The use of multi - channel visible spectrum imaging for pigment identification

Many paintings require visual compensation for losses in their paint film, and for many conservators the goal is to treat these losses so they are indistinguishable from the surrounding undamaged surface when viewed under typical museum conditions. In order to achieve this level of reintegration, the inpainted area should have nearly identical optical properties. Specifically, there should be close matches in spectral (colour and transparency) and geometric (gloss, texture and impasto) properties. The conservator controls these properties through the choice of fill material, pigments, binder, varnish and application technique. Occasionally, there are limited options in selecting the binder and method of application, particularly when the painting is not varnished or the paint film is sensitive to solvents, although there can be a variety of pigments capable of matching a specific colour. The choice of pigments has a dramatic effect on whether the treated area is indistinguishable in colour. An indiscriminate selection can result in severe metamerism. Recently, a new technique has been developed for pigment selection for inpainting. A small-aperture, portable spectrophotometer is used to measure the spectral reflectance properties of similarly pigmented adjacent areas. Using a predetermined database of pigments, also evaluated spectrally, a set of pigments are selected that when combined result in a close spectral match with minimal metamerism (Berns et al. 2002). The technique is a simplification of instrumentalbased colour matching (Berns 2000), practised routinely in the paint, plastics and textiles industries, among others. The specific mathematics is often referred to as a ‘spectral-matching algorithm’. The simplification involves using single-constant Kubelka-Munk theory and limiting the wavelength range such that differences in the absorption properties of white pigments (e.g. titanium, zinc or lead white) do not influence the spectral-matching outcome. Kubelka-Munk theory is a turbidmedia theory that considers light travelling in only two opposite directions. A colorant is characterized by its absorption and scattering properties as a function of wavelength. The pigment-selection technique has been used successfully at the National Gallery of Art in Washington D.C., which selects pigment mixtures for difficult inpainting where any colour (and spatial) mismatch is readily visible. The Gallery does so because the paint losses are large and correspond to uniform regions of colour. Examples include Dionysius by Barnett Newman (1949, gift of Annalee Newman, in honour of the 50th anniversary of the National Gallery of Art) and Siout, Egypt by Sanford Robinson Gifford (1874, New Century Fund, gift of Joan and David Maxwell, National Gallery of Art). Minimizing metamerism was a critical restoration requirement. During development of the pigment-selection technique, a pilot experiment was performed. Green paint-outs were prepared containing two coloured pigments and titanium white dispersed in polyvinyl acetate. The coloured pigments were selected from a database of five green, two yellow and three blue pigments, many with quite similar spectral characteristics, particularly in the long-wavelength region of the visible spectrum. The paint-outs were measured spectrally and The use of multi-channel visible spectrum imaging for pigment identification