The influence of pollutants on accelerated ageing of parchment with iron gall inks

Moist heat (100 °C in closed vessels) and pollutants (SO2 or NOx, 100 °C) techniques of accelerated ageing were applied in stability investigation of iron gall inks on parchment. The measured characteristics on parchment samples coated with inks (made of iron(II) sulfate and gallic acid or tannic acid in various ratios) reflected their chemical, optical and spectroscopic attributes. Decrease of surface pH values was measured for all samples, especially after ageing with SO2. The results obtained, comparing the non-aged samples with those aged upon heat and pollutants, revealed the decrease of lightness L* and the increase of chromatic coordinate b* for all six ink layers. These differences in b  * were dominantly reflected in values of total color difference (ΔEab*, CIE L*a*b  *). The variations in UV/VIS reflectance spectra and ΔEab* evidenced the considerable damage of inks exposed to accelerated ageing, especially for inks prepared with excess of acid to iron. FTIR spectroscopic measurements of parchment showed that accelerated ageing procedures caused changes in structure and arrangement of collagen, and the formation of oxidation products in parchment during the accelerated ageing was stimulated by presence of inks. EPR spectra of parchment coated with inks showed the paramagnetic signals of various Fe(III) species ions in different coordination.