Historic Paint Analysis - Laboratory Analysis

Laboratory Analysis

Although there are many tests which can be undertaken on paint in a laboratory setting, only basic analysis will be discussed here. The primary purposes of basic analysis are to determine historic finishes and to determine principle componenst such as media or basic pigments.

There are two methodologies in practice at present 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 R. 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.