Pigments in the battle against forgery

Abstract

Brass instruments of the 19th and early 20th centuries get more and more used to be played in historically informed performance practice. Monitoring of the corrosion state inside these historical brass instruments before and after being played is essential to check the efficiency of preventive conservation protocols. As corrosion of metal artifacts is an electrochemical process, electrochemical techniques are the method of choice, especially since nowadays these measurements can be carried out in situ and are not destructive.In this work open circuit potential (OCP) and polarization resistance (Rp) measurements were carried out. A special electrochemical sensor combining an Ag/AgCl (pseudo) reference electrode and a small platinum grid as counter electrode, both embedded in a thin cylindrical sponge mounted on a flexible tube, was developed for the in-situ measurements. The sensor had a surface of about 2 cm2 and could be pressed against the inside of the brass tuning slides by a small balloon that could be pumped or emptied.The inside of nine different tuning slides of four ancient brass instruments (horns) was characterized by OCP and Rp measurements; in addition photographs of inner walls were taken with an endoscope.A very promising representation for diagnostic purpose is the polarization resistance (log Rp) vs open circuit potential (OCP) plot, which allows assign groups of OCP/Rp data from the tuning slides to different surface conditions that were established on laboratory experiments. As a result, the surface condition at the point of measurement is more important than the bulk alloy composition.