Shades of violet: Study of the compositional variability of historical Methyl violet dyes

Bright violet-coloured aniline dyes were developed and produced from the late 19^th century onwards. Primarily used for dyeing of textiles, they were also employed in inks for letters and drawings. The industrial production of these dyes comprised numerous companies in Germany, France and England and concurrent chemical synthesis methods rapidly evolved. ‘Methyl violet’, also known as ‘Violet de Paris’, is an example of an aniline dye that made a large impact in Europe and was widely used. This study aims to provide insight into the composition of historical ‘Methyl violet’ dyes to gain a better understanding of the formulation of past inks, their degradation and to support colour reconstruction of old drawings and manuscripts. Thirty-nine ‘Methyl violet’ reference samples, dating from the late 19^th century until the 1930′s, originating from leading dye producers in Europe, were analysed. UHPLC-PDA-HRMS was applied for the identification of the dye compounds. It was revealed that a significant part (ca. 35%) contained not only methylated, but also benzylated pararosanilines. Although benzylated compounds are mentioned in historical sources as ‘Methyl violet’, ‘Violet the Paris’, and ‘Methyl violet 6B’, they have only once been reported in modern literature and are now elucidated in detail for the first time. The relative amount of highly substituted pararosaniline increases when benzylation is performed resulting in an overall “bluer” dye mixture. The benzylated Methyl violets were mostly found in ‘Methyl violet 6B’ samples, but not exclusively. This exploration of the composition of a large number of historical ‘Methyl violet’ samples highlights the complexity and compositional variety of early synthetic dyestuff productions and the importance of large-scale studies of historical references.