A Closer Look at Nineteenth Century Cochineal Lake Pigments Through Historical Recipes Reconstructions

• Main Author: Vitorino, Tatiana Mendes Ferreira
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Dyes; Paints; Recipes; Textile Research
• ISBN: 9798544213055

Cochineal lake pigments belong to an important class of lightparticular focus. Their fading results in undesirable changes in the artworks their identification is a challenging task. In this work, the Winsor & Newton (W&N) 19th Century Archive Database, a primary source on the practices of time, was investigated in relation to their manufacturing processes for cochineal lake pigments. Four hundred and forty-three (443) production records pertaining to Carmine (28%), Crimson (30%), Scarlet (22%) and Purple (20%) cochineal products were studied, which revealed the commitment that W&N made to the quality and durability of their pigments Two main methods of synthesis were identified, which difference lies on the pH of extraction. Seven selected representative manufacturing processes were reconstructed with as much historical accuracy as possible and the majority of the resulting pigments were successfully compared with historical samples. A multianalytical approach, which included Inductively Coupled Plasma Atomic Emission Spectrometry, microEnergy Dispersive X-ray Fluorescence Spectrometry, UV-Vis-NIR Fibre Optic Reflectance Spectroscopy, microspectrofluorimetry, microFourier Transform Infrared Spectroscopy, microRaman Spectroscopy, and Surface Enhanced Raman Spectroscopy, was used to characterise the reconstructions and historical samples. Different chromophores were detected, each presenting an elemental and molecular fingerprint that can be further characterised by an amorphous manufacturing processes. Experimental evidence suggests, for the first time, that the presence of aluminium and calcium in the molecular structure is essential to obtain organised complexes which correspond to bright carmine pigments, while the absence of calcium most probably results in a mixture of isomers of smaller complexes and purplish hues. Selected reconstructions and historical samples painted with gum-arabic and poly(vinyl acetate) were also subjected to an accelerated light ageing experience using a solarbox. These samples were investigated with conventional analytical techniques as well as with microfading spectrometry and Vis-NIR reflectance hyperspectral imaging, contributing to the demonstration of the potentialities of these relatively innovative techniques. Overall, the carmine references turned out to be the most lightfast when mixed with gum-arabic while the scarlet pigment with poly(vinyl acetate) showed a slightly smaller colour change when compared with the almost complete loss of colour of the other poly(vinyl acetate) paints. This work has provided an important breakthrough together with a valuable set of reference materials and spectral data for the further study of cochineal lake pigments, their photodegradation and identification in artworks.