5.3.3.1. Optical Microscopy

Optical microscopy is a simple but useful analytical method. Microscopy is the ability to view small areas in great detail by using magnification. This technique will be used to view the marble samples that have undergone accelerated weathering. This technique will help determine if there is any visible deposition on the stone. If efflorescence is present due to salt content it will be viewable using optical microscopy. Salt crystals grow in different patterns with varying shapes depending on their composition. The salt crystals shape will be viewable under magnification this will help to determine its composition. The surface of the field stones will be viewed with optical microscopy to check for any visible depositions or crystallization before further testing takes place.

For this analysis NCPTT will make use of its two in-house microscopes; a Leica MZ8 boom microscope with a total magnification range from 6.3X to 50X, and a Leica DMRX polarized light microscope with a magnification range from 50X to 500X. All samples under magnification can be photographed using the microscopesí digital Spot camera attachment. The photographs can provide important visual and comparative documentation.

X-ray Fluorescence analysis is a non-destructive method used to determine the elemental composition of a sample. This is done by generating elections using an an X-ray tube. The generated electrons of specific energy bombard the sample. The x-rays can either be absorbed or scattered through the material. The way in which the atom absorbs the x-ray is by transferring the energy to its innermost electron. After this is done the electrons are pushed back from the inner shell causing vacancies. The atom to becomes unstable, and outer shell electrons cascade into the vacancies. This causes the release of energy in the form of X-rays of characteristic energy. Since each element produces x-rays that have a unique energy the elemental composition of the sample can be determined.

This process is accomplished with the use of an XRF Spectrometer and its supporting software. The XRF Spectrometer reads the characteristic energy levels and maps them into a spectrum chart where the elements can be labeled and compared. A comparison of the untreated marble and the marble sprayed with the selected cleaners may show chemical residue left behind due to the cleaners.

A handheld XRF Spectrometer will be used to analysis both marble samples in the laboratory from the accelerated weathering studies as well as the field stone samples that were treated in the cemeteries. Due to the handheld XRF Spectrometerís portability it will also be used to analyze chemical deposition on whole headstones cleaned in the field. NCPTT uses a Tracer III portable X-Ray Fluorescence Spectrometer with a Rhodium target.

Scanning electron microscopy with electron microprobe capabilities permit the observation and characterization of materials.¹⁶ Both techniques are based on irradiating the samples with a finely focused electron beam, which may be swept across the surface of a specimen. Different types of signals, including secondary electrons, back-scattered electrons, and characteristic x-rays are produced when the electron beam impinges on the surface of the sample.

The surface topography of a sample can be imaged by collecting secondary and back scattered electrons as the electron beam scans the surface of the sample. This rastered image produces a three dimensional appearance of the surface. Thus, the technique can help elucidate changes in surface texture such as pitting or sugaring.

Additionally, Scanning electron microscopy with energy dispersive spectrometry (EDS) permits the identification of elements present on the surface in major, minor, and trace concentrations. Identification is based on the specific energy of characteristic x-ray peaks for each element and is similar to x-ray fluorescence spectrometry. Also, the surface of the sample can be scanned for these characteristic x-rays, and maps of a specific element can be made on the surface of the sample.

Scanning electron microscopy may be used in this study to provide additional information about possible chemical and physical changes to the field test stones and the artificially weathered stone samples.

While the presence of soluble salts contributes to weathering and decay of porous stone, the decay mechanisms are complex.¹⁷ Soluble salts, such as sodium chloride or calcium sulfate, may damage stone as a result of crystallization pressure or hydration pressure. Crystallization pressure can develop when a supersaturated solution occupies a smaller volume than the precipitating crystals and residual solution. This pressure pushes out on the pores of the stone and causes damage. Alternately, hydration pressure is developed when a salt collects water molecules around itself. Again, the volume needed for hydrated salts is larger than the restrictive pores. Salts push out against the walls of the pores and enlarge the pore space.

Clifford Price points out in his review on stone deterioration that salts represent one of the most important causes of stone decay.¹⁸ Salts may be introduced into stone through rising damp, or blown by the wind. Use of deicing salts can be a problem in colder climates. Unsuitable cleaning may leave salts that ultimately damage the stone. Based on this knowledge, it is important to determine if any of the cleaning test products leave significant soluble salts on the headstones. Tests to determine total soluble salts in stone include gravimetric and conductivity techniques. NCPTT staff will use one or both methods to evaluate the presence of soluble salts after cleaning in the field and after accelerated studies in the lab.

This test uses weight measurement to determine the soluble salts found in a stone sample. The test method is described in Boyer (1987) as: A crushed masonry sample of known weight is allowed to interact with distilled water for 24 hours. The sample is then filtered, dried and the precipitate weighed. Water soluble figures are then calculated based on the ratio of weight loss of the precipitate to the original sample. A high water-soluble content would indicate the masonry to be composed of highly water-soluble materials which would reduce its resistance to weathering.¹⁹

A second method that can be used to investigate salts and other soluble contents within the stone is the use of electrical conductivity measurements. Electrical conductivity is directly related to the concentration of dissolved ionized solids in a wash solution. Again, the sample is ground, then soaked in distilled water for 24 hours. The solution is filtered through a filter paper of 2 micrometer pores. Then an electrical conductivity meter is used to measure the conductivity of the solution in micro-siemens. Higher electrical conductivities indicate greater total dissolved solids.

¹⁶ Goldstein, Joseph I., Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiorl, and Eric Lifshin, 1981, Scanning Electron Microscopy and X-ray Microanalysis, New York, NY: Plenum Press, Chapter 1.
¹⁷ Charola, A. Elena, 2000, ìSalts in the Deterioration of Porous Materials: An Overview,î Journal of the American Institute for Conservation, Vol. 39, No. 3. (Autumn-Winter, 2000), pp 327-343.
¹⁸ Price, C.A., 1996, Stone Conservation, an Overview of Current Research. Santa Monica, CA: Getty Conservation Institute, J. Paul Getty Trust, pp 7-9.
¹⁹ David W. Boyer, 1987, ìA Field and Laboratory Testing Program: Determining the Suitability of Deteriorated Masonries for Chemical Consolidation,î APT Bulletin, Vol. 19, No. 4, 1987, pp. 45-52.