Over the course of the last fifteen years, I have worked on numerous marble monuments and buildings and in that time, have gained tremendous insight into the relationships between weathering, lichens, biofilms, and treatments.
In locations where fine-grained white marble is exposed to frequent rain, surfaces are slowly eroded. Marble in urban and industrial environments, subject to historic rainfall far more acidic than that of today (prior to the Clean Air Act of 1970), is noticeably more eroded than marble in rural environments. In northern locations, marble subject to a winter covering of snow is noticeably less eroded than that left uncovered.
Over time, with erosion and daily and seasonal temperature fluctuations, the cohesive mass of fine-grained white marble becomes weakened. Surface fissures indicate this condition and fragments of gravestones lying on the ground are often the result of a catastrophic failure where the gravestone could no longer support itself. Often the repair methods used to unite such fragments contribute to another catastrophic failure.
The “pitting” created by surface erosion, and the associated periods of increased surface wetness, provides an environment attractive to lichens and biofilms. Lichen species are environment specific (urban, rural, marine, etc.) and the makeup of a biofilm (algae, fungi, and bacteria cells) is also environment specific. Determining the potential contribution to erosion caused by lichens and biofilms, and the potential protection offered by their covering are topics that are the subject of current on-going studies. Observations suggest that in the Northeast, lichens and biofilms do not have an impact on the deterioration of marble.
“Cleaning” marble, whether to rid a surface of atmospheric soiling or lichens and biofilms, is an erosive action. The use of soft scrub brushes and low pressure water will remove calcite crystals—amounts depending upon the extent of surface erosion—from the stone mass. The extent to which “cleaning” removes deteriorative agents and the extent to which it contributes to erosion are also the subjects of current on-going studies.
Climate change, bringing warmer temperatures and more frequent storms, will have an impact on the erosion rates of marble. As conservators, addressing the need to lower our carbon footprints will have an impact on all treatment decisions. In this presentation, I will be discussing my observations of the relationships between weathering, lichens, biofilms, and treatments and how these relationships can affect treatment decisions. I will also discuss how climate change will affect these relationships and treatment decisions.
