Slate

• 1. Introduction

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  The phrases 'clean slate' and 'blank slate' trace back to slate itself: a small, smooth piece of the rock, often framed in wood, that pub keepers once used with chalk to record a customer's tab, and that schoolchildren used the same way as a portable notepad. That object came from slate, a fine-grained metamorphic rock formed from an original shale-type sedimentary rock made of clay or volcanic ash, shaped by low-grade regional metamorphism. It ranks as the finest-grained foliated metamorphic rock there is, and while it usually reads as grey, especially en masse across a roofline, slate from a single quarry in North Wales can range from pale to dark grey and also turn up purple, green or cyan. How does compressed clay turn into a rock that can be split thin enough to write on, and why does the same stone come out such different colors from one quarry to the next?

• 2. How Slate Forms

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  Slate's defining feature is 'slaty cleavage', the tendency to split along flat planes that form when strong compression forces fine clay to regrow as flakes aligned perpendicular to the pressure, rather than along the rock's original sedimentary layering; fresh cleavage surfaces appear dull, in contrast to the silky sheen of phyllite, the next-higher grade of metamorphic rock derived from the same kind of mudstone. This cleavage is continuous, meaning the individual planes sit too close together to make out in a hand sample, so the rock's texture ends up entirely dominated by them rather than by any trace of the original mudrock layering. Converting mudrock into slate involves losing up to 50 percent of its original volume as it compacts, rotating platy clay minerals into parallel layers and gradually converting them into chlorite and mica, while any organic carbon present turns into graphite. Under a microscope, the finished rock consists of thin lenses of quartz and feldspar less than 100 micrometres thick, separated by layers of mica; because the process happens at relatively low heat and pressure compared to other metamorphic rocks, slate sometimes preserves fossils, occasionally even the microscopic remains of delicate organisms.

  Quartz, illite and chlorite together make up as much as 95 percent of slate's composition, with iron oxides such as hematite and magnetite, iron sulfides such as pyrite, and carbonate minerals as the main accessory minerals; feldspar can also appear, usually as albite and occasionally as orthoclase. In the purple slates of North Wales, ferrous iron reduction spheres form around iron nuclei, leaving a light-green spotted texture that can be stretched by later stress into ellipses visible on a cleavage plane, though some of these spots may form after the deformation itself, so geologists remain cautious about using them to estimate how much a rock has deformed.

• 3. What Counts as Slate

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  Before the mid-19th century, the words 'slate', 'shale' and 'schist' were not sharply distinguished from one another, and the confusion outlasted the science: in coal mining in the United States, 'slate' remained the common term for shale well into the 20th century, so a 'roof slate' meant the shale layer above a coal seam and 'draw slate' meant shale that fell from the mine roof as coal was extracted. The British Geological Survey now recommends reserving the word 'slate' in scientific writing for cases where too little is known about a rock to classify it more precisely; a rock proven to have formed from metamorphosed shale should be called a metashale instead, and a mica-rich rock of uncertain origin should be called a pelite.

• 4. Roofing, Floors and Walls

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  Slate's two lines of breakability, its cleavage and its grain, let it be split into thin sheets that stay flat and stack easily, which is why roofing slate installed by a slater became one of its dominant uses; a wave of 'slate booms' swept Europe from the 1870s until the First World War as railway, road and waterway transport improved. Its water absorption index runs below 0.4 percent, making it resistant to frost damage, and natural slate needs so little processing that its embodied energy compares favorably with other roofing materials. Building professionals favor it for its beauty and durability as much as its practicality: a slate roof can last several hundred years with little or no maintenance, and the material is also fire resistant and energy efficient. UK roofers typically fix slate with double nails onto timber battens in England and Wales, or nail it directly onto sarking boards in Scotland and Northern Ireland, using copper nails traditionally and modern alloy or stainless steel alternatives today, giving a weathertight roof that lasts roughly 60 to 125 years. Some mainland European suppliers instead use hook fixing, which avoids drilling holes, suits narrow tiles for features like valleys and domes, and resists wind uplift better by securing the slate's lower edge, though the visible metal hooks can look wrong on historic buildings. When broken by hand, slate keeps a natural-looking edge while staying flat enough to stack, and tiles destined for flooring are often sold 'gauged', meaning the back surface has been ground flat to simplify installation.

  In slate-quarrying towns of Wales such as Blaenau Ffestiniog and Bethesda, entire buildings still stand built wholly of slate, a legacy of 19th century construction that also used slate tiles indoors for floors, stairs, walkways and walls, typically set in mortar and grouted, sometimes sealed with chemical treatments to boost stain resistance or adjust surface smoothness, though slate flooring can turn slippery outdoors in the rain. Slabs of slate were also set into walls as a rudimentary damp-proof membrane, used as shims to level floor joists, or, in areas where the stone was plentiful, built directly into walls and hedges alongside other kinds of stone.

• 5. From Blackboards to Battle Calls

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  Because it is both a good electrical insulator and fireproof, slate was used to build early 20th century electric switchboards and relay controls for large motors, and its thermal stability and chemical inertness made it a natural choice for laboratory bench tops and billiard table tops. Earlier cultures used slate as a whetstone for honing knives, before quartz took over that role, and 18th and 19th century schools relied on slate for blackboards and individual writing slates, worked with slate or chalk pencils. High-quality slate has also served as tombstones and commemorative tablets, and the ancient Maya civilization used it to fashion stelae. In Japan, slate was the traditional material for black Go stones, paired with clamshell for the white ones, and today a slate and clamshell Go set is considered a luxury item. In modern homes, offcuts of slate are commonly repurposed as table coasters.

  Pennsylvania slate, meanwhile, is the material of choice for turkey calls used in hunting, since scratching it with a striker made from various woods reproduces the calls of all four wild turkey species found in North America: eastern, Rio Grande, Osceola and Merriam's.

• 6. Where Slate Comes From

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  Spain supplies most of Europe's slate today, as the world's largest producer and exporter of the natural stone, and it accounts for 90 percent of the roofing slate used across Europe. Smaller producing regions remain active in Wales, which holds UNESCO landscape status and a museum at Llanberis, in Cornwall around the village of Delabole, and in Cumbria at the Burlington, Honister and Skiddaw quarries; some slate from Wales and Cumbria comes out colored rather than the usual blue-grey, purple and formerly green in Wales, green in Cumbria. Scotland's West Highlands once quarried slate around Ballachulish and the Slate Islands. Continental Europe adds Anjou, the Loire Valley, the Ardennes, Brittany and Savoie in France, the Ardennes in Belgium, Liguria in northern Italy near the town of Lavagna, whose name Italian still uses for the word chalkboard, Portugal around Valongo, and in Germany the Moselle River region, the Hunsrück, where a former mine near Fell now operates as a museum, the Eifel, the Westerwald, Thuringia and north Bavaria; Alta, Norway also produces a slate-like stone that is technically schist rather than true slate. In North America, slate comes from Newfoundland, eastern Pennsylvania, Buckingham County in Virginia, and the Slate Valley region spanning Vermont and New York, where colored slate is mined around Granville, New York; a major slate operation also ran in Monson, Maine, through the late 19th and early 20th centuries, producing a distinctive dark purple to blackish stone that still roofs many local buildings.

  Inuit communities in the Arctic used slate to make blades for ulus, and China now holds vast slate deposits with growing exports of finished and unfinished stone. Australia quarries slate in the Adelaide Hills, at Willunga, Kanmantoo, Mintaro and Spalding, while Brazil, the world's second-largest producer, extracts 95 percent of its slate from around Papagaios in Minas Gerais, though not every slate product from Brazil qualifies for the CE mark used across Europe.