Chemical characterization of wood samples colored with iron inks: insights into the ancient techniques of wood coloring

In the past, the practice of dyeing wood was very widespread, for instance for obtaining substitutes for rare and expensive ebony, which was used for all marquetry techniques and especially for making inlays to decorate musical instruments. This study focuses on two traditional wood-dyeing procedures carried out with iron–gall dye and iron–logwood dye, respectively. In order to reproduce the ancient techniques, preparation of dyes and their application on two different wood species (poplar and pear) were carried out according to typical ancient recipes and traditions. Wood samples were dyed using mixtures similar to two of the most important historic iron inks, and several specimens (approximately 7 mm in size) were taken and investigated in order to: (1) gain a better insight into the processes involving the dyes and the wood matrices, (2) analyze the effects of the treatments on the anatomical wood structures, (3) evaluate the level of penetration of the dyestuffs into the samples. The analyses were performed after each step of the dyeing procedures, both on the wood samples and on the solutions. Various reaction processes were hypothesized after a micro-Fourier transform infrared spectroscopy analysis, performed in transflection and attenuated total reflection mode. The effects of the various treatments on the wood samples were studied by means of stereomicroscopy, polarized optical microscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results obtained suggested that both treatments cause alterations in the outer 300 µm of the wooden samples, and that a higher level of penetration was obtained with the hematein complex than with the iron-gall ink.