An investigation of the removal of iron stains from Globigerina Limestone surfaces

Abstract

This study focuses on the removal of orange-brown iron oxy-hydroxide stains from the surface of Globigerina Limestone of the type franka tal-bajda used for building and statuary. In accordance with conservation ethics, iron stain removal tests were performed on Globigerina Limestone slabs that were subject to artificial staining with a ferrous chloride solution in order to simulate real iron stains observed in the field. Two cleaning treatments were evaluated: (i) a 1:1 mixture of sodium dithionite: tri ammonium citrate buffered at pH 8.5 with ammonium carbonate/bicarbonate and (ii) thioglycolic acid buffered to pH 8.5 with ammonium carbonate/bicarbonate. In both cases the treatment solution consisted of a reducing and chelating agent. The thioglycolic acid molecule functions both as chelating and reducing agent. The concentration of each cleaning solution, and the contact time between poultice and stained surface were varied in an attempt to determine the best application conditions for these treatments. Stain removal was evaluated through visual observation, standard photography and via a colorimetric technique. The most promising treatment system was identified. Alkaline-buffered 1% (v/v) thioglycolate solution applied for 30 minutes in a paper pulp poultice gave the best result on artificially stained limestone with minimum effects on the underlying surface. Buffered solutions of combined sodium dithionite and tri-ammonium citrate treatment might be more effective at 1% (w/v) concentration with repeated 30 min poultice applications. For both treatment systems tested, it was noticed that a high concentration of treatment solution leads to the mobilization and extraction of intrinsic iron. This was particularly exemplified with the result of the 5% (v/v) thioglycolate treatment on blank unstained limestone slabs. The application of a high concentration of treatment solution and lengthy poultice contact times should be avoided.