How Surface Analysis can Contribute to an Understanding of the Preventive Conservation of Brass Instruments

Brass instruments of the 19th and early 20th centuries are getting more and more used to being played in historically informed performance practice. Monitoring the corrosion state inside these historical brass instruments before and after being played is essential for checking the efficiency of preventive conservation protocols. The measurements inside the tuning slides, however, lack any information on the surface state and composition at the point of measurement (except for endoscope images). In order to correlate the results of electrochemical measurements with the surface state and composition, information from surface analytical experiments on model alloys in controlled environments is required. In this research, we report on the results of an x-ray photoelectron spectroscopy (XPS/XAES) surface analysis of brass (Cu-Zn alloy) model samples exposed to a mild environment (phosphate buffer pH 7) and to a quite aggressive solution (artificial saliva) that represent the two extremes regarding liquids possibly present in the tuning slides after playing. Brass alloys exposed to the phosphate buffer solution showed a rapid formation of a thin film composed of copper and zinc oxide that limits the corrosion rate but is not protective. In the artificial saliva solution, the corrosion rate of brass was initially very high but decreased rapidly to values below 1 μm/year. XPS surface analysis has shown the formation of a thick protective film of CuSCN (thiocyanate) and zinc phosphate. Combining electrochemical and surface analytical data obtained on model brass alloys allows us to rationalise the different corrosion behaviour found in the tuning slides of the brass wind instruments. The direct link between electrochemical behaviour and the surface composition of brass alloys exposed to the atmosphere or to the environment present in the tuning slides is under investigation.