First evidence and characterisation of rare chrome-based colourants used on 19th-century textiles from Myanmar

First evidence for the use of the chrome yellow dyeing method was obtained on late 19th-century Karen textiles from Myanmar. Non-invasive observations obtained by digital microscopy and fibre optic reflectance spectroscopy (FORS) provided hints of the possible presence of non-conventional organic colourants in yellow, orange and green threads used to produce three textiles under investigation. Micro-samples analysed by scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX) and micro-Raman spectroscopy enabled PbxCrOy formulations directly precipitated onto cotton fibres to be identified and characterised in these threads. Complementary analyses by high pressure liquid chromatography coupled to diode array detector and tandem mass spectrometry (HPLC-DAD-MS/MS) revealed that in the orange and green threads the chrome-based colourants were over-dyed with local natural dyes, thus revealing an unprecedented dyeing scenario, in which cotton threads imported from Europe were over-dyed locally by Karen people to obtain desired colour shades. Additionally, while yellow threads showed the presence of chrome yellow (lead chromate - PbCrO4), orange and green threads mostly contained chrome orange (basic lead chromate - Pb2CrO5 or PbCrO4 center dot Pb(OH)(2)), which led us to hypothesise that chrome yellow partially transformed into chrome orange due to the over-dyeing process performed in alkaline conditions. This study raises awareness of the need for detailed spectroscopic characterisation coupled with high resolution mass spectrometric analysis to study complex mixtures of dyeing materials in historic textiles and has strong implications for advancing our understanding of the impact of British colonisation on traditional textile-making in Myanmar.