Oxidation of dithionite treatment solutions and the effects of pH on fading textile dyes. Issue 1 (December 2014) - MacLeod, Ian
Abstract : Treatment of complex textiles with heavy loadings of corroded embroidered gold and silver threads has been problematic until recent developments with the use of neutral pH, weak solutions of sodium dithionite. From an understanding of the way in which the oxidation steps of dithionite vary with pH it is possible to determine the nature of the reactive species in solution. The data on the kinetics and characteristic oxidation potentials of dithionite in solutions with 4.0>pH<13.6 confirmed that the solutions at neutral pH were still a powerful reducing agent, which is suitable for non-cellulosic textiles such as silk. The reducing power was sufficient to get electrochemical reduction of copper and silver corrosion products on the surface of the textile without causing damage to the underlying fabric. Visual examination and simple mechanical strength testing (the capacity to withstand pulling stresses) showed that there was no measurable difference between the treated and untreated silk in the tested strips from an 18th century textile. Unless sufficient buffer capacity is present in solution the release of protons from aerial oxidation of the dithionite will cause the solution to become more acidic over time. Under this experimental environment the treatment provides an accelerated ageing method. The full impact of the pH on the treatment method for silk and metal garments was gauged through a series of exposures of strips of contemporary pink textile for different lengths of time, over which the pH became more acidic. The strong correlation of pH with the degree of fading in the test strips of 18th century silk indicates that the fading reaction is caused by oxidative acidic decomposition of the dye.