Chemometric tool for identification of iron-gall inks by use of visible-near infrared fibre optic reflection spectroscopy

A method has been developed for identification of corrosive iron-gall inks in historical drawings and documents. The method is based on target-factor analysis of visible-near infrared fibre optic reflection spectra (VIS-NIR FORS). A set of reference spectra was obtained from model samples of laboratory-prepared inks covering a wide range of mixing ratios of basic ink components deposited on substrates and artificially aged. As criteria for correspondence of a studied spectrum with a reference spectrum, the apparent error in target (AET) and the empirical function SPOIL according to Malinowski were used. The capability of the proposed tool to distinguish corrosive iron-gall inks from bistre and sepia inks was evaluated by use of a set of control samples of bistre, sepia, and iron-gall inks. Examples are presented of analysis of historical drawings from the 15th and 16th centuries and written documents from the 19th century. The results of analysis based on the tool were confirmed by XRF analysis and colorimetric spot analysis.