Historic Paint Analysis

Prime Testing Group Ltd. offers the testing services of Historic Paint Analysis:

Paint analysis is about determining first, the number of coatings (including primer and finish coats), the original colors used in a building (often recorded in notations of the Munsell Colors System, a standardized color identification system used commonly in the paint industry), and the types of coatings (i.e. oil or water-based paints, stains, glazes, varnishes, or wallpapers). Analysis can also reveal which colors and paint types are used where in the building, the presence of decorative painting (i.e. graining, marbleizing, stenciling), the physical characteristics (i.e. gloss, texture), and the appropriate date or period of each layer.

Under a laboratory microscope, the analyst records all the layers and matches them to a system of color notation (usually the Munsell Color or the Plochere Color Systems). Through microscopic examination, the analyst can also identify special finishes, such as graining and marbleizing, and can differentiate between primers and finish coats.

A stereomicroscope enables the analyst to count the number of coating layers seen in cross-section, and is also useful to measure relative paint thickness and for observing accumulations of grime and soot between layers of finish paints. A stereomicroscope is especially well suited for finding sample areas that are the least discolored and, therefore, best for color matching.

Another instrument used by the paint analyst is the polarized light microscope. It is used to analyze and identify the pigments used in a paint film, in micro-chemical media tests, or to analyze wallpaper fibers found during investigations. The polarized light microscope is also used to photograph pigment particles or crystals found in a preparation. The cross-sections of paint are often embedded in a "tablet" of acrylic resin. When wet and covered with a cover slip, the cross-section is ready for photography using the top light. Polarized light microscopy (PLM) techniques allow for the identification of different pigment particles based on the characteristics of particle shape, color, refractive index, and optical properties. If PLM requires further confirmation, energy-dispersive x-ray analysis (EDX) may be carried out using a scanning electron microscope.

Ultraviolet light microscopy is also used, especially to identify media. Illumination of different materials with ultraviolet light causes them to auto-fluoresce, or glow, with characteristic colors. For example, shellac will fluoresce orange or yellow-orange, and some plant resins (amber, copal, sandarac, and mastic) fluoresce bright white. Exposing the paint layer to long-wave ultraviolet light can usually reduce the discoloration of oil media. This effect is called “bleaching”. The length of exposure time required varies with different paints, and can be difficult to measure. Often a portion of the sample is covered with an opaque material, which is moved slightly every 24 to 48 hours, until the coloration differences between the sections become negligible.

Chemical tests can be done to determine the actual pigment, or pigments, used as well as to determine paint media. For example, the presence of lead in paint samples is tested for by applying a tiny amount of diluted sodium sulfide reagent through a micropipette onto a specific paint layer or onto all layers in a cross-section. Any black sulfide discoloration reactions that occur clearly indicate which layers contain lead and which do not.

All of the information is then organized in a paint seriation chart, which describes the chromo-chronology, literally the color history of the building. Finally, the paint analyst will recommend the type of modern paint, which may be used to create the correct surface and texture of the original or, in special cases; it may be desirable to mix the paints according to historic recipes, or to increase the paint’s texture through the addition of sand.