Historic paint analysis

Historic paint analysis is the scientific analysis of architectural finishes, including not only paints but also metallic finishes and clear and translucent finishes used on historic buildings. The primary purpose of such analysis is to determine the color of the finish used at a particular time in the building's history, usually the original construction, but not always. Secondary purposes include determination of ingredients such as media (water, oil, latex, etc.) and pigments (organic pigments, inorganic pigments, dyes, etc.). Paint analysis is also used at times as a dating technique for various building elements.
Typical problems encountered in historic paint analysis include such things as paint loss, surface deterioration, newer materials, substrates, delamination, media and pigment deterioration, and alligatoring.
History.
Historic architectural paint analysis finds its roots in the early twentieth century in the United States. The historic preservation movement began in 1849 with the preservation of Mount Vernon, the home of George Washington. Early preservationists began to realize that paints and finishes which had survived were very important but may not have been the original, or historic, finishes. Interest in historic wallpapers also developed with the interest in historic paint and color. One of the earliest endeavors came with the restoration of Williamsburg, Virginia funded by John D. Rockefeller in the 1920s. Early investigations by simple scraping of the finishes by Susan Nash of the surviving original buildings yielded a palette that became popularly known as Williamsburg colors. 
In the 1950s and 1960s, serious efforts at investigating original paint colors were underway at Independence National Historical Park in Philadelphia by architect Penelope Hartshorne Batcheler. Her pioneering efforts introduced, for the first time in this country, the use of a stereo microscope to more closely examine the 18th century paints at Independence Hall. Batcheler also introduced the use of the Munsell Color System for matching and referencing original paint colors. Her landmark publication, "Paint Color Research and Restoration", was the very first publication concerning the analysis of historic architectural paints for determination of original colors. At the same time, in the United Kingdom, microscopy of paint samples was developed by Joyce Plesters of the National Gallery, London who worked mainly with easel paintings but also with samples from wall-paintings.
In the 1960s and early 1970s, Morgan W. Phillips at the Society for the Preservation of New England Antiquities (SPNEA) became involved with historic paint and color analysis, specifically at the Harrison Gray Otis House in Boston. At the same time, E. Blaine Cliver, Historical Architect, who originally worked with Batcheler at the National Park Service (NPS) in Philadelphia, then with the National Trust for Historic Preservation in Washington, DC and later at the Northeast Regional Office of the NPS, became involved with historic paint analysis, especially in a laboratory in Building 28 of the former Boston Navy Yard. In the early 1970s, Frank S. Welsh joined the NPS in Philadelphia and began his research and study of historic paints with Penelope Batcheler, where he introduced the use of the National Bureau of Standards Color Name Charts, (NIST) for naming the colors matched to the Munsell Color System. Welsh also was the first to introduce the term "paint analysis" into the lexicon of historic preservation. As an independent historic paint color consultant one of his first major projects was Monticello, the home of Thomas Jefferson. In the mid 1970s, Matthew J. Mosca started working for the National Trust with Blaine Cliver. Later, as a preservation consultant, Mosca researched the historic colors of Mount Vernon.
The advances in the science of paint color research by these individuals suggested that the popular Williamsburg colors had been matched to faded and aged finishes. During the 1980s and 1990s, Colonial Williamsburg consulted with Welsh to undertake a comprehensive paint and color analysis on numerous buildings in the historic area. In his research, "the first modern scientific paint analysis" there, confirmed that the Williamsburg color palette did not represent the actual historic colors. In addition he found that in many cases their early efforts had mistakenly matched later paint layers, some nineteenth-century.
Historically, paint analysis was done on site by making exposure windows and carefully removing later paint layers to reveal a sequence of finishes down to the substrate. This was the methodology employed during the early restoration of finishes at Historic Williamsburg. Although this method is employed by a few practitioners, it is not common because of its inherent problems of misinterpretation and failure to address issues such as paint ageing and discoloration. Because finishes analysis is performed under laboratory conditions samples are collected in the field for later analysis and can be collected by the analyst or by his client who then ships them to him.
Laboratory analysis.
The primary purposes of analysis are to determine historic finishes and to determine principle components such as media or basic pigments. There are two methodologies in the preparation of paint samples for microscopic analysis. The first, which is derived from the medical world, is to treat the sample as a specimen and set it into a fixed position in a permanent medium such as paraffin. The specimen is then ground to a flat finish, providing a horizontal surface for viewing under a microscope. The second is to leave the samples in a loose condition with their broken surfaces which then can be manipulated under the microscope to permit a variety of views of the layers. The primary disadvantage to the first method is that the grinding process tends to blur layers together, especially layers of similar or identical colors. It also provide only a single, fixed point of viewing. The second method lacks these disadvantages, although skill and experience is required to manipulate the samples effectively.
Following preparation of the samples, they are typically viewed under an optical microscope using either natural north light or polarized artificial light simulating natural north light. North light is essential in order to render the colors accurately without the effects of the yellow spectrum of direct sunlight. Each individual layer is identified and, typically, matched to the Munsell color system. The Munsell color system is a scientific system in which colors have been ranged into a color fan based upon three attributes: hue or color, the chroma or color saturation, and the value or neutral lightness or darkness. Unlike color systems developed by paint manufacturers, the Munsell system provides an unchanging standard of reference which is unaffected by the marketplace and changing tastes in colors.
The hue notation, the color, indicates the relation of the sample to a visually equally spaced scale of 100 hues. There are 10 major hues, five principal and five intermediate within this scale. The hues are identified by initials indicating the central member of the group: red R, yellow-red YR, yellow Y, yellow-green YG, green G, blue-green BG, blue B, purple-blue PB, purple P, and red-purple PR. The hues in each group are identified by the numbers 1 to 10. The most purplish of the red hues, 1 on the scale of 100, is designated as 1R, the most yellowish as 10R, and the central hue as 5R. The hue 10R can also be expressed as 10, 5Y as 25, and so forth if a notation of the hue as a number is desired. Chroma indicates the degree of departure of a given hue from the neutral gray axis of the same value. It is the strength of saturation of color from neutral gray, written /0 to /14 or further for maximum color saturation.
Value, or lightness, makes up the neutral gray axis of the color wheel, ranging from black, number 1, to white at the top of the axis, number 10. A visual value can be approximated by the help of the neutral gray chips of the Rock or Soil Color chart with ten intervals. The color parameters can be expressed with figures semi-quantitatively as: hue, value/chroma (H, V/C). The color "medium red" should serve as an example for presentation with the three color attributes, 5R 5.5/6. This means that 5R is located in the middle of the red hue, 5.5 is the lightness of Munsell value near the middle between light and dark, and 6 is the degree of the Munsell chroma, or the color saturation, which is about in the middle of the saturation scale.