The problem of reproducing the synthesis of metal nanoparticles is well known among practitioners of modern nanoscience, even if it is not often mentioned in the literature (see, e.g., Moshofsky and Mokari, Chem. Mater., 2013). The poster child for such difficulties should probably be the gold-based pigment, Purple of Cassius. Known since the 17th century, Purple of Cassius' identity as a nanoparticulate gold hydrosol supported on tin oxide was not universally accepted until the work of Richard Zsigmondy in the 1890s. Zsigmondy, in turn, recognized that his results basically reaffirmed the conclusions of Michael Faraday, who with the aid of light-scattering experiments had deduced the same in 1857. Remarkably, eighty years before Faraday's work, Pierre Joseph Macquer wrote in the Dictionnaire de chymie (Paris, 1778) that the color of Purple of Cassius was due to gold that was finely divided (�extr�mement divis�). Up until the time of Zsigmondy, however, many chemists, including J. J. Berzelius, held an alternate view: Purple of Cassius was considered to be a compound, probably derived from a gold oxide with tin oxide. The latter interpretation was not unreasonable, as the pigment failed classical chemical tests for gold (e.g., the gold was not extracted with mercury). The fact that the exact conditions under which the pigment was formed led to variations in its color made the replication of experimental results difficult, further complicating its study. In this presentation, we examine the arguments put forth in the Dictionnaire de chymie in defense of the presence of metallic gold in Purple of Cassius, which given the nature of the material and the limitations of 18th century chemical theory, was a remarkably perceptive interpretation of the available data.

Hidden gold: The nature of Purple of Cassius in the <i>Dictionnaire de chymie</i>

246th National Meeting (2013)

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Hidden gold: The nature of Purple of Cassius in the Dictionnaire de chymie