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Authors would like to mention that aditional experimental details, spectra and pictures are available from the corresponding author on reasonable request.
This article discusses the discovery of phosphorescent lithopone on watercolor drawings by American artist John La Farge dated between 1890 and 1905 and the history of lithopone in the pigment industry in the late 19th and early 20th centuries. Despite having many desirable qualities for use in white watercolor or oil paints, the development of lithopone as an artists’ pigment was hampered by its tendency to darken in sunlight. Its availability to, and adoption by, artists remain unclear, as colormen's trade catalogs were generally not explicit in describing white pigments as containing lithopone. Further, lithopone may be mistaken for lead white during visual examination and its short-lived phosphorescence can be easily missed by the uninformed observer. Phosphorescent lithopone has been documented on only one other work-to-date: a watercolor by Van Gogh. In addition to the history of lithopone's manufacture, the article details the mechanism for its phosphorescence and its identification aided by Raman spectroscopy and spectrofluorimetry.
Lithopone is chemically inert and practically insoluble in acids, alkalis and solvents. The optimized particle-size distribution of Lithopone attained by means of co-precipitation and calcining permit the achievement of a high apparent density, which imparts to Lithopone its low resin requirement and its excellent rheological behaviour.
