of the lasers, called the GC-1, Bartosz built himself using a new design. With the other lasers, the pulses are emitted as line scanning, with a mirror alternating left and right, so the pulses come out like a machine gun firing back and forth. This creates hot spots at the edges, which can result in overcleaning or damage. Optical shields and other methods can be used to cut off the hot spots, although this decreases efficiency. In Bartosz’s new design, the laser pulses are emitted in a circular ring pattern, so the beam is constantly moving around in a circle. In this formation, there are no hotspots, and when the instrument is moved across the surface, the coverage area is exposed twice, which increases efficiency. Since this unit is also smaller, Bartosz was able to take it up to the top of the scaffolding. As others on his team used the line scanning lasers and worked from the bottom of the monument up, Bartosz worked from the top down. With his new laser, he was able to clean the same sized area in half the time!
With lasers, the level of cleaning is controllable, and Bartosz mentioned that the team was asked to try leaving some soiling in the recesses of the carving to increase the legibility of the glyphs. However, the soiling was uneven, and the carving had suffered previous damage, so it was not always easy to distinguish. An overall cleaning was carried out, although a few areas of soiling were left near the top of the monument for future analysis.
Following laser cleaning, fragile and unstable areas of the stone monument were consolidated. The 3,500 year old, 220 ton obelisk now stands cleaner and more stable for the future. In a fitting end to the talk, Bartosz noted the appropriateness of using light to clean and revive an ancient monument that was originally built to honor the sun.