Covid regulations proved fatal for a 1918 biplane. They greatly accelerated inevitable chemical reactions between the skin and the paint, Italian researchers report in Scientific Reports.
The aircraft in question is an Ansaldo A.1 Balilla (pictured) from the Caproni Aviation Museum near Trento, once flown by Natale Palli. Almost all the other aircraft from that era have been thoroughly restored, but this one is still in its original condition. It is also a work of art: the fuselage is decorated with a large portrait of St George and the Dragon, painted especially for Palli by the later famous artist Amos Nattini.
In recent years, the museum piece has deteriorated remarkably fast. The fuselage is mainly made of plywood, but the wings and the tail are wooden frames covered with fabric, and more and more holes are appearing in the fabric. At the end of 2020, Jacopo La Nasa and colleagues from the University of Pisa therefore launched an investigation into the chemistry behind this deterioration.
It now appears to be due to an unusual choice of material. Almost everyone at the time used linen for upholstery, but Ansaldo chose natural silk, which was stronger and lighter, but also much more expensive. The stretch lacquer (dope), used to stretch the fabric tightly over the wooden frames as it shrank, was a solution of the plastic cellulose acetate rather than the cheaper (but highly flammable) cellulose nitrate. Camphor was used as a plasticiser. Finally, oil paint gave the plane its final colours.
The Nasa group has now chemically analysed a number of samples. This was mainly done by pyrolysing them and passing the volatile products released through a gas chromatograph. The results largely confirmed the existing model, with some additional information on oil paints and pigments. But the researchers also measured the volatiles that had already evaporated naturally from the wings. And this showed that two degradation processes were taking place simultaneously. First, camphor is escaping, but even worse, acetic acid is also being released.
The latter is caused by the slow hydrolysis of the plastic. This process is autocatalytic: once started, it is self-perpetuating. In the film world it is known as the vinegar syndrome. Older film material is usually also made from cellulose acetate, and if it starts to smell like vinegar, you have a problem.
This process cannot be completely prevented, but it can be greatly reduced by storage in controlled and, above all, dry conditions. And this is what went wrong here. At the beginning of the corona pandemic, the museum was closed for months and the ventilation was switched off. As a result, the humidity increased and the acetic acid vapour lingered longer.
The researchers took out a pH meter: in the raw silk they measured 6.1, but under the coating the pH was a mere 4.5. So the protein fibres in natural silk cannot cope: they hydrolyse themselves.
When the research started, there were plans to restore the aircraft. But this publication makes it sound as if this will be very difficult: in retrospect, the Ansaldo was irretrievably lost from the moment it was built. In any case, other aircraft museums have been warned.
La Nasa, J. et al. (2023) Sci Rep 13, DOI: 10.1038/s41598-023-39164-9