Colonial Williamsburg Digital Library

Purpose

The goal of this project is to use cross-section microscopy to identify the early finishes, if they are present, in exterior paint samples from the William Finnie House. A variety of techniques are used, including cross-section microscopy with reflected visible and ultraviolet light, pigment identification with transmitted polarized light microscopy, binder identification with fluorochrome staining, scanning electron microscopy with elemental dispersive spectroscopy, and colorimetry.

Previous Research

This report builds on an unfinished draft from 2004 by Peggy Olley, a Winterthur/University of Delaware Program in Art Conservation Graduate Fellow, on the exterior finishes of the Finnie House that was written as part of her third-year internship with Colonial Williamsburg under the supervision of Susan Buck.¹ Mark Kutney, a former CWF conservator, also undertook a study of the exterior finishes in 2003, but his data could not be located.

Historical Background

It is not known exactly when the Finnie House was constructed, but a house of the same shape is drawn on this lot in the 1782 Frenchman's Map. Archeological investigations in the 1930s found no other foundations near the house that would indicate an earlier structure.² The current name of the house comes from Quartermaster General Colonel William Finnie who is althought to have owned the land during the period of the Revolution. The insurance documents from 1801 suggest the house had achieved its current form by at least that date.³ The front porch may be of a slightly later date, but was added before 1801 when the house was insured. There are changes in the framing, windows, and weatherboards above the porch pediment that suggest that the porch is an addition. The property was purchased by Colonial Williamsburg in 1928 and was restored in 1932. At that time a southern wing dating to between 1806-1823 and extending off the back of the main block of the house was removed. Some restoration was undertaken on the front porch as well. The fretwork on the frieze was restored based on traces of original ornament and the freestanding columns that had been replaced with piers were changed back to columns.⁴

Sampling Procedures

Thirty-three exterior samples were taken from the Finnie House by Susan Buck in consultation with Edward Chappell on December 8, 2003. The samples were collected with a scalpel and placed in numbered bags corresponding to the recorded sample locations. These samples were cast in cubes labeled "Finnie" followed by a sample number. During the writing of her report, Olley recast material from samples ten and eleven in cubes labeled "Fin 10" and "Fin 11". These samples were initially photographed with 35mm color print film. For the purposes of this report, all of the previously cast sample cubes were rephotographed digitally and the digital image files were named "WF", followed by the sample number. Additionally, more material from several samples was recast in cubes labeled "WF" followed by the sample number. On July 27, 2007 twenty-nine additional samples were taken from the exterior by the author and Chappell. These samples were numbered beginning with WF34. The exact sample locations for all the sampled examined in this report are provided in the following table.

Results of Cross-section Microscopy Analysis

Surprisingly, some of the best samples with the most complete stratigraphies on the Finnie house came from the weatherboards on the north elevation that have as many as twenty-one generations of finish. The northern weatherboards have six generations of early lead white-based paints before a characteristic zinc white-containing layer was applied in generation seven. In cross-section there are large yellow and red pigment particles present in many of the early layers that were identified as yellow and red ochre with polarized light microscopy. There are also small black particles that appear to be lampblack, although these could also be embedded grime. The colored pigments are present in very low concentration but they would give the early paints a warmer and darker tone. Colorimetry indicated that the firstgeneration paint was actually a warm light gray in color, although it looks cream-colored in the magnified cross-sections. The colors of the other early paint layers were not measured but they are probably cream or light gray as well.

The weatherboards on the south elevation have only three early cream or light gray generations of paint. The first layer on the southern weatherboards seems to correspond to the second-generation paint on the northern weatherboards, based on the appearance of that layer in ultraviolet light. Either the southern weatherboards are slightly later, or earlier layers were lost due to wear. It is also possible that the rear elevation was not painted as often as the front in the early period.

Three door architraves were sampled for analysis, including the north front door, the south door on the main block, and the south door on the east wing. Of these, the northern door architrave has the most complete stratigraphy with five early cream or light gray layers, one less than found on the weatherboards. The front door architrave is either slightly later than the weatherboards, has layers lost through wear, or was not painted as often. Some of the early layers on the architrave also have a few colored pigments. The upper portion of the frame around the transom has the same stratigraphy as the rest of the architrave, which suggests the transom is contemporary with the door architrave. The south and southeast door architraves have one and three early cream or light gray layers respectively. Again, these door architraves are either later, have lost layers due to wear, or were not painted as often.

One window frame and one window sash yielded good early stratigraphies with six early cream or light gray paint generations. Thus, these elements are contemporary with the north elevation weatherboards. Not all of the windows were sampled, but the profile of the proven early sash and frame could be used to help date the others windows on site in the future.

On the north elevation, the cornice of the front porch and the west wing were sampled. The cornice of the west wing has six early, cream or light gray layers with some colored pigments before a characteristic zinc white-containing layer was applied. This suggests it is contemporary with the north elevation weatherboards. The cornice of the front porch also has six early layers, which reinforces the idea that it the porch is early, if not original. Inside the porch, the fascia on the reverse of the cornice yielded unusual layers not found in other samples that may be finishes applied to the ceiling of the porch.

Results of Scanning Electron Microscopy with Elemental Dispersive Spectroscopy

In order to better understand the stratigraphy of the early generations of paint, three samples with evidence of early layers were taken to the Applied Research Center (ARC) at the Department of Energy Thomas Jefferson National Accelerator Facility in the Jefferson Center for Research and Technology, in Newport News, Virginia. The cast samples were sputter coated with a gold palladium mixture prior to analysis with the help of Olga Trofimova, Laboratory and Research Technician at the College of William & Mary. The scanning electron microscopy with elemental dispersive spectroscopy (SEM-EDS) analysis was done by Brandt Robertson, Lab Specialist with Old Dominion University.

SEM-EDS backscattered electron images were created of a few samples that show the cross-sections in gray scale. In these images, elements with higher atomic numbers are lighter in color and elements with lower atomic numbers are darker. Thus, layers with a large organic component like varnishes appear dark while paints with significantly quantities of lead white pigment appear much lighter. Because the backscattered electron images remove color and instead show differences in elemental composition this technique can make the differences in finish layers more apparent, and, by extension, the boundaries between the layers can become more apparent. Since many cross-sections from the Finnie house have jumbled early layers with similar cream or light gray paints, the SEM-EDS backscattered electron images were used to more clearly define the number of early layers in a few key cross-sections. The SEM-EDS images on the next three pages show the early layers as different shades of gray which, in conjunction with the visible and ultraviolet light images, makes the boundaries between the layers more clear.

This analysis was applied to key cross-sections from the window and the weatherboards. In all three cases, the SEM-EDS analysis indicated that there were indeed six early cream or light gray layers present in the cross-sections. Since this is the same number of early layers found on elements from the porch, the SEM-EDS analysis supports the theory that the porch is original.

Sample WF47, north elevation, west wing, east window, inner beaded head at junction with plain outer frame, west end of frame
Clockwise from top right:
SEM-EDS backscattered electron image
(60x magnification and 15kV energy)
Ultraviolet light (100x magnification)
Visible light (100x magnification)

Sample WF24, north elevation windows, west window in main block, north sash, top rail 8" east of west side
Clockwise from top right:
SEM-EDS backscattered electron image
(130x magnification and 15kV energy)
Ultraviolet light (200x magnification)
Visible light (200x magnification)

Sample WF10, north (front) of central block, weatherboards inside porch, second board down from top, 2" east of door architrave
Clockwise from top right:
SEM-EDS backscattered electron image
(43x magnification and 15kV energy)
Ultraviolet light (100x magnification)
Visible light (100x magnification)

SEM-EDS analysis was also used to do specific spot analyses of the elemental composition in certain layers and a line scan feature was used that shows how the elemental composition of layers changes as it moves across the cross-section. The spot analyses, as shown in the upper set of images, indicated that the early paint layers contain carbon, oxygen, and lead which are probably present in the form of lead carbonate, the main component in lead white pigment. Elemental calcium was detected and may be present in the form of calcium carbonate, which is a common paint extender. A very small amount of aluminum was detected in the early layer as well. This element is commonly present in dirt and clay, which may suggest there is embedded grime in this layer.

SEM-EDS backscattered electron image (left) of sample WF24 from a window with locations of spot analyses and spectrum (right) of spot 2 from generation one

A spot scan of generation seven confirms that this layer contains zinc, probably present as zinc white pigment. This dates generation seven to after 1845 when zinc white became commercially available for use in oil paint. This layer also contains silicon, which is often present in dirt and clay, and barium, which is part of a common paint extender.

SEM-EDS backscattered electron image (left) of sample WF47 from a window with locations of spot analyses and spectrum (right) of spot 2 from generation seven

A line scan across all the layers in sample WF10 from the weatherboards indicates how the elemental composition of the layers changed over time. The graph below has the early layers on the right and the modern layers on the left. Note that there is a point on the left near around 160 microns where the level of titanium (shown in olive green) surpasses the level of lead (shown in orange). This probably represents paints applied after 1978 when lead paint was outlawed by the Federal government. The vertical black line in the middle of the graph represents roughly where the zinc white-containing layer is located in the cross-section. At this point the amount of lead is greatly reduced and there is a peak in the zinc (shown in pink) concentration.

SEM-EDS backscattered electron image (left) of sample WF10 from the weatherboards with the location of the line scan and spectrum (bottom) of scan showing location of elements in the sample

Results of Binding Media Analysis with Fluorochrome Stains

Several samples from various exterior elements were selected for staining with biological fluorochrome stains that mark out the presence of proteins, carbohydrates, oils, and zinc (Zn²⁺). There were no positive reactions for proteins when the fluorochrome stain Alexafluor 488 was applied to door or trim samples. There were positive reactions for zinc (Zn²⁺) beginning in generation seven, as shown by a bright blue reaction color. Since a feasible commercial method for delivering zinc white pigment in oil paint was not developed until 1845, generation seven must postdate this. These results were confirmed with SEM-EDS analysis.

Sample WF10, weatherboards inside porch, second board down from top, 2" east of door architrave

Most of the layers on the weatherboards, including the early layers, reacted positively for carbohydrates, as shown by a dark red reaction color when the fluorochrome stain TTC was applied. In the early paints the carbohydrates may be present as a gum component added to the paint or used to grind the pigments. In the modern paints the carbohydrates could be cellulosic bulking agents.

Sample WF10, weatherboards inside porch, second board down from top, 2" east of door architrave

There were weak reactions for saturated lipids in the early cream or light gray paint layers on the weatherboards, as shown by a pink reaction color when the fluorochrome stain DCF was applied. This type of reaction is typical of aged oil paints.

Sample WF13, north elevation, weatherboards 5" west of west porch pilaster, 5th board up from windowsill

The cross-section below also shows positive reactions for oils in a sample from the weatherboards after the fluorochrome stain DCF was applied. The early layers reacted positively with a pink reaction color that indicates saturated lipids and aged oils. The later layers reacted positively with a yellow reaction color that indicates unsaturated lipids and less aged oils. All the layers had at least a weak reaction when DCF was applied suggesting that they are all oil-bound paints.

Sample WF10, weatherboards inside porch, second board down from top, 2" east of door architrave

In samples from the weatherboards enclosed in the porch, generations seven, eight, and nine are somewhat translucent layers that appear similar to limewashes. However, these layers stained strongly positive for oils when the fluorochrome stain DCF was applied so they are probably oil paints despite their translucency. Perhaps these are cheaper paints with a higher proportion of chalk extenders which could explain their translucency. These layers could also have a resin component that would explain their translucency.

Sample WF43, north elevation, main block, weatherboards in porch, 3rd down from roof of porch, 1" west of west side of door architrave

Cross-sections from the front door were also stained for binder analysis. As with the other exterior samples, most of the layers stained weakly positive for oils with the fluorochrome stain DCF. The early layers have a pink positive reaction color for saturated lipids and aged oils. The later layers have a yellow positive reaction color for unsaturated lipids or less aged oils.

Sample WF44, north elevation, front door, upper west panel, underside of raised panel, on right side

Cross-sections from the front door were also stained with the fluorochrome stain Alexafluor 488 that marks out proteins, but there was no obvious reaction. However, as with the other exterior samples, the samples from the door stained positive for carbohydrates with the fluorochrome stain TTC, as shown by a dark red reaction color throughout the sample.

Sample WF44, north elevation, front door, upper west panel, underside of raised panel, on right side

Results of Pigment Identification with Polarized Light Microscopy

A few colored particles were detected in some of the early layers in some cross-sections. However, the very low concentration of colored particles made it difficult to isolate them for pigment analysis. The first layer in samples from the weatherboards, windows, and elements of the porch were isolated and five dispersed pigment samples were analyzed. By examining the uncast samples under a binocular microscope an effort was made to collect some of the colored particles for the analysis in each of the dispersed pigment samples. Nonetheless, in two of the five dispersed pigment samples only chalk and lead white were detected. This indicates that the concentration of colored pigment particles is indeed low. The red and yellow particles visible in cross-section were identified as red and yellow ochre. The black particles present in some of the layers were more difficult to isolate and may be lampblack or grime. Since the paint has a uniform grayish cast, the black particles are probably intentionally added lampblack pigment.

Dispersed pigments from sample WF56, north elevation, front porch, west pilaster capital, cavetto of astragal, east side 1" from south of corner

The generation one dark gray finish coat from the front door was found to contain lead white, chalk, and lampblack pigment. These are all common fairly inexpensive pigments that are not particularly useful in dating this layer.

Dispersed pigments from sample WF44, north elevation, front door, upper west panel, underside of raised panel, on right side

Results of Color Measurement

The Finnie house is currently painted with a multi-colored scheme, as shown in the color swatches below. For the sake of comparison the colors of the current paint scheme were measured. The weatherboard paint had a CIE L*a*b* value of L* = 64.81, b* = +0.19, and a* = +26.19. The exterior trim had a CIE L*a*b* value of L* = 78.07, b* = +1.29, and a* = +21.87. The current door paint had a CIE L*a*b* value of L* = 31.30, b* = +4.41, and a* = +4.78.

To determine whether some elements on the exterior were indeed picked out in a different color in the eighteenth century, several color measurements were taken from the first-generation paint in uncast samples from the weatherboards, windows, and elements of the porch. The paint on all the elements was found to be fairly close in color. Before light bleaching, none of the averages of the first-generation paint measurements in any of the samples was different by ΔE value of more than 6.00. After light bleaching, the greatest difference between the average measurements of any two first-generation paints was no greater than ΔE value of 4.23. (Generally, ΔE value of around 2 to 4 is considered acceptable in the color printing industry for standard color deviation.) This slight discrepancy in color seems more likely to be due to natural variation in hand-ground paint and aging conditions than to an intentional color difference. Thus, this examination of the first-generation paint on the exterior of the Finnie House suggests that the house was originally painted with a monochromatic scheme quite unlike the current multi-colored scheme.

All the exterior woodwork at the Finnie house appears to have originally been painted a warm, light gray with the doors dark gray. Before light bleaching eight color measurements were taken from the firstgeneration paint in the uncast samples and a sample from one of the pilaster capitals on the porch was found to have the closest measurement to the average. The paint had a CIE L*a*b* of L* = 69.47, b* = -0.09, and a* = +13.66. Under magnification with a color-corrected light source, the best commercial paint match to this degraded, unbleached paint was Sherwin Williams 2072. This modern paint has an L* value of 70.81, a b* value of -0.02, and a* value of +12.97. This is different from the color of the degraded first-generation paint on the pilaster capital of the porch by ΔE value of 1.51 which is very good.

Sherwin Williams 2072, best commercial match to aged, first-generation paint before light bleaching

After exposing the samples to sunlight in an east-facing window sill for more than six months, the average of all measurements of the first-generation paint had changed by a Δ value of 5.06, becoming lighter and less yellow. The light bleaching exercise was conducted to compensate for any darkening caused by the degradation of the oil binder. All the measurements from the light-bleached samples were averaged and the closest measurement to the average was from a sample taken from a window frame. That sample had a CIE L*a*b* of L* = 71.60, b* = -1.20, and a* = +10.40. Under magnification with a color-corrected light source, the best commercial paint match for the light-bleached, first-generation paint is Martin Senour CW 819 "Pelham Gray Light" from the Williamsburg Color Collection. This paint has an L* value of 71.04, an a* value of -0.68, and a b* value of +9.35 which is different from the lightbleached original paint by a low Δ value of 1.53.

Martin Senour CW819, best commercial match to aged, first-generation paint after light bleaching

The image below shows an uncast portion of sample WF56 from a porch pilaster capital which had a good accumulation of the generation one warm light gray paint. The sample shown below is upside down so the first-generation paint is on top. To the right of the original paint is a chip of the commercial color match for comparison with the light-bleached original paint.

Sample WF56 from the north elevation, front porch, west pilaster capital, cavetto of astragal, east side 1" from south of corner
Uncast sample in reflected visible light after light bleaching, 40x magnification

The generation one dark gray finish paint applied to the front door of the Finnie house was also examined. Since this paint is very worn and fragmentary, there was not a large enough sample to use the colorimeter microscope for measurement. Instead, the dark gray paint was matched under magnification with a color-corrected light source to a commercial paint. The best commercial paint match found was Benjamin Moore "Midnight Dream" BM 2129-10. This paint has a color value of L* 23.72, a* -0.11, b* -1.58 in the CIE L*a*b* system. When the door is repainted a glossy paint should be used since the cross-sections showed evidence of a plant resin varnish over the dark gray finish coat. This dark gray paint could be extrapolated to the other doors on the house and possibly the shutters.

Benjamin Moore 2129-10, best commercial match to first-generation door paint

Conclusion

This examination of the exterior finishes on the Finnie house involved analysis of sixty-two samples from various elements on the north and south elevations. These samples were used to understand the comparative stratigraphy of the finishes on the exterior over time. In the vast majority of samples from the exterior trim it was found that the earliest finishes were six generations of similarly colored cream or light gray paints. The same paints were found on all the exterior elements, except the door, so the house appears to have had a monochromatic color scheme originally, unlike the current two-tone system.

When pigment identification with polarized light microscopy was undertaken on the first-generation warm light gray layer, it was found to be pigmented mostly with lead white and chalk. Some red and yellow ocher particles were also detected in small amounts. Black particles are visible in this layer in crosssection, but were difficult to isolate for pigment analysis. The black particles are either embedded grime or particles of lampblack added to intentionally darken the paint. Fluorochrome binding media stains also suggest that the early layers have oil binders, perhaps with carbohydrate components.

In both generation one and two the front door was painted with a cream-colored base coat, a dark gray finish coat, and a plant resin varnish. Thus, the front door originally had a glossy dark gray surface. The dark gray paint in generation one was found to be composed of lampblack and lead white in an oil binder with some chalk particles.

On top of the six earliest generations of paint there is a characteristic white paint layer that contains zinc white pigment (identified by SEM-EDS and fluorochrome staining). This presence of this pigment dates this layer to after 1845. Since the early paints are all very similar the zinc white-containing layer in generation seven was useful in aligning the stratigraphies between samples.

A few elements were missing some of the early layers, which may suggest that they were painted less often, lost some layers due to wear or weathering, or are of a later date. The weatherboards on the south elevation and the rear door to the east wing only have three cream or light gray paint generations before the zinc white layer was applied. Perhaps the rear of the house was painted less often than the front. The rear door to the main block of the house has an abbreviated stratigraphy that suggests modifications were made to this entrance when the rear wing was either added or removed. The front door frame and the frame of the rear door to the east wing have a gray paint in generation seven that has some red pigment particles. This colored paint is found only on the door frames and the weatherboards enclosed by the front porch. At the same time, it appears that the doors were given a decorative treatment, probably a faux grained scheme.

One of the most interesting findings is that the front porch is indeed contemporary with the rest of the house, or at least was added before the first generation of paint was applied, as its finish history matches that of other elements on the exterior with six early cream or light gray paint generations. The relative stratigraphy of the porch and other elements on the front elevation was determined with cross-section microscopy and SEM-EDS. Cross-section microscopy also indicates that the transom over the front door is original since it has the same finish history as the rest of the door elements.

A few samples from the porch yielded some unusual paints. The weatherboards enclosed by the porch have two dull yellow paints in generations eight and nine that were not found on any other element. It could be that these paints only survived in this sheltered area, or that the porch enclosure was picked out in a contrasting color in this period. A sample taken from near the ceiling of the porch has a few unusual layers, as well, including some finishes that may be limewashes, and a light blue paint applied 47. before generation eleven.

Commercial paint matches were generated for the first-generation, warm light gray paint on the exterior trim before and after light bleaching that can be used in repainting the house. Although there was very little evidence remaining of the first-generation paint on the front door, a commercial paint match was found that is a good match to the dark gray finish coat. On the door, a glossy paint should be applied since the cross-sections show evidence of a plant resin varnish over the dark gray finish coat. No samples were taken from the rear doors, but the generation one dark gray paint finish was presumably applied to the other entrances as well and could be extrapolated to the shutters.

Cross-section Preparation Procedures

The samples were initially examined with a stereomicroscope under low power magnification (5 to 50 times magnification) and divided as needed. When possible, a portion of each sample was kept in reserve for future analysis and a portion cast in a labeled cube of a commercial two-part polyester resin manufactured by Excel Technologies, INC. (Enfield, CT). The resin was cured under an incandescent lamp for several hours. The resin cubes were then ground on a motorized grinding wheel with 400 grit sandpaper to reveal the cross-sections. Final finishing was achieved using a Buehler Metaserv 2000 grinder polisher equipped with abrasive cloths from Micro Mesh, INC. with grits of 1500 to 12,000.

Cross-section microscopy analysis was performed using a Nikon Eclipse 80i microscope equipped with an EXFO X-Cite 120 Fluorescence Illumination System fiber-optic halogen light source. The cross-sections were examined at magnifications of 40x, 100x, 200x, and 400x using reflected visible light and a UV-2A fluorescence filter cube with a 330-380nm excitation. The cross-sections were photographed digitally using an integral Spot Flex digital camera with Spot Advanced (v. 4.6) software. The light levels of the images were adjusted in Adobe Photoshop CS2. The color on the digital images is somewhat indicative of the actual color of the paints, but cannot be used for color matching as the printing process can cause color shifts.

Under ultraviolet light many materials have characteristic autofluorescence colors that can suggest their composition. For example, most natural resin varnishes have a bright whitish autofluorescence while oil varnishes tend to be darker in ultraviolet light. Visible light microscopy can also yield valuable information. The presence of soiling layers or weathering can indicate that the finish layer existed as a presentation surface for a period of time. Since many interior finishes, such as faux graining, make use of a predictable sequence of layers, it is important to determine which layers were meant to be final presentation surfaces.

Scanning Electron Microscopy Procedures

SEM-EDS analysis was used to determine the number of early layers and their elemental composition. Before analysis the cast cross-sections were sputter coated to reduce charging. A Hummer 6.2 Sputter Coater located in the College of William & Mary Applied Research Center Laboratory was used and operated by Laboratory and Research Technician, Olga Trofimova. The samples were coated with a goldpalladium mixture approximately 15nm thick layer. The SEM-EDS analysis was executed with a JEOL JSM-6060LV variable pressure scanning electron microscope with energy dispersive x-ray spectroscopy Thermo Electron System and a Pioneer detector. The machine was operated by Brandt Robertson, Lab Specialist with Old Dominion University Applied Research Center Laboratory. This machine was used to create backscattered electron images of the cross-section with the relative densities of the elements visible by the brightness of the layers. Elements with high atomic numbers appear bright on a backscattered electron image and lower atomic number elements dark. Mainly organic layers, such as varnishes, will appear dark in the SEM-EDS so their presence was suggested at as well. Elemental analysis was also done in specific locations and as a line scan running across the cross-section. This SEM-EDS is able to see elements as light as carbon. All the analysis was run at an accelerating voltage of 15kV.

Binding Media Analysis Procedures

To better understand the composition of the paint binders, selected cross-sections were stained with biological fluorochrome stains to indicate the presence of carbohydrates, proteins, oils, and zinc in the paint binders. The stains used were ALEXA (0.1% w/v Alexafluor 488 in dimethylformamide brought to 54. a pH of 9.0 with 0.5% borate) which marks proteins bright green, TTC (4% w/v triphenyl tetrazolium chloride in methanol) which marks carbohydrates dark red-brown, DCF (0.2% w/v 2,7 dichlorofluorescein in ethanol) which marks saturated lipids pink and unsaturated lipids yellow, and TSQ (0.2% w/v N-(6- methyl-8-quinolyl)-p-toluenesulfonamide in ethanol) which marks zinc (Zn²⁺) blue-white.

Pigment Identification Procedures

Samples with good accumulations of early paints identified through cross-section microscopy were scraped with a scalpel under magnification to reveal the target paint layer. A small amount of this layer was then scraped onto a glass microscope slide, dispersing the pigments. The dispersed pigments were permanently embedded under a cover slip in Cargille Meltmount (Cargille Labs., Cedar Grove, NJ). The Meltmount used has a refractive index of 1.662. The prepared slides were then examined under the microscope with transmitted visible light using a polarizing filter at a magnification of 1000x with an oil immersion objective. The morphological and optical properties of the pigment particles was observed and compared to reference pigment samples.

Color Measurement Procedures

Color measurements were made using uncast samples that were selected under magnification. An effort was made to find a clean, unweathered ares for measurement whenever possible. All color measurements were made using a Minolta Chromameter CR-241 with a measurement area of 0.3mm for uncast samples and a measurement area of 1.8mm for paint swatches. This microscope has an internal, 360° pulsed xenon arc lamp and can measure color with five color systems. The color system used in this report is the CIE (Commission International de l'Eclairage) L*a*b* color system. In the CIE L*a*b* color system L* represents lightness from 1 to 100 with 100 being the lightest, a* represents red to green with positive numbers being more red and negative numbers more green, and b* represents yellow to blue with positive numbers being more yellow and negative numbers more blue. This system was adjusted from the CIE Yxy system developed in 1931 to better represent the human eye's sensitivity to color. Color measurement values are compared to each other using the Δ formula which calculates the difference between two color measurements.¹ The Δ formula equals the square root of the sum of the differences in the L* value squared, the differences in the a* value squared, and the differences in the b* value squared. Generally, Δ value of less than two represents colors that are difficult for the human eye to differentiate.

Commercial color matches were found using a database of CIE L*a*b* values of paints from Benjamin Moore, Sherwin William, Martin Senour, and Pittsburgh Paints that calculates Δ values. The matches with the lowest Δ values were compared to the uncast sample under magnification with a color-corrected light source to choose a final match.

Color Measurement Data

First-Generation, Warm Light Gray Exterior Trim Paint
Before light Bleaching
Date: 11/1/2007

Color Matches

Photobleaching
Date: 2/3/08

Photobleaching
Date: 4/28/2008

Photobleaching
Date: 5/19/2008

Photobleaching
Date: 6/13/2008

Color Matches

First-Generation Exterior Dark Gray Door Paint Match

Scanning Electron Microscopy Data

Accelerating Voltage: 15.0 kV Magnification: 60

Accelerating Voltage: 15.0 kV Magnification: 130

Accelerating Voltage: 15.0 kV Magnification: 43