Digital image processing techniques as a tool for evaluating and mapping patinas on granite monuments

Monument conservation is related to the interaction between the original petrological parameters of the rock and external factors in the area where the building is sited, such as weather conditions, pollution, and so on. Depending on the environmental conditions and the characteristics of the materials used, different types of weathering predominate. In all, the appearance of surface crusts constitutes a first stage, whose origin can often be traced to the properties of the material itself. In the present study, different colours of “patinas” were distinguished by defining the threshold levels of greys associated with “pathology” in the histogram. These data were compared to background information and other parameters, such as mineralogical composition, porosity, and so on, as well as other visual signs of deterioration. The result is a map of the pathologies associated with “cover films” on monuments, which generate images by relating colour characteristics to desired properties or zones of interest.

Estimate from Josiah Bemis to John Lowell and Loammi Baldwin for Chelmsford granite, [ca. 1813]

Bemis, responding to an advertisement from Lowell and Baldwin requesting bids for building materials, outlines his proposal for using Chelmsford granite for the construction of University Hall. Bemis lists 12 stipulations in his proposal including costs per bricks, techniques for constructing the upper storeys, and labor costs.