Historic paint analysis is the scientific identification of finishes applied to architectural features at some point in time identified as being historic. This might be the original construction, as is often the case, or some subsequent period in time, usually associated with the life of a historic person or a particular event (e.g. the signing of the Declaration of Independence). It is not limited to paint only, but includes a wide array of other finish types such as varnishes, shellacs, metallic leafs, etc.
Historic paint analysis can be used to determine a large amount of information regarding finishes. This includes information regarding surviving colors, original colors, sheen, lead content, finish type, media employed, pigments, application techniques, and weathering characteristics. Typically, however, color determination is the goal of the analysis. There is a common misunderstanding that the surviving color is exactly the same as the original color and that matching that color will result in a correct restoration. Unfortunately, this is infrequently the case. Most paints do change over time in the following ways:
Fading. The more intense the pigment the more likely fading will be an issue. For example, certain red pigments will fade to the point where they disappear entirely. On the other hand, certain inorganic pigments, include some red pigments, are no affect by fading.
Color shifting. Certain pigments will change color over time. For example, Prussian blue will change from an extremely strong green-blue to a softer and lighter blue within a relatively short time. One significant question that should be addressed in these cases is whether the original design intent was for the initial color or the shifted color.
Yellowing. Linseed oil, among several media, will yellow over time, shifting the color toward the yellow end of the spectrum. This is particularly marked in white paints resulting in the question as to whether the color should be restored to an originally bright white or to an aged, mellower appearance Chalking. Lead based paints were often formulated to chalk over time and thereby create a rough surface to which later paints would readily adhere. This change in sheen poses a dilemma in restoration because modern paint do not chalk, thereby leaving the choice to recreate a new, eggshell or glossy finish, or a flat finish resembling a chalked surface.
The purpose, then, in historic paint analysis, usually, is to recreate the original color and sheen of the historic finish in question. Typically, this is accomplished with modern materials and with modern methods. Therein lies the challenge.