Micro-XRF and FT-IR/ATR analyses of an optically degraded ancient document of the Trieste (Italy) cadastral system (1893): a novel and surprising iron gall ink protective action

This paper aims to identify causes and processes of an undesired age-related optical phenomenon in which two kinds of paper – awhite paper and a green one – and an iron-gall ink are involved togetherwith unavoidable environmental agents. Documents under examination are dated 1893 and come from the Trieste cadastral system archive (Ufficio del catasto tavolare di Trieste). The green paper, with a high content of lignin and kaolin is a preprinted payment order used in accounting operations and it is adjacent to a white lignin-free paper. Diffused brown stains appear on the white paper mainly from being in prolonged contact with the green one which was handwritten using iron-gall ink. The green page induces a strong browning effect on the contact side of the white one, except for inked areas. The manuscript black lines of the lignin-containing page seem to protect the adjacent surface, where a mirror image appears. A particular type of mirror effect, which we propose to call negative mirror effect, is clearly evidenced. Micro-XRF (micro-X-ray fluorescence spectroscopy) and FT-IR ATR (attenuated total reflectance Fourier transform infrared spectroscopy) techniques were used to achieve information on both diagnostic inorganic and organic components. By way of our chemical model we can hypothesise the migration of oxidised brown low molecular weight extra-cellulose compounds (from lignin and/or hemicelluloses) from green recto to white verso pages. The browning process is only hindered in sharp correspondence with the areas of the green recto folio written with the iron-gall ink: this ink acts as a physical barrier to the migration or, as a second hypothesis, it is able to withhold the mobile organic compounds by way of a specific but still unknown interaction. In the field of scientific research on metal-gall ink corrosion this is a novel, interesting and surprising case of the reverse, in which the ink itself is not the cause of the corrosion, but the unforeseen preventive agent.