Hidden in plain sight: revisiting the synthesis, characterisation, degradation and the intricate relationship between Scheele’s green and Emerald green

Abstract

Carl Wilhelm Scheele's notorious toxic pigment, commonly referred to as “Scheele's green” often resulted in a mixture of products with unknown chemical composition. Positive identification of the pigment has been limited to Raman spectroscopy and indirect analysis using FTIR and XRD methods. Despite these techniques, reported occurrences of the pigment in heritage samples are scarce, suggesting that Scheele’s green is rarely reported due to challenges in its characterisation rather than infrequent use. Regarding the degradation of Cu-As green pigments, common assumptions suggest dissociation in acidic pH conditions, generating mobile arsenic and copper ions, followed by oxidation to As(V), which can co-precipitate with Fe, Al, and Ca ions. This study reproduces the fabrication of Scheele's green using historical recipes, focusing on maintaining the pH of the arsenite solution at 9.3. The research explores its relationship with Emerald green, the challenges associated with their identification, and addresses common misconceptions about the degradation of such pigments. Maintaining the pH at 9.3 proved influential in obtaining a crystalline product with an intense Raman signal, aligning with the widely accepted spectra of Scheele’s green. However, Raman spectra from amorphous Cu-As samples consistently exhibited broad bands at 288 and 845 cm−1, prompting a proposed modification for a dual representation of the pigment: the “common” form with broad bands and the “uncommon” or crystalline form as reported in the literature. Demonstrating that the crystalline form shares nearly identical Raman and FTIR spectra implies an identical chemical composition to Trippkeite. Evidence presented highlights that Cu-As based pigments contain free copper, arsenite and arsenate ions prone to migration, challenging commonly described degradation pathways. The hypothesis presented here, that Emerald green synthesis may inadvertently yield small amounts of Scheele’s green urges caution in pigment identification using Raman spectroscopy. Additionally, the study reveals, for the first time, the occurrence of Scheele’s green in a book, with particles exhibiting a spherulite form, challenging identification of Emerald green solely based on morphology.