Slate is formed when high pressures are exerted on a sedimentary rock like shale. It is a dark, fine-grained stone made up of thin, sedimentary layers that allow it to be split into thin slabs, often used for roof tiles. Slate has been used in early electronic devices and on a bridge to a fairy tale location in Cornwall, as well as extensively by the construction industry.
Image Credit: Javier Ruiz/Shutterstock.com
Slate performs well in all weathers including frost, which makes it well-suited for exterior cladding and roofing applications. Its high durability and aesthetic appeal have seen it used in interiors around hearths, on kitchen worktops, and as floor tiles.
Viewed under the microscope, slate contains thin lenses of quartz and feldspar in so-called QF-domains. These are separated by mica layers in M-domains, typically only a few dozen microns thick.
Striking slate with a specialist tool and a skilled hand results in the material shearing off into tiles along these mica layers. However, the layers are not visible to the naked eye.
As well as quartz and feldspar, slate is usually composed of illite and chlorite. Other accessory materials make up less than 5% of slate’s composition. These include iron oxides like hematite and magnetite, iron sulfides like pyrite, and various carbonate materials.
Slate has a water absorption index of under 0.4%. This is extremely low, and it enables slate tiles to withstand all kinds of inclement weather including frost.
It is also highly durable. Slate tiles can last for hundreds of years, and a traditionally constructed slate roof should perform for 60 to 120 years with next to no maintenance required.
In the early 20th century, slate was used as a base material for the first electric switchboards. Its naturally occurring electrical insulation, thermal stability, and chemical inertness also led to slate being used for laboratory benchtops throughout the 20th century, and even for tabletops for billiards.
Today, slate is also prized for its potential green credentials. In a 2011 study published in Resources, Conservation and Recycling, researchers conducted life cycle assessments (LCAs) of several dimension stone materials used in the UK construction industry.
On a cradle-to-site basis, slate’s embodied carbon was calculated at just 251 kg of CO2e per tonne of material. The report also stressed the importance of local supply of dimension stone, as transportation considerably increased the figures.
How is Slate Being Used Today?
Many traditional domestic buildings and “non-designed” or vernacular dwellings around the world have made use of slate for exterior roofing and cladding, flagstone flooring, hearthstones, and boundary walls.
Its desirable properties, unique aesthetic appearance, and this cultural heritage mean that slate is still prized by architects and designers for new projects the world over. Below are a few rock-solid examples of modern slate installations.
The 2019 Serpentine Pavilion, London (UK)
The Serpentine Pavilion – a public gallery and cultural space in London, UK – features a fresh architectural design each year. In 2019, Japanese architect Junya Ishigami adorned the Pavilion with a “hilly” roof made up of slate.
The roof used 61 tonnes of slate from Cumbria in northwest England. A basket roof and slender steel columns supported the massive amount of stone.
Therme Art Celebrates Junya Ishigami's Serpentine Pavilion—London 2019
Video Credit: Therme Art/Youtube.com
Yang Liping Performance Arts Center, Dali (China)
Studio Zhu-Pei, an architectural firm based in Beijing, China, also used a slate roof to evoke steep nearby terrain in the Yang Liping Performance Arts Center in Dali, China.
The roof’s organic profile is informed by the silhouette of surrounding mountains, and slate is the material of choice to top it off.
Bruno Stephens’s Mother’s Studio, Sterrebeek (Belgium)
Belgian architect Bruno Stevens, working with Belgian studio LAVA Architecten at the time, used slate extensively for the exterior of a ceramics studio he attached to his mother’s house in Belgium.
Tintagel Castle Bridge, Cornwall (UK)
William Matthews Associates, a British architecture studio, joined forces with engineering firm Ney & Partners to build a bridge to a fairy tale island in Cornwall, in southwest England. The bridge uses slate tiles stacked vertically for the footpath.
The slate compliments the impressive engineering of the bridge and highlights a unique feature: a small gap in the middle of the span. The gap is there to allow for movement from the two large, steel cantilevers that make up the bridge’s structural skeleton.
The Making of Tintagel Bridge
Video Credit: English Heritage/Shutterstock.com
The footpath bridge looms 58 meters above the sea between two cliffs on the Cornish north coast. On the island side is a medieval ruin, which legend says was the site of the mythic King Arthur’s conception.
The slate that visitors walk over to get to the island was quarried from Delabole quarry, also in Cornwall. Delabole is England’s oldest working quarry, and it played a significant role in the history of slate in the UK over the centuries.
Slate was also used for the footpaths on either side of the bridge, but laid in a more traditional manner. The architects wanted a more unique display of slate on the bridge itself, so they had the idea to turn the tiles on their sides.
The result is effective: the texture and look of the vertically stacked slate tiles draw your attention to the staggering 58-meter drop to crashing waves below, and ready visitors for a fairy tale adventure on the legendary island.
References and Further Reading
Block, I. (2019). Tintagel Castle Bridge in Cornwall has a gap where it meets in the middle. Dezeen. Available at: https://www.dezeen.com/2019/08/09/tintagel-castle-bridge-william-matthews-associates-cornwall/.
Borne, A. (1988). Blue Slate Quarrying in South Devon: An Ancient Industry. Industrial Archaeology Review. Available at: https://doi.org/10.1179/iar.1922.214.171.124.
Cárdenes, V. et al. (2016). Roofing slate standards: A critical review. Construction and Buildings Methods. Available at: https://doi.org/10.1016/j.conbuildmat.2016.04.042.
Crishna, N., P. F. G. Banfill, and S. Goodsira (2011). Embodied energy and CO2 in UK dimension stone. Resources, Conservation and Recycling. Available at: https://doi.org/10.1016/j.resconrec.2011.06.014.
Frearson, A. (2019). More details of Junya Ishigami's craggy Serpentine Pavilion revealed in full photo set. Dezeen. Available at: https://www.dezeen.com/2019/06/18/junya-ishigami-serpentine-pavilion-in-pictures/.
Griffiths, A. (2021). Curving roof incorporates seating at Yang Liping Performing Arts Center. Dezeen. Available at: https://www.dezeen.com/2021/09/26/yang-liping-performing-arts-center-studio-zhu-pei/.
Healy, P. F., et al. (1995). Pacbitun (Belize) and ancient Maya use of slate. Antiquity. Available at: https://doi.org/10.1017/S0003598X00064735.
Klein, K. (2020). Architect Bruno Stevens designs slate-covered studio for his ceramist mother. Dezeen. Available at: https://www.dezeen.com/2020/05/03/architect-bruno-stevens-slate-studio-for-ceramist-mother/.
Mourdikoudis, S., R. M. Pallares, and N. T. K. Thanh (2018). Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties. Nanoscale. Available at: https://doi.org/10.1039/c8nr02278j.
Raimondo, R. et al. (2019). Carbon-Based Aeronautical Epoxy Nanocomposites: Effectiveness of Atomic Force Microscopy (AFM) in Investigating the Dispersion of Different Carbonaceous Nanoparticles. Polymers. Available at: https://doi.org/10.3390/polym11050832.
Walsh, J. A. (2007). The use of the scanning electron microscope in the determination of the mineral composition of Ballachulish slate. Materials Characterization. Available at: https://doi.org/10.1016/j.matchar.2007.04.013