An investigation of alternative reducing agents to sodium dithionite for solubilising rust stains on cotton

Abstract

Rust stains can be problematic for historical textiles, weakening stained areas and eventually leading to losses as iron ions catalyse polymer chain scission. The reducing agent sodium dithionite effectively removes rust stains from cotton textiles, but its rapid aqueous degradation and health and safety restrictions have limited its use in textile conservation. Sodium metabisulfite and ascorbic acid were identified as potential conservation appropriate reducing agents. Test treatments were
carried out with each reducing agent in combination with the chelator tri-ammonium citrate to determine effective solution concentrations. An evaluation framework was devised to compare the working properties of each reagent. Stain removal was evaluated using spectrophotometry and bathophenanthroline test strips. Reagents' effects on the cotton fibres were evaluated by measuring treated stained and unstained samples' surface pH and using optical microscopy. A rust stained historical sampler was treated with the most successful concentration of each reducing agent. Sodium metabisulfite removed little staining throughout testing. Ascorbic acid removed lighter rust stains but the low pH of higher concentration solutions made it an inappropriate reducing agent for acid-sensitive cellulosic textiles. Sodium dithionite was the most successful reducing agent. Recommendations for its use were made based on the findings of this research.