How researchers will ensure Norway's Nidaros Cathedral's new doors last 'forever'

Norway’s national shrine ,Nidaros Cathedral, is getting new doors.

The three large portals on the cathedral’s west front will be fitted with doors made of English oak (Quercus robur) from Denmark, painted red on the outside and decorated with wrought-iron fittings and ornamentation.

Danish artist Fredrik Tydén designed the doors, which will be crafted in the Nidaros Cathedral Restoration Workshop (NDR).

But replacing the doors involves more than just going out into the forest and cutting down any oak tree, and hoping for the best.

This type of wood is particularly rich in tannic acid, which reacts with the wrought iron that will be mounted on the doors. It can also discolour the wood surface.

Some trees contain more tannins than others, depending on factors such as where they grew, the climate, their age, and even which part of the tree is being examined.

Doors with a long-term guarantee

Each door will be around six metres tall and weigh well over a tonne. These are clearly not ordinary front doors. The quality has to be as high as possible.

“The quality is important when crafting doors intended to have a lifespan of several hundred years,” says research manager Lars Gullbrekken.

He works with building materials at SINTEF, which has been commissioned by NDR to analyse the acid level in the oak.

The goal is to ensure that only the very best parts of the wood are used and that the researchers can determine the correct treatment for the door surfaces.

“Acid damage shows up as black ‘rust’ on the wood, and the goal is for the doors to maintain their red colour,” he says.

The oak samples are testing in the lab

Research manager Anna Nordborg and researcher Julia Halang are now analysing the oak pieces at SINTEF’s mass spectrometry lab.

Mass spectrometry is a method used to determine what a substance is made of by weighing its smallest particles.

“Nidaros Cathedral is a signature building for Trondheim, so being involved in this process is very exciting. It's an important building, and an honour to contribute to ensuring that the doors last a long time,” says Nordborg.

To determine the tannin level, the tannins from the oak are first extracted from the oak using alcohol, transferring them into a solution that can be analysed. A dye is then added that reacts with the tannins, allowing the researchers to measure the amount.

Tannins function as a natural defence against fungi, bacteria, and herbivores. They also strengthen the structure of the wood and make it more resistant to decomposition.

“There will always be a certain amount of tannins in oak, but once we know the level, we can figure out which treatment will work best. Alternatively, the wrought iron can be treated so that the two materials won’t react with each other,” says Nordborg.

The researchers have also tested the tensile and compressive strength of the iron that will be used in the forged hinges.

A groundbreaking project

Both wrought iron and oak are well-known and traditional building materials. But combining them in the way planned for Nidaros Cathedral is less common.

“I think it’s great that NDR and the technology community in Trondheim are coming together to share new knowledge about old methods and traditional materials,” says Marie Louise Anker.

She is the director of cultural heritage protection at NDR.

“The work on the new doors of Nidaros Cathedral is a groundbreaking project, where we combine medieval sources and historical experiences with today’s most advanced analysis methods. The goal is to be confident that the solutions we choose will last ‘for eternity,’” she says.

What did the original doors look like?

Today’s simple, grey-painted portals were created in 1930 as a temporary solution in connection with the 900th anniversary of the death of St. Olav.

What was intended to last for just a short period has now remained standing for an entire century.

The cathedral has been exposed to several major fires, including in 1328, 1531, 1708, and 1719. These destroyed much of the original interior and furnishings.

The west front was significantly restored and later rebuilt in the 19th and 20th centuries, but the medieval doors were already gone when the restoration began.

Nevertheless, researchers can make an educated guess about what the doors most likely looked like, based on European cathedral doors and traces in the masonry.

The doors were probably constructed of heavy oak planks, like those found in other Gothic cathedrals in Northern Europe. They may have had wrought-iron fittings that were both functional and decorative.

These features are in line with the Norwegian medieval tradition and with traces of hinges and fasteners in the stone portals.

The west front had three portals, as it does today, and each of these would have had heavy, double-leaf wooden doors. They were probably dark in colour, either tarred or naturally aged oak.

Medieval church doors were often richly decorated with Christian symbols, but there are no descriptions of the decoration on the original doors of Nidaros Cathedral.

The Middle Ages meet modern poetry

Now the doors will once again be richly decorated, with respect for their cultural history.

When announcing the winner of last year’s artistic competition, jury leader and art adviser Ida Højgaard Thjømøe praised the winning designer.

“Fredrik Tydén’s winning proposal distinguishes itself by combining the rich idiom of the Gothic with contemporary, poetic expression. The proposal shows a thoughtful combination of ornamentation, symbolism and craftsmanship,” she said.

The doors will be completed for the National Jubilee in 2030

Tydén and craftsmen from NDR will collaborate in constructing the new doors.

They are scheduled to be completed in time for the National Jubilee in 2030, which marks one thousand years since the Battle of Stiklestad.

This is where the Viking king Olav Haraldsson, later known as St. Olav, died.

The year 1030 was a milestone for the formation and Christianisation of the kingdom, and the emergence of Norway as we know it.

If you visit Nidaros Cathedral in 2030 and enter from the west side, know that you have likely passed through Norway’s most thoroughly tested doors. Thanks to modern technology and laboratories, generations after you will be able to do the same.

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Read the Norwegian version of this article on forskning.no

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