The phrase clean slate comes from pub tabs chalked on stone
Innkeepers once kept customers' charges on small framed sheets of slate, written in chalk and wiped when the debt was paid. That habit gave English the clean slate and the blank slate. The rock behind the idiom is a squeezed, baked mud that splits into sheets thin and tough enough to roof a house for centuries.
Slate starts as shale-type mud or volcanic ash and is transformed by mild regional metamorphism. Strong compression makes the tiny clay flakes regrow at right angles to the squeeze, creating slaty cleavage, planes so densely packed that they cannot be seen individually by eye. That direction has nothing to do with the original sediment layers; it records the push of mountain building. The mudrock can lose up to half its volume in the process, clay turns to chlorite and mica, and organic carbon becomes graphite.
Because the heat and pressure are modest, the crystals stay microscopic, which gives slate its dull split surfaces compared with the silky sheen of phyllite, the next grade up, and can even preserve delicate fossils. Quartz, illite and chlorite make up to 95 percent of it. Colour varies widely: North Wales yields many greys plus purple, green and cyan, and its purple slates are sometimes dotted with pale green spheres where iron was chemically reduced. Until the mid-19th century, slate, shale and schist were used almost interchangeably, and American coal miners still called shale slate well into the 20th century.
As a roof, slate is hard to beat. It absorbs less than 0.4 percent water, so frost barely harms it, and it resists fire. A properly nailed or hooked roof lasts roughly 60 to 125 years, while the stone itself can endure for several centuries. Better railways, roads and canals set off slate booms across Europe from the 1870s until the First World War, and in Welsh quarry towns such as Blaenau Ffestiniog whole buildings are made of it.
Its stability and insulating power found other jobs: early 20th-century electrical switchboards, laboratory benches and billiard tables. Schools used it for blackboards and writing slates, the Maya carved stelae from it, Japanese players traditionally used it for black Go stones, and Pennsylvania slate is still prized for turkey calls.
Source: Slate
