THE TREATMENT OF IRON-STAINED MARBLE: TOWARD A" GREEN" SOLUTION.

In the field of stone restoration, an unresolved issue is represented by the removal of iron stains from stone substrates. This study deals with a comparative study of the efficacy of several formulations in removing iron-stains from marble. These formulations are mostly based on chelating agents (ammonium thioglycolate, ammonium citrate, L-cysteine and DL methionine), which can form stable complexes with iron. Two sets of experiments have been carried out. Powdered calcium carbonate has been mixed with ferric hydroxide, then the mixture has been put in contact with the formulations, then the amount of removed iron has been evaluated. Another experimentation has been carried out on marble specimens artificially stained with rust. Removing tests have been performed, and their efficiency has been evaluated by measuring the colorimetric variations of the surfaces and the variation of the porous structure. The use of cysteine together with sodium dithionite solution showed the best results; moreover this formulation is the most eco-friendly solution, both for the restores and for the environment.