Methods of Measurement

Below are some of the methods used to quantify weathering of gravestones. If visitors to this page are interested I can begin to formalize some of the more descriptive methods such as Rahn's index. This would involve a number of interested workers comparing photographs of the decay of lettering and categorizing its severity. Combining a large number of these assessments it should be possible to derive a common set of images that reflect what most people regard as weathering of a particular severity. If you are interested in contributing to such a study then please e-mail me.

Lead Lettering Index

The lead lettering index (LLI) has been used by a number of workers to measure the amount of material removed usually from marble gravestones [1-4]. The index is assumes that the lead lettering often found inserted into gravestones was originally polished flush to the surface. Over time the marble has been eroded and left the lead lettering proud of the surface. It is assumed that the lead lettering has not been eroded to any significant degree over the exposure of the gravestone. Similar protrusion of lead can be found on building ( Images 1-4 ) and have been used at St Paul's Cathedral, London to quantify weathering loss over a 250 year period [5]. The marble is assumed to have eroded at a relatively uniform rate across the whole gravestone. Measuring the height difference between the lead lettering and the marble surface indicates how much material has been removed since the gravestone was exposed. Repeated measurements across the gravestone (between 10 and 25) is assumed to provide a representative sample of height change. If the same letters are selected, for example the '1' in the date or the 'I' in died, then comparisons are possible between gravestones keeping the letter location approximately constant. As the marble erodes, however, the lettering itself can become loose and begin to peel off. Often the lettering is attached to the gravestone by small lead pegs and these become loose. The lead lettering begins to curl away from the surface and eventually drops out. The impression of the lettering, complete with peg holes, is often visible in old gravestones. This means that there is a limit to the age of gravestones that can be measured using this method. Once peeling of lettering begins the method becomes difficult to apply with any consistency.

Width Differences

Meierding [6] has suggested that marble gravestones are finished with parallel surfaces. This means that the gravestones are originally the same width over their whole length. If any original polished surface can be found on the gravestone on both parallel sides, this can be used to determine the original width of the gravestone. Meierding suggests that the width of the gravestone base often retains polish and indicates the original width. Comparing the width at the base with widths measured at different heights on the gravestone indicates not only the average loss from the gravestone, but also the differences in loss at different heights.

Lettering Alteration

Rahn [7] and Meierding [8] suggested that the alteration of carved lettering on gravestones follows a sequence of deterioration. The lettering becomes progressively roughened and difficult to read. The edges of the carved surface at first become less distinct as granular disintegration occurs. The whole letter begins to merge into the surface of the gravestone as the agents of erosion remove the surface. Eventually the lettering becomes almost indistinguishable from the surface of the gravestone itself. Once the lettering is indistinguishable the method become inappropriate to use. Although this method can not provide any absolute measurements of the amount of material lost, it can provide a relative measure of deterioration. This is often referred to as a semi-quantitative index to reflect this. Meierding ensured that this type of index was applied consistently in his study by using a set of photographic standards against which to assess the weathering found on each gravestone observed. An example of the type of visual classification that could be used is given below. Further subdivisions could be made on the basis of upper and lower case lettering. It is important to note, however, that the classes give an impression of a steady linear change in weathering. Weathering of lettering may not be linear. Once weathering has began it may rapidly deteriorate the visual appearance of the lettering. This may mean that the initial classes are indicative of a long period of weathering, whilst the latter classes represent the rapid loss of lettering over a shorter period. Alternatively, lettering may become rapidly rounded and remain in class 2 or 3 for a long time. Rapid removal of the surfaceafter thsi stable period may then mean a very short time span for classes 4-6. The linearity or otherwise of this method is one aspect that requires further analysis.

Lettering Depth

Variations in the depth of carved lettering have also been used to indicate the amount of material eroded from the surface of a gravestone [9]. It is assumed that the lettering was originally cut to the same depth and any differences between the depths of carving are the result of weathering. This is a large assumption. It implies that even if the same person carved all the gravestones measured, they carved the lettering to exactly the same depth each time. The assumption of the same person carving the lettering also becomes difficult to sustain if the age range of gravestones measured is great or if different graveyards are compared. Mechanical carving may alleviate some of these problems, but it still assumes different machines cut to the same, standard depth over the whole time they are used.

Percentage Cover

Different forms of weathering can occur on gravestones. The methods above do not really address these forms of weathering. The extent and intensity of these forms can be measured by using the simple idea of percentage cover. The extent of a particular form can be estimated as the percentage cover of a given area. The area may be a quadrat used to ensure a standard area or given portion of the gravestone. Either way the observer needs to define a classification scheme for weathering forms, identify them and then estimate their cover. The intensity of specific forms, such as pits, can be further quantified by measuring their depth and width using a pair of calipers. Combined these measurements provide a standard quantification of different weathering forms.

Close-Range Photogrammetric Analysis

If you have the time, funds and equipment it is also possible to obtain extremely detailed measurements of the surface of the gravestone. This requires specialised camera equipment (a calibrated camera with a fixed focal length) to take the iamges as well as an analytical or digital photogrammetric station to analyse the images. Using such images, however, you can measure surface forms and produce digital elevation models to an accuracy of between 50 and 100 microns in the x, y and z directions ( Image 5 ).

References

1 Dragovich, D. 1986. 'Weathering rates of marble in urban environments, eastern Australia', Zeitschrift fur Geomorphologie , N.F. 30 , 203-214.

2 Cooke, R.U., Inkpen, R.J. and Wiggs, G.F.S. 1995. 'Using gravestones to assess changing rates of weathering in the United Kingdom', Earth Surface Processes and Landforms , 20 , 531-546.

3 Inkpen, R.J. 1998. 'Gravestones: Problems and potentials as indicators of recent changes in weathering', in Jones, M. and Wakefield, R. (eds.), Aspects of stone weathering, decay and conservation , Imperial College Press, London, 16-27.

4 Attewell, P.B. and Taylor, D. 1988. 'Time-dependent atmospheric degradation of building stone in a polluting enviornment', in Marinos, G. and Koukis, G. (eds.), Engineering Geology of Ancient Works, Monument and Historical Sites , 739-753.

5 Sharp, A.D., Trudgill, S.T., Cooke, R.U., Price, C.A., Crabtree, R.W., Pickles, A.M., Smith, D.I., 1982. 'Weathering of the balustrade on St Paul's Cathedral, London', Earth Surface Processes and Landforms , 7 , 387-389.

6 Meierding, T.C. 1981. 'Marble weathering rates: A transect of the United States', Physical Geography , 2 , 1-18.

7 Rahn, T. 1971. 'The weathering of tombstones and its relation to the topography of New England', Journal of Geological Education , 19 , 112-118.

8 Meierding, T.C., 1993. 'Inscription legibility method for estimating rock weathering', Geomorphology , 6 , 273-286.

9 Matthais, G.F. 1967. 'Weathering of Portland Arkose tombstones', Journal of Geological Education , 15 , 140-144.