Evaluation in laboratory and in situ of bioproducts for cleaning rust stains on stone surfaces

Removing rust stains from stone surfaces is a very serious problem in the restoration field. The need to use less toxic products both for the restorer's health and for the environmental preservation has led to experiment bio-based products. In particular, a mixture of two amino acids has been used for cleaning tests both in laboratory, on marble specimens, and in situ, on artworks in different limestone stones. Selection of a bio-based treatment to replace toxic chemicals used to remove chromatic alterations induced by corrosion products of iron and its alloys on stone substrates. Two sulfur-containing essential amino acids, such as cysteine and methionine, and a reductant such as sodium dithionite, were mixed and applied on different limestone stones in poulticing materials. These amino acids - that are safe, cheap and commercially available - form complexes with Fe(III) and Fe(II) with high stability constants and very weak complexes with Ca(II) and Mg(II), by acting as ligands for rust removal. This cleaning method was preliminarily tested in laboratory on marble specimens stained of rust by artificial aging. The mixture was also applied to several artifacts in situ by means of agar gel. The efficiency of the cleaning was evaluated by measuring the colorimetric variations of the treated surfaces with a spectrophotometer. The results show the efficacy of bio-based treatments to remove the rust stains