In the mid 1990s, a heavy corrosion attack was discovered inside the Principal 16’ façade pipes in the Stellwagen organ (Die kleine orgel) in St. Jakobi Church in Lübeck.

The corrosion had started inside the lower part of the foot and moved gradually upwards in the pipe foot towards the mouth area. The corrosion had begun to cause cracks and holes in some of the pipe feet and something had to be done in order to save these invaluable and wonderful sounding pipes from 1467.
The questions were:
Why did these old pipes suddenly start to corrode in the last decades?
How should the corroded pipes be treated and how can they be kept from corroding further?

These questions were the point of departure for the COLLAPSE project (Corrosion of Lead and Lead-Tin Alloys of Organ Pipes in Europe, EVK4-CT-2002-00088). COLLAPSE was a research project supported by the European Commission under the Fifth Framework Programme.
Partners in the project were

• University of Gothenburg, Göteborg Organ Art Center (GOArt), Gothenburg, Sweden. 
• Chalmers University of Technology, Department of Environmental Inorganic Chemistry, Gothenburg, Sweden.
• University of Bologna, Department of Metals Science, Electrochemistry and Chemical Techniques, Bologna, Italy.
• Evangelisch-Lutherische Kirchengemeinde St. Jakobi in Lübeck, Germany.
• Marcussen & Søn, Orgelbyggeri A/S, Aabenraa, Denmark.
  

Field studies were performed, studying organs affected by corrosion in Italy, the Netherlands, and Belgium and comparing them with organs in similar locations not being affected by corrosion:

• Basilica di S. Maria di Collemaggio, L'Aquila, Italy, second half of 17th century (corroded) <> Church of Madonna di Campagna, Ponte in Valtellina, Italy, 1518 (not corroded).
• The Koninklijk Conservatory, Brussels, Belgium, 1880 (corroded) <> The Jezuietenhuis, Heverlee, Belgium, 1880 (not corroded).
• Groene of Willibrorduskerk, Oegstgeest, the Netherlands, 1976 (corroded) <> Waalse kerk, Amsterdam, the Netherlands, 1734 (not corroded).

The field studies included the following work:

1. The room and the condition of the organ were both documented using a form containing a set of documentation parameters. Documentation of the damage from pipe corrosion was performed through written descriptions and photos of the pipes. A boroscope with attached digital camera was used for taking photos inside of the pipe feet. The inside of the pipe feet were also inspected manually using the boroscope.

2. Metal samples were taken from corroded and non-corroded pipes. The samples were investigated regarding the microstructure, and analysis of the chemical composition of the metal was performed. Phase composition and morphology of corrosion products on the surface of the samples were also identified.

3. Coupons of polished metal that mimic the material used in the historical organ pipes were exposed for a longer period in the organ by placing them inside the pallet box in the windchest. The field exposures were intended to provide information on the corrosivity of the environment and, especially, on the soluble anions deposited on the metal surface. The coupons were analyzed in the lab after exposure.

4. Gas analysis of the church environment and the organ wind supply.
Filters for passive sampling were placed inside and outside the organ for a couple of weeks before analysis. Filters for active sampling were also used for analyzing the organ wind: The air from the organ wind system (windchests, windtrunks) was pumped slowly through a filter over a period of several hours using a pumping device. The filter content was later analyzed in the laboratory.

5. Temperature and relative humidity were measured over a period of more than one year at two positions: On the windchest close to the pipes and at the bellows close to the organ wind inlet. These positions were selected with the intention of calculating the possibility for condensation inside the pipes.

The gas analyses showed that high concentrations of organic acid vapors (especially acetic and formic acid) were found in the organ wind flowing through the pipes in the heavily corroded organs. Typical concentrations of acetic acid were in the range from 150 to 1500 ppb (parts per billion). Comparing the corroded and nearly uncorroded organs in each region always showed a significantly higher concentration of organic acid vapors in the corroded organ.

In order to investigate the influence of temperature on the emissions of organic acids, the measurements were performed during summertime (August, 26 °C) and wintertime (February, 7 °C) in the Stellwagen organ in Lübeck. The concentrations of all organic species were considerably lower in wintertime compared to the measurements during summer. During the warm summer period, the concentration of acetic and formic acid vapor in the pallet box was 5 to 8 times higher compared to winter conditions.

 

Laboratory experiments were performed under controlled environmental conditions in order to measure the corrosion effect on pipe metal of environmental factors and substances found in the field studies.

The corrosion rate of lead in the presence of low concentrations of nitrogen dioxide, NO2 , and sulphur dioxide, SO2 , is very slow and only slightly higher than in clean humid air. This implies that emissions of NO2 and SO2 from traffic and combustion of fossil fuels are of minor importance to the atmospheric corrosion of lead. The combination of SO2 and NO2 does not produce a synergistic effect on the lead corrosion.

Laboratory investigations in well-controlled conditions show that low concentrations (170– 1100 ppb) of acetic acid vapor in humid air are very corrosive towards lead. The rate of corrosion is constant in time and proportional to the acetic acid vapor concentration.
The concentrations of the acids in the laboratory studies are in the same range as to the measured values in the field studies.

Lead corrosion slows down immediately when the supply of acetic acid vapor is discontinued.

In order to decrease the corrosion rate and to prevent further corrosion in an organ pipe, one possible conservation procedure could be to clean the corroded parts of the pipe with water. By removing corrosive compounds (salts) from the surface, the corrosion rate can be decreased. The insoluble corrosion products (e.g. lead white) are not to be removed. Of course, the cleaning should not affect the metal itself.
Curve (1) shows mass gain in the presence of acetic acid (HAc) without water-cleaning. Mass gain is linear with time in the presence of acetic acid. Curve (2) shows the mass gain of samples that were first exposed to acetic acid for two weeks. Then the supply of acetic acid was interrupted, resulting in a sudden decrease in the corrosion rate. The effect of water- cleaning after two weeks’ exposure to acetic acid is illustrated by curves (3) and (4); note that the mass gain after water-cleaning is reset to zero. When the water-cleaned samples are again exposed to acetic acid (curve (3)), the mass gain curve has the same slope as without cleaning, indicating that water-cleaning does not affect the corrosion rate (compare the slope of the mass gain curves nr. (1) and (3)). Curve (4) shows the mass gain of samples that were exposed to humid air after water-cleaning. The flat mass gain curve indicates that the corrosion rate is very low in this case.
The laboratory results show that if acetic acid is present, water-cleaning does not decrease the corrosion rate of lead samples. This means that the success of the water-treatment of the corroded pipes from, e.g. the Stellwagen organ, is connected to the simultaneous removal of corrosive gaseous species, e.g. acetic acid, in the wind system.

 

 

When the results from the field study and the laboratory study were combined, they reveal that there are several factors that are important for the emergence of pipe corrosion. If the source creating the corrosive environment was located outside the organ one would expect to find corrosion damage both inside and outside the pipe body, as well as on the pipe foot. However, in this study corrosion was only found inside the pipe foot, which indicates that the main source creating the corrosion is located in the organ.

The wood in the organ
The field study shows that there are unexpectedly high concentrations of organic acids and especially acetic acid in the organ wind system for some of the field study organs. It is known that acetic acid is corrosive to lead and the laboratory experiments show that acetic acid is corrosive to lead also in very low concentrations. Comparing the corroded and the non corroded organ in each geographical region, the wind system in the corroded organ always contains higher concentration of organic acids compared to the non corroded organ.
The organic acids are emitted from the wood used in the organ (in windtrunks, windchests and toeboards). In old organbuilding traditions, and also often today, oak has been used extensively in organbuilding (in Italy walnut is also common). Oak is known to emit large amounts of organic acids and especially acetic acid.
There are two different situations in which the organic acids could enter into the pipe foot: (i) when the pipe is played, the wind containing the acids will flow through the foot; (ii) when the pipe is not played, the organic acids emitted from the wooden parts just below the toe hole (especially from the wall in the toeboard hole) will slowly enter into the foot through the toe hole. It is not easy to measure how much the "sounding" or the "silent" situation will contribute to the corrosion situation. However, from the corrosion documentation of the Stellwagen organ in St. Jakobi Church, it is possible to draw some conclusions from the distribution of the corrosion damages in the pipe foot and between different pipe feet.

It is important to understand that the pipe foot volume constitutes an almost closed volume. Besides the toe hole there is only the very narrow opening, the windway in the top of the foot, where the wind leaves the foot and enters into the bottom of the pipe body. Besides this opening, the top of the foot is covered by the languid acting as a lid.

(i) When the organic acids are transported with the wind into the foot when the pipe is played, the foot is filled with air containing the acids and one should expect a rather uniform distribution of the corrosion on the metal surface inside the foot. It should also be possible to see some differences between the pipes; pipes that are often played should be more corroded, like pipes in the middle part of the tonal range, and small pipes in the high treble range should be less corroded.
(ii) When the organic acids from the wooden parts under the pipe slowly enter the foot through the toe hole while the pipe is not played, one would expect most corrosion close to the toe hole in the foot tip where the acids enter the foot. Also pipes with small foot volumes would be more corroded because higher concentrations of acids would more easily build up in these feet.
The documentation of corrosion damages shows that the corrosion distribution is very similar to the (ii) "silent" situation.

The corrosion starts inside the lower part of the foot close to the toe hole and then moves gradually upwards in the pipe foot towards the mouth area. Also pipe feet with the smaller volumes were much more corroded than the larger pipe feet. If nothing is done, there will be cracks and finally holes in the pipe foot wall. If the corrosion reaches the mouth, the sound properties will gradually change and finally the pipe will be silent. This is of course very serious because the historical sound quality will be lost and the sounding cultural heritage will be gone forever.

Also, the fact that a pipe is not played most of the time, and several of the silent facade pipes in the Stellwagen organ were highly affected by corrosion indicates that the "silent" situation contributes significantly to the corrosion attack.
Another observation in this organ supporting the assumption that the organic acids are the major problem is that one of these silent facade pipes has no languid, creating excellent ventilation for the open pipe foot and this foot is not corroded at all!

All silent pipes are standing on the same wooden toeboard with drilled holes underneath as the sounding pipes.

The question remains about how old organs could survive several hundreds of years without any corrosion problems when oak has always been used for making windchests. However, there are historic documents from the seventeenth and eighteenth centuries describing frequent corrosion problems. There are historical reports of mice chewing on the corrosion products, called lead sugar because of the sweet taste, and at the same time damaging the pipes further so that they had to be repaired. This means that the corrosion problem is not new, but today there can be a problem if new wood is introduced into an old organ during restoration or repair work.

The glue in the organ
Knowing that acetic acid creates lead corrosion, the use of white glue (polyvinylacetate glue) in the organ can be another factor that can increase risk for corrosion.

There are many different types of white glue, but they all emit acetic acid. Some of them emit acetic acid when they are fresh, and others emit acetic acid when they are ageing. Some of them release large quantities of acetic acid. In general white glue has frequently been used when building organs and there have also been cases where white glue has been painted inside the windtrunks and windchests in order to protect from wind leakage. White glue has been used by most organ builders since the beginning of the 1960s.

The pipe metal
The composition of the pipe metal is an important factor influencing the corrosion situation. The alloy consists of lead and tin and also some trace elements. In a corrosive environment, metal containing less than about 1% tin, like the corroded pipes in St. Jakobi Church, is very sensitive to corrosion, but more tin in the alloy makes the metal more corrosion-resistant.

An observation from the field study illustrates this situation: the corroded and uncorroded block in reed pipes in the organ in Heverlee in Belgium. This is unfortunately a rather frequent corrosion problem in organs from the 19th century. The higher tin content (3.9%) protects the D# pipe block from corroding in the corrosive environment. Tin will have a protecting effect only at lower humidity. The protecting effect from tin will gradually disappear if the air humidity increases. This could explain observations that pipes containing 20-30% tin also showed corrosion.

The environment
The humidity and temperature condition in the organ will have an influence on the corrosion situation. Especially higher humidity will increase the emission rate of organic acids from the wood, and higher humidity will also speed up the corrosion process itself in the pipe.
In addition, higher humidity will decrease the protecting effect from the tin.
It has sometimes been suggested that condensation inside the pipe foot is the reason for the corrosion. It would certainly speed up a corrosion process, but possible condensation is not considered to be the key factor creating pipe corrosion.

To summarize:
The corrosive environment inside the pipe foot is created by the emission of organic acids from the wood and the white glue in the organ.
Different types of wood emit different amounts of organic acids and there is no “perfect” wood without any emissions. However, oak is known for its high emission of organic acids, especially of acetic acid.

For a corrosive environment to create conditions of corrosion in the pipes will mainly be dependent on the pipe metal alloy and the humidity in the organ.

The described situation is very similar to the problem of corrosion of metal objects in wooden showcases in museums, creating the emission of organic acids from wood into a closed or almost closed space, corresponding closely to the conditions observed in the pipe foot.

Considering the reasons for the corrosion, the only sustainable way to solve a pipe corrosion problem in the long-term perspective is to change the environment in order to decrease the concentration of organic acids in the pipe foot. It is also important not to create a corrosive environment when repairing or restoring an instrument.