Historical formulations of lake pigments and dyes derived from lac: A study of compositional variability

“Lac” is a term referring to a pink-red-purple organic colourant derived from an insect and used as a lake pigment or a dye. Although historical sources indicate extensive usage, findings in historical samples remain scarce and are based on the detection of laccaic acids A and B, which are the main colourant molecules in lac. In this work various lake pigment and dye production methods were investigated, with the aim to explore the compositional variability derived from changing fundamental parameters, such as temperature and pH. Exhaustive research focusing on historical recipes was fundamental in order to create a series of 16 mock-up samples. Preliminary investigations were carried out using colorimetry and photo-induced luminescence imaging to evaluate the differences in colour and luminescence behaviour. Visible-induced visible luminescence imaging (VIVL) in the range c. 540–700 nm was particularly efficient at detecting the presence of included shellac in lac lake pigments. High performance liquid chromatography coupled with an online diode array detector followed by electrospray ionisation and quadrupole time-of-flight detection (HPLC-DAD-ESI-Q-ToF) was ultimately applied to assess the chemical composition of the samples and principle component analysis (PCA) was used to establish correlations between the chemical composition and the production methods. The results demonstrated that, regardless of the pigment/dye production method, a transfer of shellac components always occurs from the sticklac raw material into the lake pigment or dye. More specifically, although an increase in temperature and alkaline pH during the extraction promote the inclusion of high molecular weight shellac components, a variety of shellac components are also found when mild conditions, such as room temperature and neutral pH, are used. The results obtained for artificially aged lac-dyed textile samples showed that lac colourants are more prone to degradation than shellac components. The use of mass spectrometric detection for the identification of lac in historical objects is therefore recommended, as this technique is able to detect non-colourant molecules, which may be present in the pigment or dye as a result of the production methods.