Natural slate comes from sedimentary rock formed some 500 to 600 million years ago during the Cambrian, Ordovician and Silurian periods. Volcanic action, compression and movement transformed the original clay constituents into slate and gave the rock its essential characteristics - a tendency to split into thin sheets.
Chemically, slate is composed of silica and alumina compounds with a smaller proportion of lime, magnesia and iron in the form of sulphide.
Most of today's slate quarried in North Wales and Spain consists of large boulders which are blasted or hewn out of the mountainside and brought to the surface.
These boulders , part of naturally occurring pillars of slate, are machine sawn into cubes, the size of which determines the ultimate width of the slate. The next stage is usually carried out by hand, the rock being riven by hammer and chisel down the grain until the required thickness of slate is achieved. The four edges are then dressed (guillotined) by machine or hand to give the finished length and width (typically 515 x 260 mm in Australia).
The dressing produces a bevelled edge which is attractive in appearance but also prevents capillary action of water running off the roof.