The corrosion of historic organ pipes continues to be a major problem for older historic organs in Europe. This is particularly true in the pipe feet of the larger bass pipes which are made primarily of lead. Corrosion in the pipe foot reduces the load-carrying capacity of the pipes, and makes them more difficult, if not, impossible to tune as corrosion eats away at the pipe wall and eventually breaks through. A number of recent major research projects including the EU COLLAPSE project and a project in Bremen, Germany indicate that the cause is corrosion of lead by acetic and formic acids emitted from the wooden windchest. Several measures have been suggested for dealing with the problem including neutralizing or removing residual acid in the corrosion product, coating the insides of the pipe feet, or replacing the feet with lead-tin alloys.
This research has been primarily chemical in nature. While it provides evidence for the cause of corrosion, the proposed measures have a number of logistical and conservation ethics problems. The production of the acids is continuous, which means that pipes have to be treated regularly with some kind of aggressive solution. Coatings must be regularly maintained and replaced. Given that the affected pipes are usually the large bass pipes in a complex organ structure, such treatments would be a difficult operation to carry out regularly. Furthermore, the proposed measures only treat the symptoms and not the source of the problem, the acid emission into the pipes.
It was noted in the previously mentioned projects that corrosion tended to be worse in organs which were not played as often, and in silent pipes. This led to a multidisciplinary pilot study conducted by the Cultural Heritage Agency of the Netherlands (RCE) to investigate the possibility of venting the pipes when the organs are not played. A team including an organ expert, a Dutch organ builder, chemists and fluid mechanics experts is studying the airflow through pipes as they are played, to see if that correlates with the corrosion in the pipe feet. Three-dimensional (3D) computer modelling and high-speed smoke visualization techniques are being used to determine the airflow within a transparent organ pipe, and locate eventual “dead” zones where corrosive gas concentrations may be higher. Endoscopic techniques are being used to determine the location of the corrosion in pipe feet. A sensor is being developed to measure the acid concentrations in the air in pipe feet.
The results of the fluid mechanics studies and initial endoscopic work indicate that corrosion correlates with dead air zones in the pipe foot. Venting the pipes is possible, and would be best accomplished by reversing the air flow in the pipe, that is, in the opposite direction to playing. Further work is planned to determine how often venting is required, and the most efficient way of doing this using the existing blower, as well as dealing with the fact that the organ will be continuously producing tones while being vented.
