Síntese, identificação dos produtos de degradação e importância do pigmento azul da Prússia para o patrimônio cultural no Brasil

Abstract

This work carried out extensive bibliographic research in sources from the 18th century to the present day regarding the pigment Prussian blue. This pigment was synthesized using the methodology described in the English treatise The Handmaid to the Arts Teaching, v.1 and 2 (1758) and modern methods. The first syntheses of Prussian blue published at the beginning of the 18th century were based on five main reagents: animal matter, alkali, alum, Mars vitriol, and spirit of salt. To achieve reproduction closer to that which was carried out at that time, natural ox blood, pearl ashes, alkali obtained in this work from wood ash lye, and an iron vessel during the calcination step were used, which was not reported in any other work consulted in the literature. In the synthesized pearl-ash samples, the compounds K2CO3, KHCO3, K2SO4, K2O, KCl, and SiO2 were characterized by infrared spectroscopy (IR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD) and energy-dispersive X-ray fluorescence (EDXRF). In the syntheses carried out following 18th-century methodology, the Prussian blue pigment was obtained with the highest yield using an iron vessel in the calcination step. The yield is also related to the HCl:alum ratio used in the last step of the synthesis; however, the yields obtained were lower than those described in the 18th-century literature. The presence of amorphous carbon in Prussian blue was observed using Raman spectroscopy, and KCl, alum, sulfate/sulfide compounds, and aluminum hydroxide were identified as impurities using SEM-EDS, XRD, thermogravimetric analysis (TGA), and IR. In modern syntheses, KCl was also a synthesis byproduct, with insoluble and soluble Prussian blue were obtained by simply varying the proportions of the reagents. Thus, alum, amorphous carbon, and aluminum hydroxides may be indicative of a pigment produced by 18th-century methodology. Watercolor on paper (19th century and commercial watercolor) and oil on wood prototypes using Prussian blue were prepared and subjected to anoxic atmosphere testing for 45 days and accelerated aging testing. After accelerated aging tests, the prototypes showed irreversible changes in the inks and supports, while colorimetric measurements of the prototypes subjected to an anoxic atmosphere indicated reversible changes. However, the spectra of Prussian blue obtained by Raman spectroscopy indicated possible degradation of the pigment. Since it is sensitive to radiation, laser powers below 0.067 mW should be used to analyze Prussian blue by Raman spectroscopy. A bibliographic study of trade records from the 18th century and physical-chemical analyses of several works of Brazilian cultural heritage were carried out. In this context, portable equipment for performing analyses in situ, such as EDXRF, macro mapping by X-ray fluorescence (MAXRF), and FTIR were essential. The oldest source that can be associated with the trade of Prussian blue is the book “Pauta e Alvará: de sua confirmação do Consulado Geral, da Saída, e Entrada na Casa da Índia. Lisboa, do ano de 1744” and the oldest works containing Prussian blue in Brazil identified in this work are the Compromisso da Irmandade de N.S. do Pilar do Mosteiro de São Bento do Rio de Janeiro (1740), and the Compromisso da Irmandade de São Miguel e Almas de Prados da Comarca do Rio das Mortes (1722-1746). The presence of Prussian blue in cultural heritage presents particularities regarding conservation procedures that can cause color changes in the paints, such as anóxia treatment. In addition, the identification of elements such as K, S, and Al in works dating from the 18th century can help in dating works of cultural heritage and be associated with impurities in Prussian blue, affecting the pigment degradation process and color fading.