The spatially resolved characterisation of Egyptian blue, Han blue and Han purple by photo-induced luminescence digital imaging
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The photo-induced luminescence properties of Egyptian blue, Han blue and Han purple were investigated by means of near-infrared digital imaging. These pigments emit infrared radiation when excited in the visible range. The emission can be recorded by means of a modified commercial digital camera equipped with suitable glass filters. A variety of visible light sources were investigated to test their ability to excite luminescence in the pigments. Light-emitting diodes, which do not emit stray infrared radiation, proved an excellent source for the excitation of luminescence in all three compounds. In general, the use of visible radiation emitters with low emission in the infrared range allowed the presence of the pigments to be determined and their distribution to be spatially resolved. This qualitative imaging technique can be easily applied in situ for a rapid characterisation of materials. The results were compared to those for Egyptian green and for historical and modern blue pigments. Examples of the application of the technique on polychrome works of art are presented.
KeywordsArcheometry/fine arts Fluorescence/luminescence Egyptian blue Han blue/Han purple Imaging Infrared
This study was partially sponsored by the Andrew F. Mellon Foundation and was made possible thanks to the invaluable collaboration of the scientists, conservators and curators of the British Museum, the Courtauld Institute of Art and the National Gallery in London, particularly David Saunders, Janet Ambers, Catherine Higgitt, Tracey Sweek, Michelle Hercules, Thorsten Opper, Peter Higgs, Sharon Cather, Maram Na'as and Marika Spring. Dr Emily Murray kindly provided a sample of shellfish lake. Pro-Lite Technology LLP kindly provided samples of the Spectralon® references.
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