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Raman spectroscopy on pigments from 17th century portrait miniatures
The art of painting portrait miniatures developed from mediaeval book illumination techniques during the seventeenth century. Portrait
miniatures are painted in watercolour on thin parchment or on ivory, protected by a cover glass and enclosed in a locket. Miniatures were originally used as diplomatic tokens of the royal courts over Europe. In the mid seventeenth century the art was introduced to the Swedish court by Queen Kristina and soon became a much appreciated art in the country. As of today the National Museum of Fine Arts in Stockholm holds one of the largest collections in the world, counting 5000 pieces.
The court miniaturist Elias Brenner was active in Stockholm during
the
end of the seventeenth century. In 1680 Brenner wrote a document on
pigments recommended for miniature painting, containing actual
samples of each pigment. This rather special primary source offers a
unique possibility for cross disciplinary research. By analysing the
chemical composition of the samples in the document the historical
references on nomenclature can be identified scientifically.As part of a research project by conservator Cecilia Rönnerstam at National Museum of Fine Arts in Stockholm the samples are currently being analysed at K-Analys AB using Raman spectroscopy. The Raman technique has found wide use in analysis of museum objects, the main reason being that no destructive sampling is needed from the original object and therefore the artefact under investigation is not damaged.
The instrument used for this project is an inVia Raman microscope provided by Renishaw and fitted with two lasers; an Ar-ion laser at 514 nm and 20 mW and a diode laser at 785 nm and 300 mW. A grating with 2400 grooves per mm is used for 514 nm and 1200 grooves per mm for the 785 nm. Spectra from the samples are scanned continously within 100-2000 cm-1, except for a few cases which require 100-3500 cm-1. The objective in use is a x50 NA 0.45 Leica, mounted on a telescope extension tube fixed to an automatic stage in horizontal position. Using a custom fit holder, the document with the pigments can be held in an almost vertical position in front of the microscope. The laser focus, which can be seen via a video image, can be adjusted by moving the stage in x/y direction.
At the moment of writing the research project is ongoing. Preliminary results, however, show findings of traditional mineral pigments like lead white, vermilion, red lead, indigo, lead tin yellow and orpiment.