Electrochemical (without external current) and electrolytic (with external current) techniques are commonly used industrially: for corrosion assessment, corrosion prevention and surface finishing. They are also used for jewellery (electro-deposition) and in heritage conservation-restoration. In the following we present some fields of application; specifying the conditions of use (chosen electrolyte and applied potentials).

Surprisingly, electrochemical and electrolytic techniques were used in conservation-restoration as early as the end of the 19th century. Rathgen who worked as a conservation scientist for the museums of Berlin was utilizing them to stabilize[1] chlorinated archaeological bronzes by cathodic polarization (Gilbert 1987). In the 1920s, this treatment was pursued in the USA by Fink and Elridge who treated chlorinated Egyptian objects (Drayman Weisser 1994).

By the 1970s a large number of conservation professionals had tested these techniques, but not always with success; sometimes due to terminal errors (swapping of + and -) or inappropriate conditions (extreme negative potentials causing hydrogen bubbling, despite only aiming to stabilize the metal using a low potential). In most cases the treatments were carried out at constant current or voltage with a power supply.

Electrolytic treatments in conservation made an important breakthrough in the 1980s when the Valectra division of the company Electricité de France slowly but surely imposed the idea to treat metal artefacts at a constant potential measured with a reference electrode. This new approach enabled the operator to be more selective and to only provoke the required electrolytic reactions. Valectra worked mainly on marine artefacts and gained international recognition following their treatment of the first 2000 artefacts recovered from the wreck of the Titanic (1912) (Montluçon et Lacoudre 1989).

The laboratory of Arc’Antique in Nantes has been pursuing these efforts since the 1990s. Specialized in the electrolytic treatment of marine artefacts (chlorinated copper and iron based artefacts), Arc’Antique has since applied these techniques to the removal of tarnish from gilt silver artefacts and to the stabilization of actively corroding lead objects (Degrigny 2010).

More recently, other important research has been carried out on the qualitative analysis of metal artefacts and the treatment conditions have been refined.

Text of Christian Degrigny (christian.degrigny@he-arc.ch)

References:

Gilbert, M., Friedrich Rathgen: the father of modern archaeological conservation, JAIC, (1987), 26, 2, 4, 105-120. On line at: http://cool.conservation-us.org/jaic/articles/jaic26-02-004.html. Consulted in April 2014.