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Otta slate – from black to a beautiful golden patina

Et spisst skifertak med brun skifer fra Otta Pillarguri.
From the Pillarguri Peak above Otta, we quarry slate of which the like is not found anywhere else in the world. Slate has been quarried industrially at Otta in the spectacular mountain regions of Rondane and Jotunheimen in Norway since 1875.

Otta slate is unique in many ways, including its mineralogical structure.

It has rosettes of amphibole and garnet crystals, which give the slate a quite unique 3D appearance. Outdoors, moreover, the dark grey Otta slate will change colour over time. This is because Otta slate contains small amounts of minerals, such as iron oxides and sulphides, that gradually develop a golden patina when the slate is used outdoors.

It comes as no surprise that this Norwegian natural stone is regarded as an exclusive material all over the world.

Rust development

Most of the quarried slate is dark grey in colour. Some of it may also be rust-coloured.

Otta slate roofing tiles and facade slate are usually supplied with a ratio of between 5 and 10 per cent slate that has already developed a golden patina. The rest is dark grey or black. When Otta slate is exposed to moisture and other climate effects, it gradually changes colour. How long this process will take depends on the environment and variations in the slate itself. Rust develops more quickly in a damp coastal climate than in a dry inland climate. The process is slower for cladding than for a roof, and even slower for outdoor paving.

Black Otta slate used in interiors will not become rust-coloured.

The images from the Scandinavian Golf Club in Farum outside Copenhagen, Denmark, are a good illustration of how the colour has changed since the building was new. The images were taken with a gap of eight years.

Scandinavian Golf Club med skifertak og tørrmur i brun og sort glimmerskifer i Ottaskifer.
Scandinavian Golf Club med et skifertak i brun skifer fra Otta Pillarguri. Takskiferen har endret farge fra sort til rust gjennom 8 år.

Golden brown Otta slate adorns buildings all over the world

In Massachusetts, USA, the striking Boston Public Library extends for almost 2000 sqm along one of the city’s central avenues. The building is clad entirely in Otta slate, of which the exquisite colours were created by nature and refined by the talented local stonemasons at Otta in Gudbrandsdalen, Norway.

The library won the prestigious “International Award for Architecture in Stone” in 2005.

Gylden brun skifer fra Otta Pillarguri dekker Boston Public Library, også kalt glimmerskifer
Boston Public Library, USA | Architect: Machado and Silvetti Associates

Since the 1870s, Norwegian royalty, heads of state and tourists have all visited Elvester Art and Culture Hotel in Lom. Beneath its beautiful Otta slate roof, people, nature, art and food meet in spectacular surroundings.

The hotel gained its beautiful Otta slate roof soon after the Otta Quarry was opened in 1899, since when the roof has resisted wind, weather and frost for almost 120 years. During that time, the roof has changed character from dark grey to golden, a natural colour change that we’ll be able to admire for at least another 100 years.

Clad in golden slate from the Pillarguri Peak above Otta, the Tautra Maria convent lies on the island of Tautra in Trondelag, Norway. Production of the façade cladding of the award-winning building designed by Jensen and Skodvin Architects began in the mountains above Otta 460 million years ago.

The building has won a number of prizes, including Building of the Year in 2006. It really is an amazing building, with roof structures of laminated wood and glass, and exterior façade cladding of Otta slate.

Since 2006, after exposure to weather, wind and seasonal conditions, the Otta slate has changed character from dark grey to golden.

Portrett av arkitekt Helen Hyllseth

Helene finally realized her dream of a slate roof from Otta

– I expected more rust colour right away. Then I realised it came from the iron in the stone as it oxidises. Now I’ve already seen some change!

Helene Hyllseth
Civil architect MNAL, Kvernaas Arkitekter

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