Naphthol Green – a forgotten artists’ pigment of the early 20th century. History, chemistry and analytical identification

Abstract The synthetic nitroso dyestuff Acid Green 1 (hereafter AG1, C.I. 10020) and its lake (C.I. Pigment Green 12, hereafter PG12), today known under the trade name Naphthol Green B, was identified for the first time on six historical colour charts of commercial brands of artists’ paints. These comprise water and oil colours produced between 1910 and 1932 by the companies Winsor & Newton (London), G. Wagner (Hannover), Fr. Schoenfeld (Dusseldorf), Redeker & Hennis (Nurnberg) and Talens & Zoon (Apeldoorn). Because the use of AG1 (first synthesis in 1883) is well-known for textile dyeing, but not at all as a lake pigment for artists’ paints, these findings stimulated a comprehensive source research on the early production history, fastness properties and acceptance of the lakes of AG1 in the chemical and paint-technological literature from its invention to c. 1950. To summarize the findings, lakes of AG1 are regularly mentioned for paint purposes soon after the dyestuff's invention, mainly for house paints, paints for wall papers or as pigment for printing inks. In 1892, a lead lake of AG1 is described explicitly also for artists’ paints and a certain use in artists’ water colours, but also oil paints of higher quality could be proved until at least the 1930ies by the source research presented here. However, the pigment's fastness to light is unsatisfactory according to modern standards, and lakes of AG1, since 1956 denoted as PG12 (barium lake), are no longer of relevance in artists’ paints. It was presumably replaced by more stable organic green pigments such as those from the copper phthalocyanine group that have been introduced since the mid-1930ies. In this work, the non-destructive or minimally invasive analytical identification of AG1/PG12 in works of art is shown to be possible by Raman spectroscopy, SERS, FTIR and/or UV/VIS/NIR reflectance spectroscopy, accomplished by XRF for the lake substrate characterisation. Reference spectra of modern and historical specimens of AG1 (as lake and dye) produced in 1893 and 1900 are also presented here. Knowledge of the application and chronology of this today almost unknown pigment in art is of importance e.g. for dating purposes in authentication cases as well as under preventive conservation aspects, e.g. to define appropriate conditions for lightning conditions.