Sheet Scaling in Mohegan Granite

de Karen Stone

In the late nineteenth and early twentieth centuries, dimension stone construction was widely popular for monumental structures in the United States. The rise of steam-powered quarry machinery, as well as expedient transportation options, increased access to building stones such as Mohegan granite, extracted from the Peekskill area of New York. The Cathedral Church of Saint John the Divine in New York City, the George T. Bliss Mausoleum in Woodlawn Cemetery, the Westchester County National Bank in Peekskill, and the First Church of Christ, Scientist in Reading, Massachusetts are but a few of the structures that utilized this medium-grained, yellowish-gray granite. Approximately one century later, many of these stones are consistently displaying evidence of sheet scaling deterioration. This condition not only degrades the aesthetic integrity of a structure, but it has the potential to pose safety concerns as surface layers can easily dislodge from great heights. Unimpeded, sheet scaling could cause structural instabilities as ever more material scales or is disaggregated from a receding surface. Perplexing to many in the conservation field is the fact that very little is understood about how and why sheet scaling occurs. Are intrinsic factors, such as mineralogy, the root cause of this mode of deterioration? Are extrinsic factors, such as incompatible in situ conditions, the true culprit? Even further, could the predisposition for sheet scaling be traced back to aggressive quarrying techniques? In this research, various structures composed of Mohegan granite in Massachusetts, Connecticut, and New York were studied for their sheet scaling conditions, while core samples were acquired from five of these sites, including the original quarry. Thin section petrography, x-ray diffraction, and scanning electron microscopy with energy dispersive spectroscopy were utilized to research whether particular factors could be identified as contributing to the observed sheet scaling. While definitive conclusions may require further testing, data generated from this study could be used to exclude potential factors, or perhaps identify those that reoccur in other granites known to exhibit this mode of deterioration. This line of inquiry may reveal whether sheet scaling is the result of an isolated factor, or rather as a confluence of factors that consistently combine to produce the same unfortunate effect. Once the agent for this mode of deterioration is defined, better strategies for slowing, halting, or preventing this condition can be sought.