A closer look at brazilwood and its lake pigments

Abstract

Brazilwood is a soluble redwood that was used as source of organic dye and for lake pigments. It was widely applied in medieval times to colour textiles and in the form of pigment for manuscript illumination and, although it is relatively unstable, it is still found in the 19th oil palette. However, this colorant has received little attention. In this sense, the present work aims to contribute knowledge on the characterisation of brazilwood and its lake pigments. The flavonoid brazilein is the red chromophore that can be extracted from the brazilwood tree. To study with more detail its behaviour, the molecule was isolated from brazilwood scrapes. As confirmed by NMR and HPLC-DAD, a high purity compound was obtained (for which a pKa1 of 7 was determined) and a pure brazilein-Al3+ complex was produced. Characterisation with UV-Vis spectroscopy and μ-spectrofluorimetry revealed that in its acidic form, the chromophore presents absorption, excitation and emission maxima at 446, 475 and 536 nm, respectively, and at 540, 548 and 560 nm when deprotonated. When complexed with Al3+ these values are changed to 510, 520 and 585 nm. Brazilwood lake reconstructions with as much historical accuracy as possible were also prepared based on recipes from the 15th century documentary text Livro de como se fazem as cores and the Winsor & Newton 19th century archive. This led to a greater understanding of the manufacture of the lakes and its evolution through time. Furthermore, in order to investigate the best strategy to characterise such pigments and paints, and to provide reference samples, the lakes produced and applied as temperas were then analysed by colorimetry, FTIR, SEM-EDS, FORS and UV-Vis μ-spectrofluorimetry. Despite some differences exist between the two sources, mainly with respect to the dye extraction method, both are guided by the same principles and are constructed in order that the same final pH, adequate for pigment precipitation, is obtained. In particular, the 15th century Livro de como se fazem as cores, so distant in time, allowed for the successful preparation of brazilwood lake pigments, which can be obtained in a variety of different hues from light pink to dark red and purple. Concerning the analysis carried out, FTIR was essential to characterise extenders, while FORS spectra (λabs 560 nm) and μ-spectrofluorimetry (λex 553 nm for excitation and λem 585-90 nm for emission) data were very consistent and fundamental to identify the brazilwood chromophore. These three techniques were essential and complementary in the characterisation of the paints produced. Medieval reconstructions were compared with pink and red colours of illuminations found in three French books of hours (Palácio Nacional de Mafra, Portugal) from the 15th century. Good correlation between FTIR, FORS and μ-spectrofluorimetry spectra of the reconstructions and historic samples suggest that the illuminations colours were prepared with brazilwood lakes.