When Uppsala Cathedral’s two 10-metre long tower crowns needed to be renovated, the task was accomplished with help from, among others, RISE’s anti-corrosion painting experts.
When Uppsala cathedral’s tower crowns were climbed and inspected in 2013, corrosion damage was discovered when a piece of the ornamentation suddenly came off in an inspector’s hand. Swerim and RISE were contacted to investigate the damage and assist with the extensive repair work by providing expertise in materials, casting, corrosion protection and strength calculations.
The top adornments comprise various ornate mouldings in grey cast iron, along with a wrought iron cross. Since there are gaps between different parts of the structure, moisture has been able to accumulate, which caused the adornments to rust. The only way to repair the damage was to take down the more than 4 tonnes of heavy adornments piece by piece using a giant crane.
When the parts were examined, cracks were detected in certain exposed places. The cast iron used for the spires, which were made in the late 1800s, is very brittle and has poor fatigue properties. For this reason, the decision was made to replace several of the parts with newly manufactured ones. The various parts were scanned in order to perform accurate strength calculations, taking into account the wind loads to which the crosses are subjected. To reduce stresses and prevent future moisture penetration in sensitive areas, some geometric modifications were undertaken in the new parts, but only internally so the outward appearance will not be affected.
True detective work
The forge will produce moulds and then cast the new parts. This time, ductile cast iron is being used instead of grey cast iron to ensure that the adornments last a long time.
“It’s been true detective work,” says Pontus Rydgren, the person in charge of Swerim’s side of the project. “There was hardly any documentation preserved from the 1800s. When we started the work, no one knew what kind of materials were up there or what rustproofing had been used. Working with such a well-known cultural heritage has been really exciting and very different to the investigations we usually carry out on modern materials. The Church of Sweden is inspiring to work with. They are meticulous and have a great interest in technology and its history.”
Working with such a well-known cultural heritage has been really exciting
Rustproofing and reinstallation
The next substantial job will be to rustproof and reinstall the massive adornments. The investigation carried out by RISE shows that, in the 1970s, a zinc ethyl silicate was used as a primer with chlorinated rubber as the topcoat. The primer has fared very well and provided good protection for the top adornments over the last 45 years. The topcoat on the other hand had completely degraded and only primer remained on much of the top adornments.
“Rusting of the top adornments was limited to columns and hard-to-reach surfaces, which were probably not treated during the previous renovation,” says Björn Tidbeck Senior Researcher in anti-corrosion painting at RISE. “Dismantling all the parts this time has provided good opportunities to treat all the surfaces.”
The new anti-corrosion paint will consist of the same primer used in the ’70s or metallisation by means of a galvanizing spray, while a grey polyurethane with good weather-resistance will be used for the topcoat. Painting and reinstallation are scheduled for spring 2021.