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Sensor system for detection of harmful environments for pipe organs

Final Report Summary - SENSORGAN (Sensor system for detection of harmful environments for pipe organs)

The organ belongs to the core of European culture reflecting a diverse history of traditions and stylistic periods. The European heritage of the organ is preserved in more than 10 000 historical instruments. A major threat to this heritage is indoor harmful environments. The organ contains many different types of materials, like different types of wood, leather, different types of lead-tin alloys, from pure lead to almost pure tin, brass and iron. It also contains a complex system of moving parts and the bellows, the wind channels and the wind chests need to be airtight. This makes the organ to an object being very sensitive to microclimate changes and harmful environments. Organic acids, also in combination with condensation phenomena, create pipe corrosion causing serious damage to the pipes. Harmful humidity conditions can create cracks in the wooden parts of the organ, making the instrument unplayable. The SENSORGAN project objectives were to make available new instrumentation for monitoring and detection of harmful environments for organs through development of sensors for real time measurement.

The results from the previous European Commission funded COLLAPSE have shown that the escalating problem of corrosion inside organ pipes is caused by organic acids, especially acetic acid, emitted from the wooden parts in the organ. The organic acids in the organ wind enter into the pipe foot and creates a corrosive environment inside the foot. When an alternating electrical field is applied on a piezoelectric quartz crystal, the quartz crystal starts vibrating at its resonant frequency. If a coating is applied to the crystal surface the crystal frequency will decrease. This property is used for development of a dosimeter for detection of organic acids being corrosive to organ pipes. A coating of pipe metal (lead) is applied to the crystal. When the coating reacts (starts to corrode) with the organic acids, the mass of the coating increases and a frequency shift of the crystal vibration can be detected.

For validation and calibration purpose the dosimeter was exposed to corrosive environments with defined different temperatures, humidities and acetic acid concentrations. The dosimeter response was measured in real time and the dosimeter coating was evaluated and analysed before and after exposure by different methods such as Raman and impedance spectroscopy. A miniaturised holder containing the necessary electronics for driving the crystals was developed. The holder was used both for exposures in the laboratory and in the field study. For the application of the dosimeter in the organ an adaptor was designed and built. The adaptor ensures that the pipe can be played also during the monitoring and that the dosimeter is exposed to the organ wind flowing into the pipe foot. In addition to the electronics in the crystal holder, a unit containing electronics and software for data storage and readout was developed.

Fluctuations in ambient relative humidity are considered to be one of the main factors which contribute to the deterioration of wooden cultural objects. Wood responds to these fluctuations by gaining moisture when the humidity is high or losing moisture when the surrounding air is dry. Wood shrinks as it loses moisture and swells when it gains moisture. When wood is restrained in its movement, it can experience high stresses within the material, which can cause significant damage. Cracks in the wooden parts of the organ can cause serious damages making the instrument unplayable. The organ facade, often containing invaluable art handicraft like wood carvings and sculptures, can also be damaged by a harmful environment. A new and innovative method of recording acoustic emission (AE) activity has been employed to trace microfracturing of wooden parts of the organ thereby constituting an early warning for emergence of cracks in the wood. When a microcrack develops, a sound pulse is emitted from the wood. This sound is not in the audible frequency range but it is detected by the sensor. A prototype sensor including signal acquisition system and software to process and evaluate the data have been developed and successfully tested. A new version of the sensor system adapted for the market is ready for commercialisation and will be introduced into the market soon after the project end.

The results from the field study show that the environmental situation in and around the organ was rather satisfying during the monitoring period. No high concentrations of organic acids in the pipe, low microcrack activity in the wind chest and no condensation situation detected in the monitored pipe. This could be one factor contributing to the 400 years old organ well preserved condition.

The results from the project will contribute to preserving European organ heritage, developing improved organ restoration policies, EU policies that bring socio-economic, environmental and technological approaches together, scientific grounds to assist the CEN in setting normative standards for conservation of cultural heritage, especially the microclimate inside churches, a general understanding of microclimatic factors that create harmful environments for organs, whose impact on European building environments will contribute directly to the quality of life and health of the members of the community, detecting harmful environments and assessing methods to impede them, e.g. organ restoration, church restoration, installation of friendly heating systems and, supporting and improving organ builders' restoration practices.