Gouache, from the Italian guazzo, is a method of painting with opaque colors ground in water and thickened with gum and honey. The Head of Conservation at The Bergen Museum of Art, MSc. Yngve Magnusson, was so kind to send me over a Winsor and Newton Color Chart of Designer’s Gouache to be tested by Multispectral Imaging. I thought it useful to share this information that complements the previous post on historical pigments. Fortunately, Winsor & Newton provide detailed notes on the pigments in this color chart, here.
The following multispectral imaging documentation was done as described in the just mentioned post. The only difference is that the reference swatches zinc white and cadmium red are put aside on the far left.
Some highlights from the Ultraviolet Fluorescence imaging: overall this pigments are pretty dull but some pigments such as Bengal rose show a mild red fluorescence (indeed it contains a fluorescent dye).
More useful is the UV Reflected Imaging. Indeed, Cerulean blue and synthetic ultramarine mildly reflect UV. But cobalt blue is a strong UV reflector. This turns useful for its identification.
Looking at the Infrared Photo I was caught by the IR absorbency of Indigo which in my collection is actually IR transparent. Indeed, The indigo in the color chart is not a real indigo as stated here.
The Indigo swatch looks suspicious also in the infrared false color where it should be red instead of pitch black.
The cadmium pigments (red, orange, yellow) in this color chart look intensely IR fluorescent as expected.