| Problems in the Pipeline |
| Thursday, 19 January 2006 | |
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The churches of continental Europe harbour some of the world’s finest and most ancient pipe-organs. These instruments, originally built in the fifteenth, sixteenth and early seventeenth centuries by such renowned organ builders as Friederich Stellwagen and Arp Schnitger were, in their time, some of the most sophisticated machines ever built and stand as shining examples of both technological and artistic achievement. Instruments such as the 1467 Stellwagen organ in the parish church of St Jakobi’s in Lübeck, Germany are prized for their unique tone qualities: aficionados rate this organ as one of the finest in the world for the performance of Renaissance and early baroque repertoire. The news therefore that the largest pipes in this important instrument were quite literally losing their voice, sent shockwaves through the organ playing and building community. Inspection of the instrument revealed the cause: small holes in the walls of the pipes. The organ had an acute case of lead corrosion.
As word of the Lübeck Stellwagen organ’s symptoms spread throughout Europe, it became apparent that this was not an isolated case: similar rapid corrosion was afflicting historic organs across the continent. The pipework of these organs is crafted from lead, which is well known to corrode gradually under atmospheric conditions. However, the corrosion pattern in Lübeck was entirely new, characterised by the formation of a white chalky residue in the pipe interior which eventually eats through the pipe entirely. In 2003, research engineer Carl Johan Bergsten from the Organ Art Centre at Sweden’s Gothenburg University (GOArt) assembled a team of chemists, metallurgists, organ builders and historians, founding the Corrosion of Lead and Lead-Tin Alloys of Organ Pipes in Europe project, or COLLAPSE, to investigate. Chemical analysis of corroded and uncorroded pipework samples quickly began to yield clues suggesting that, of all things, well-meaning restoration may lie at the heart of the problem. The corroded lead contained higher than normal levels of organic acids— known to cause the rapid oxidation of lead to lead hydroxycarbonate and lead hydroxyacetate, responsible for the observed white residue. Further investigation and air sampling indicated the presence of acetic acid in the airflows of the stricken instruments. Why had this rapid corrosion suddenly appeared in instruments, some of which had otherwise survived for five hundred years? A possible source of acetic acid in pipe-organs is the oak wood used to line the bellows and the windchests on which the pipes themselves sit. In each of the corroding instruments, restoration work involving replacement of oak wood had taken place in the recent past. As wood ages, the cellulose forming its cell walls tends to break down releasing, amongst other chemicals, both formic and acetic acid.When fresh wood is used to line wind chests and bellows, this organic acid is released directly into the organ’s wind supply, from where it is carried into the pipes. Archaeologists and conservationists have long known the effect of oak on lead artefacts, and will always avoid storing such items in oak drawers or cabinets—but the organ building community has been slow to make this link. In the five hundred year lifetime of a historic organ such as that in Lübeck, the interior wood of the organ will have been changed several times. So the question rises once again, why is this corrosion only now apparent? Svensson’s work suggests that church central heating may be to blame: a recent addition to many European churches, this modern concession to parishioner comfort may be driving off acid in the new wood faster than ever before. Catherine Oertel at Cornell University, collaborating with GOArt, employed a range of spectroscopic techniques to probe organ pipe composition, focussing especially on X-ray fluorescence spectroscopy. Here, an electron microprobe bombards the sample in question with a harmless stream of electrons, causing X-ray emission, with each element in the pipe’s composition giving a characteristic wavelength fingerprint. Pipe alloy composition also seems to be an important factor in determining susceptibility to corrosion. Almost all affected instruments in continental Europe were built in the Northern German tradition, using lead alloys with a low tin content of 1.5–2%. Lead is alloyed with tin, both to harden the organ pipes and to add lustre. Tin was extremely scarce and expensive at the time, and so European builders were directed by frugality. It is indeed the low tin content in the pipes of prized historic organs that contributes to their unique tone colour. This explanation is corroborated by the fact that British historic organs have so far not been affected by rapid pipe corrosion. At the time, Cornwall was Europe’s major source of tin: British organ builders therefore had ready access to a cheaper source of the metal, and their pipework often contains up to 20% in the alloy. Interestingly, addition of tin to the pipework alloy is not thought to directly provide resistance to corrosion. Optical and electron microscope analyses by researchers at the University of Bologna have shown that pipework with a low tin content tends to have higher levels of trace impurities such as bismuth and antimony— additives which can subtly alter the microstructure of the alloy. It could well be that understanding the effect of these trace impurities holds the key to unravelling the mystery of organ corrosion. Meanwhile, can anything be done to safeguard these instruments? Catherine Oertel favours either the development of a coating to be applied to wood components inside the organ, preventing the escape of organic acids, or the use of passive filtration systems to remove them from the wind supply. Bergsten at GOArt hopes to produce a protective coating for the pipes to protect from corrosion. For now, however, the pipework remains irreplaceable; and as more and more organs gradually lose their voice, organ enthusiasts can only hope that progress is swift. Mark Turner is a PhD student in the Department of Chemistry |
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