Breathing New Life into Historical Instruments. How to Monitor Corrosion

Brass instruments of the 19th and early 20th centuries are getting more and more used to being played in historically informed performance practice. In the case of a brass instrument, a musician’s hot, wet breath could worsen the instrument’s interior corrosion, but little is known about this danger. Monitoring the corrosion state inside these historical brass instruments before and after being played is essential for checking the efficiency of preventive conservation protocols. As corrosion of metal artefacts is an electrochemical process, electrochemical techniques are the method of choice, especially since these measurements can be carried out in situ and are non-destructive. In this research, open circuit potential (OCP) and polarisation resistance (Rp) measurements were carried out inside the tuning slides of brass wind instruments in order to monitor the corrosion state and its evolution. The principle and measurement procedure will be described. The initial characterisation of the tuning slides showed a great variation among different tuning slides and also for different positions in a single tuning slide. Thus the surface condition at the (small) point of measurement is more important than the bulk alloy composition. The instruments were then played for several minutes each day: one group dried the inside afterwards each time with a fan, while the control group applied conventional practices. A special electrochemical sensor was developed for the in-situ measurements, combining an Ag/AgCl (pseudo) reference electrode and a small platinum grid as a counter electrode, both embedded in a thin cylindrical sponge mounted on a flexible tube. 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. It has been found that the polarisation resistance (log Rp) versus open circuit potential (OCP) plot is a very promising representation for diagnostic purposes that allows us to assign groups of OCP/Rp data from the tuning slides to different surface conditions that have been established in laboratory experiments. Comparing the measurements performed at the beginning, after 7 months and after 14 months of playing, the tuning slides of the instruments without drying showed a constant or increasing corrosion rate over time. On the other hand, preventive conservation, i.e. drying the instruments with the fan, showed constant or slightly lower corrosion rates. This overall positive result has been confirmed with endoscope measurements taken on the inside of the same tuning slides. Note that this result holds as an average on a statistical basis comparing all the tuning slides at the different control times.