Iron removal from waterlogged wood: Extraction by electrophoresis and chemical treatments

Publication Type:

Journal Article

Authors:

Charlène Pelé; Elodie Guilminot; Sylvie Labroche; Gwenaël Lemoine; Gilles Baron;

Source:

Studies in Conservation, Volume 60, Number 3, p.155-171 (2015)

Abstract:

The presence of iron oxides (lepidocrocite, goethite) in archeological wood may result in a degradation of the wood matrix. Extraction of these iron oxides is largely dependent on their solubility. In this study, balsa wood samples were impregnated with iron oxides to test extraction treatments. Additionally, archeological wood samples were also examined to determine treatment efficiency. Electrophoresis and simple immersion treatments were performed using various chemical solutions: a neutral and a conductive substance (potassium nitrate), an acid (acetic acid), three alkaline chelating agents (tri-ammonium and tri-sodium citrate and sodium oxalate), three acidic and slightly acidic chelating agents (ethylenediaminetetraacetic acid (EDTA), citric acid, and oxalic acid), and a reductant (sodium dithionite). Potassium nitrate did not extract sufficient amounts of iron, irrespective of whether the treatment was conducted by electrophoresis or simple immersion; any observable dissolution was attributed to protonation because of the acidic pH around the anode (as low as 3). Dissolution in acetic acid did not extract iron with either treatment. Strong chelating agents improved extraction, and these compounds gave the best results for simple immersion, particularly EDTA. This chemical is well adapted for use on archeological objects because of its chemical properties (stability constant, speciation based on pH). The addition of sodium dithionite to the solution improved dissolution. Even though electrophoresis improved extraction (in particular for tri-ammonium citrate), none of the tested chelating chemicals were suitable for electrophoresis because of a significant increase in temperature as well as high anode corrosion. The presence of iron sulfide in the archeological wood limited the effectiveness of the tested chemicals. A pre-treatment in sodium persulfate was expanded to include oxidized iron sulfide in oxy/hydroxide iron, which improved the extraction rate.