New antioxidants for treatment of transition metal containing inks and pigments

Corrosive iron and copper ions and acids in iron gall inks and in certain pigments such as verdigris and malachite lead to enhanced degradation of paper. A recent EC co-funded project "InkCor" proposed the use of tetraalkylammonium bromides in combination with magnesium ethoxide in alcoholic solution for stabilizing these media. In the present paper, the potential stabilising effect of another type of antioxidants - alkylimidazolium bromides - is compared to the one achieved using either an aqueous calcium or magnesium phytate treatment or a non-aqueous solution of tetrabutylammonium bromide in combination with magnesium ethoxide in ethanol. Two new antioxidants, 1-ethyl-3-methylimidazolium bromide and 1-butyl-2,3-dimethyl-imidazolium bromide, in combination with alkali magnesium ethoxide in ethanol, effectively stabilised iron gall ink containing model paper. The effect of stabilisation was superior to the previously studied antioxidants, i.e. tetraalkylammonium bromides and the two phytates. No negative side effects on the colour of the ink were observed, which makes the two antioxidants appealing alternatives to the currently used aqueous calcium phytate treatment. The study was extended also to the model papers containing corrosive copper pigments verdigris and malachite. The most effective treatment solution in this case was composed of tetrabutylammonium bromide and magnesium ethoxide in alcoholic solution. Although no negative side effects on paper containing iron gall ink or copper pigments were observed, it is imperative that the effects of the treatment solutions are studies also on a number of historical samples, before they are used for stabilisation of historical artefacts. © 2008 K. G. Saur Verlag 2008.