Vildmannen 7 building in Stockholm: after the 2017 fire disaster, the facade shines again in its colorful elegance (1)

Vildmannen 7 in Stockholm after the fire of 2017.Vildmannen 7 in Stockholm in 2019.

We describe the restoration steps of the sandstone ashlars, which stem from boulders, not quarries

By Björn Schouenborg*

Our Insight report is about the building’s architecture and history

In November 2017, a devastating fire broke out in the property Vildmannen 7 in the Swedish capital Stockholm during an ongoing restoration. The pictures after the fire made it hard to believe that it would be possible to save this beautiful and historical building, which is as an “extremely high cultural historical value” in the country’s list of monuments. Only the facade was still standing; the interior was completely destroyed.

For the owner, the real estate company Hufvudstaden, it was obvious to investigate whether preserving all or parts of the building was possible.

The Research Institutes of Sweden (RISE) was hired to make a comprehensive damage inventory and was also given the freedom to design the investigation as efficiently as possible. The focus was on the classic stone facade and the entire masonry since natural stones and bricks were built together as one masonry unit.

Roslag sandstone boulders.Roslag sandstone boulders.Roslag sandstone.

The natural stone used is the Roslag sandstone. It is not mined in quarries but is sawn, cut, and hewn from moraine blocks. That gives the facade a very varied character, with stones of different colors and textures – which was a real challenge in the investigation, as the technical properties of the stone would prove to vary as much as the color.

The initial assessment of the damage was done with ultrasound and Schmidt Hammer (impact hammer). But it was not very successful, not least because of the great variation in the material. Common sense, visual assessment along with tapping with a small hammer and listening to the sound, provided better answers.

A photomontage in A0 format was made of both facades where every single stone is visible and could be classified based on a rough 3-graded scale.

Fire damage.

The building’s second floor showed the most extensive damage, especially near the streets intersection between Jakobsbergsgatan and Biblioteksgatan. Here, the flames had shot out through the windows and considerably heated the vault stones. The cold extinguishing water had then cracked the rock in something reminiscent of ancient tunneling techniques.

Kurt Derfält, from Arctic Quartz company, had been responsible for the ongoing restoration, which was interrupted by the fire. In consultation with Kurt, a suitable area was selected for sampling stones to be analyzed more extensively at the RISE natural stone laboratory in Borås: the selected stones came from a window opening and comprised all color variants and degrees of damage.

One crucial analysis was assessing how deeply the visible cracks affected the stones.

Fire damage.

Using fluorescent epoxy, both macro samples (entire cross-sections of stones) and thin sections (microscopy preparations) were produced. It was quickly discovered that the damage was not deeply penetrating. It was generally a question of millimeter-deep damage.

The most damaged stones were more or less completely intact under the exfoliated, centimeter-thick outer layer! A conclusion that was also very important for the assessment of the overall damage picture and subsequent need for the replacement of stones.

Facade restoration.

The macroscopic analysis clearly showed large differences between ashlars of different colors: Yellow stones were generally more porous and softer, something that was also confirmed by Kurt Derfält’s practical experience. The red ones were much harder. The scientific analysis thus confirmed the experienced craftsman’s know-how.

Many other tests were carried out, such as frost resistance, flexural strength and compressive strength before and after frost testing.

The results varied but were clearly approved for the purpose and Hufvudstaden’s requirement of at least 100 years of durability.

Samples were also taken from the “restoration material” to be compared with nonexposed material. Stones classified as intact remain fresh and sound for a long time. The weakly affected stones were in most cases only superficially damaged, many times only cosmetic defects. Stones classified as clearly damaged (extensive cracking) were in practice intact at depth and could, in theory, be used to produce smaller bricks to replace severely damaged ones.

This extra handling made the project not so cost-efficient.

The rest of the brick and mortar masonry was also tested and deemed in good condition, with primarily superficial damage.

The final part of the survey consisted of numerically assessing what proportion of the facade stones were damaged and undamaged: The total surface of undamaged stone on Biblioteksgatan was estimated to 93 percent. The equivalent for Jakobsbergsgatan was 95 percent. In other words, only a few percent of the total number of masonry units were heavily damaged – despite the hot flames and the cold extinguishing water.

Facade restoration.

But many of the window sills had not survived. Many had already been heavily weathered before the fire.

The more ornate stones above the window arches had also been damaged in many places and had to be re-cut. Overall, it still looked so good that everyone was convinced that it was worth keeping the facade.

The restoration was completed in August 2023, and the facade now looks more like the original facade than the one before the fire.

* Björn Schouenborg, Doctor of Mineralogy & Petrology, is a Senior Scientist at the Research Institutes of Sweden (RISE). He also played an important role in finding the causes of the bending of the white marble facade slabs at the Finlandia Hall in Helsinki and is the Chairman of the Swedish Standardization Committee.

RISE, Björn Schouenborg

Facade restoration.Facade restoration.The restoration was completed in August 2023.

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(15.04.2024, USA: 04.15.2024)