An Evaluation of Laser Ablation Cleaning on Surficial Black Crust on Pennsylvania Marble: A Case Study of the Hood Cemetery Gate, Germantown, Philadelphia, PA

Black crust, or gypsum crust, is a surficial accumulation of atmospheric deposits in a gypsum matrix, which is often firmly bonded to the substrate. This condition is ubiquitous on marble buildings, not only threatening the historic fabric, but also leading to aesthetic damage. Laser ablation cleaning has become a popular method to remove black crust from masonry surface. This study focuses on the evaluation of laser ablation cleaning on the Pennsylvania marble of the Hood Cemetery Gate, which locates in Germantown, Philadelphia, PA. In advance of cleaning, the marble and the black crust was characterized through petrographic thin-sections, XRay Diffraction, and Scanning Electron Microscopy to understand the micro-structure and chemical composition of the materials, A low power portable fiber laser was used for cleaning and evaluation. Laboratory cleaning tests on small samples were conducted to determine the effective parameters, in dry and wet conditions. The cleaned area was examined with a spectrophotometer, Scanning Electron Microscopy, and Raman spectroscopy to detect potential changes in, surface morphology, and chemical composition. Insitu cleaning test was conducted using the optimal parameters based on the results of laboratory tests, and the outcome was evaluated in terms of color change. In conclusion, laser ablation cleaning was generally effective on the marble of the Hood Cemetery Gate. In wet condition, it resulted in a satisfying removal of the black crust without causing micro-scale damage or undesired discoloration.