The Science of Conservation : Colorimetry

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CCI Newsletter, No. 28, December 2001

The Science of Conservation : Colorimetry

by Nancy Binnie, Conservation Scientist, Conservation Processes and Materials Research Division

The conservation assessment of a heritage architectural building often includes a condition assessment of its interior elements including wood, plaster, metal, tile, and marble, as well as documentation of paint and surface finishes. Such assessments may also involve documentation of original colour or gloss, as well as recommendations on the selection of materials that can replicate the colours or finishes of original materials.

The simplest method of assessing colour is visual colour matching, but the accuracy of this method may be limited by the viewing conditions, sample size, background/surround colour, level and type of illumination (fluorescent, incandescent, or daylight), directional characteristics of the surface (texture, gloss), and skill of the viewer. Visual colour matching can be further complicated with original surfaces that have only a very thin paint layer, or have a texture which makes isolation of one coloured layer very difficult. In addition, because paint components have a tendency to change colour as they age, the colour that is seen likely differs from the original colour.

The use of a spectrophotometer can eliminate many of the problems associated with visual colour matching for painted surfaces, and on-site measurements with a portable machine can be almost as quick. A small window' is first opened by sanding or cutting down to the original or desired paint layer, and then the colour of a circular 4-mm-diameter area is measured with the spectrophotometer. The process is completed by repainting the window to match the current colour.

The spectrophotometer provides documentation of the colour values (Munsell numbers and CIEL*a*b* coordinates). The closest colour match to a modern commercial paint or custom-mixed paint (in whatever brand line has been specified by the client) can then be selected. But machinery cannot totally replace the human eye, and a visual colour match should also be made using ambient as well as alternate light sources (when available).

The use of a spectrophotometer combined with visual colour matching provides the best possible match to the original colour.