Ageing of flax textiles: Fingerprints in micro-Raman spectra of single fibres

Flax fibre (Linum usitatissimum) is probably the earliest textile material and holds a great archaeological interest. The possibility to define a connection between ageing and molecular characteristics is thus a concrete purpose aiming to help indirect dating. In the present work, such possibility was investigated by spectroscopic techniques that allow to examine micro-sized samples and are moreover non-destructive towards the sample itself, an important requirement when precious and ancient artefacts are analysed. Confocal micro-Raman spectroscopy and laser-excited micro-fluorescence spectroscopy were applied to 23 micrometric fibres from historical linens (dating from about 3000 BC to the XVII cent.) and 11 crude or treated modern fibres. The intensity ratio between Raman bands at 1121 and 1096 cm(-1), already suggested in the literature as a possible signature for ageing, was systematically evaluated after baseline correction, showing that modern samples exhibit a quite constant ratio value of 0.85 +/- 0.05 which diminishes up to 0.7 if the linen fibre is heated or bleached. Fibres form archaeological linen show instead a lower value for this ratio, that decreases to about 0.5 depending both on age and on conservation conditions. Laser-excited fluorescence spectra were also collected from the fibres, yielding a Pearson correlation value of about 0.7 between the intensity of the fluorescence emission and the age of the flax samples. Irregularity in the trend is mainly due to the possible influence of alien features such as contamination from organic substances.