5.3. Methods of Analysis

Staff scientists have determined multiple ways to evaluate accelerated aging samples and field stone samples, based on the criteria established at the beginning of the study by the project team. Important questions to answer include:

• Is the stone appearance changed by the chemical cleaner? If so, is the appearance acceptable?
• Are there physical changes to the stone upon cleaning? Is the stone surface physically altered during the cleaning process with the cleaner? Is the surface rougher or smoother? Is the stone porosity altered during the cleaning process? Are the pores of the stone larger or smaller after cleaning?
• Are there chemical changes to the stone upon cleaning? Does the cleaner interact with the stone to produce chemical changes on the surface? Does the cleaner interact with the stone to produce salts or efflorescence?

Appearance change is documented using photographic techniques and color measurements. Photographic images are taken of the lab test stones prior to accelerated weathering using the QUV weatherometer. Field test stones were photographed in each cemetery prior to cleaning with the test cleaners. The field tests stones were photographed again after cleaning.

NCPTT staff is photographing test samples from laboratory or field test stones under standard lighting on a Kodak gray card using a Polaroid copy stand and color balanced ECT incandescent bulbs. Digital photographs are taken using a Sony DSC-S85 digital camera at 2272 x 1704 pixel resolution. The photographs will be compared visually by at least ten unbiased observers and ranked on the basis of change in appearance.

Using a Minolta Colorimeter, CR-400, staff made color measurements on lab test samples prior to accelerated aging using the QUV weatherometer. Each measurement was repeated three times on each stone sample and averaged in order to compensate for slight variations in surface texture. The samples were then exposed to UV radiation, temperature cycling, and spray mist cycling as described in section 5.1.1.2. Samples were cleaned weekly for a total for four cleanings. Samples were cleaned either with D2, Daybreak or WEG Marble Cleaner.

Staff will make color measurements of the samples after exposure. The measurements will be taken following the procedures described in the above paragraph. Once the ìafterí pictures are taken, total color change, ΔE^*, will be calculated for each sample. Samples will be considered unchanged if the ΔE^* is 3 or less. When ΔE^* is 3 or greater, the samples will be considered to have undergone a color change. Shifts in lightness and chroma will be noted.

One aspect of stone deterioration is the change in physical properties of the stone. Physical changes commonly observed in the field include sugaring, blistering, and scaling, among others.⁷ Visual observations allow researchers to determine the state of the condition at a moment in time. However, quantification, or putting numbers to the conditions, allows observations over a time range. The NCPTT staff chose two ways to characterize physical changes in the accelerated weathering tests and in the filed test samples. Laser profilometry can be used to quantify the surface of the stone samples. Changes in the pore structure can be measured by porosimetry.

Surface texture is the local variations in the in the surface from its ideal shape. It can be characterized by a number of variables defined by international standards^{8, 9}, including

Sa ñ Average Roughness for an Area,
Sp ñ Highest Peak Surface, the height of the highest peak in the roughness profile
over the evaluation area,
Sv ñ Valley Depth from surface,
St ñ The total height of the surface, the sum of Sp + Sv,
Sku ñ The Kurtosis, a measure of the randomness of heights and sharpness of a surface,
Svk ñ The roughness of the valleys,
Sk ñ Roughness of the core
Sfd ñ the fractal dimension of the surface (complexity of the surface),
Sq ñ the root mean square of the roughness, and
Vv ñ Void volume of the valleys, among others.

In her doctoral work, ElizaBeth Bede Guin showed that surface texture and porosity can affect the deposition of air pollution on to surfaces.¹⁰ Guin characterized the porosity and surface texture of four different types of high-calcium limestone including Salem, Cordova Cream, Cottonwood Top Ledge, and Monks Park limestones. Half of the stones were chemically etched to create rough surfaces while others were semi-polished to create smooth surfaces. Next the samples were exposed to a simulated polluted sulfur dioxide environment within the NCPTT environmental exposure chamber for 24 hours. The conditions were 50 ppb SO2; 65% RH; 25∞C; 4 m/s wind speed. Deposition velocities were calculated for each stone surface. While porosity was the dominant variable influencing pollution deposition, Guinís work showed that three texture parameters including Sk, Svk, and Sq, did correlate to the deposition of sulfur dioxide on the stone.¹¹

Cleaning the surface of the stone may affect both the porosity and the surface texture parameters. The changes may lead to additional soiling by atmospheric pollutants. Alternately, changes may increase moisture retention and lead to increased biological growth.

In this work, we will characterize several surface texture parameters of both laboratory stones and field test stone prior to and after cleaning/accelerated aging or field exposure. Changes in the surface texture from cleaning and/or aging will be noted. Surface texture will be analyzed using laser profilometry on small cut samples.

Laser profilometery is based on the principle of optical triangulation. It employs a light source (a laser), imaging optics, and a photodetector. The laser is focused on to the surface of the sample. Reflected light is focused on to the photodetector, which generates a signal that is proportional to the position of the spot in its image plane. As the distance to the target surface changes, the imaged spot shifts due to parallax. To generate a threedimensional image of the stone surface, the sensor is scanned in two dimensions, thus generating a set of distance data that represents the surface topography of the stone.¹²

NCPTT uses a Solarius LaserScan, a 3-d non-contact laser profilometer, to characterize stone sample surfaces. The instrument uses a class II diode laser (670 nm wavelength) and a 2 μm spot size. The vertical resolution of this instrument is 0.1 μm. The maximum vertical range is 1 mm. This range allows for the measurement of surface peaks and valleys typically encountered on stone surfaces. The laser is scanned over an area of 31.07 mm (x-axis) by 23.02 mm (y-axis) at a scan speed of 5 mm/s and a resolution of 25 μm.¹³ The estimated run time per sample is 111 minutes.

Laboratory samples chosen for accelerated weathering were documented by laser profilometry using the conditions described in the above paragraph. The samples will be analyzed after exposure and changes in parameters will be calculated.

Field test samples will be measured upon return from the field. This will require cutting small samples from each stone cleaning space. The surface texture will be compared to control samples which have not been exposed in the field, but have been carefully stored in the lab.

Porosity is the volume of void spaces found in the stone and is expressed as a fraction between 0-1. The porosity of a stone is important consideration when determining how much water or liquid can be absorbed in a stone or how a stone might be affected by air pollution or long term weathering. Also, a more porous stone may absorb more and retain more cleaner, making it harder to rinse.

Increases in porosity may reflect erosion or material loss from the surface of the stone. This undesirable affect may come from mineral dissolution in water or cleaner. Alternately, decreases in porosity may reflect growth of salts or other residues within the stone pore system.

The voids within a stone have additional characteristics that can be described by size and shape. Large voids in the stone greater than 50 nm are considered macro-pores. Pores in the 50 nm to 2 nm range are considered meso-pores, while pores smaller than 2 nm are called micro-pores. The size of the pores affects the way fluids move through the stone.

One technique that can determine pore size in a material is called Mercury Intrusion Porosimetry (MIP). It can measure pore size in the range of meso-pores to macropores. Samples are submerged in a confined quantity of mercury and then the pressure of the mercury is hydraulically increased. This forces the mercury into the pores of the material. The results obtained from the instrument include

• pore size distribution (macro/meso range of porosity spectrum),
• hysteresis curve,
• specific surface,
• bulk density,
• total porosity (%), and
• particle size distribution.

Nitrogen gas absorption can be used to determine the micropores and the lower range of meso-pores in a material. The measurement of adsorption at the gas/solid interface is one of the most widely used techniques for the study of microporous and mesoporous solids. The gas molecule acts as a ruler for the measurement of features at the nanometric scale. Nitrogen is the gas most often used for this type of study. With this technique, a series of isotherms¹⁴ are plotted for the absorption and desorption of nitrogen onto the surface of a stone. ASTM UOP821-81¹⁵ describes a method of determining the distribution of surface area, pore volume (size) and length among the micropores, 60 nm (600 A) and smaller, as well as total surface area, total pore volume and average micropore diameter for porous substances using a Micromeritics Digisorb 2500 Analyzer.

⁷ Price, C.A., 1996, Stone Conservation, an Overview of Current Research. Santa Monica, CA: Getty Conservation Institute, J. Paul Getty Trust, pp 1-4.
⁸ International Organization for Standards, ìGeometrical Product Specifications (GPS) - Surface texture: Profile method; Surfaces having stratified functional properties - Part 2: Height characterization using the linear material ratio curve,î ISO 13565-2 1996.
⁹ American Society of Mechanical Engineers, ìSurface Texture, Surface Roughness Waviness and Lay,î ASME B46.1, 1995.
¹⁰ Bede, ElizaBeth Anne, ìThe surface morphology of limestone and its effect on sulfur dioxide deposition,î Ph.D. dissertation, University of Delaware, 2001, 327 pp.
¹¹ Bede, Ibid., Chapter 6. (N.B. S parameters reflect area measurements, while R parameters reflect line measurements).
¹² ìIntroduction to Laser-based Profilometry,î Laser Techniques Co., 14508 NE 20th St., Bellevue, WA, 98007 http://www.laser-ndt.com/LP_method.pdf (accessed 3/12/2007).
¹³ Other conditions include a row pitch of 85.95 and a column pitch of 88.33.
¹⁴ Absorption isotherms are plots of the amount of gas absorbed at equilibrium pressure at a constact temperature, usually nitrogen at its boiling point.
¹⁵ ìUOP821-81 Automated Micro Pore Size Distribution of Porous Substances and/or Desorption Using a Micromeritics Analyzerî ASTM International.