Fourier transform infrared (FTIR) is an efficient tool to analyse deposits on iron gall inscriptions that appear on historic documents. Deposits look like white powder or whitish crystals under a microscope. Crystals show cubic or needle shapes with sizes below 20 μm. Mapping infrared spectroscopy in addition allows the selective identification of mixed deposits. Selective analysis of different inorganic oxalates is possible in four regions of the mid-infrared spectrum, thus avoiding interferences with bands of other compounds present. Calcium, potassium and iron oxalates, further calcium sulphate, calcium carbonate, basic iron sulphates and iron ammonium sulphate were identified on samples taken from selected manuscripts. The results enabled to establish a correlation of ink composition and the observed intensity of ink corrosion.

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