Welsh Slate
Welsh Slate is the finest slate to be found anywhere in the world
The best Welsh slate was laid down 500 million years ago as a fine silt on the sea floor. The silt was of the eroded remains of a massive mountain range and it was slowly laid down on the sea floor to a thickness of well over a mile where it became 'mudstone'. At this time the land that is today Wales was situated 60 degrees south of the equator and was part of an ancient land mass called Avalonia. Avalonia was situated closer to Antarctica than what New Zealand is today. The southernmost part of New Zealand, Invercargill, in comparison lies at just 47 degrees south. At the time this was happening there were no animals on land. The creatures that lived in the oceans had not yet evolved to where they became land animals.
The layered mudstone that formed from those eroded sediments was gradually thrust down over six miles deep into the earth's mantle both through its own weight and that of further sediments above it, as well as through crustal movements. The mudstone was there subjected to high pressures and high temperatures which transformed it into slate.
400 million years ago further crustal movements uplifted, folded and metamorphosed it to become the slate we see today and it existed 160 million years before the Dinosaurs appeared on the earth.
Slate does not split along the plane of the layers in which it was laid down but at an angle to this, typically in the range of 30° to 90°. Slate has been used for roofing for centuries and the Roman fortress of Segontium at Caernarfon, founded in 77AD by Agricola and occupied until 394AD, is said to have been roofed with slate from the Alexandra quarry near Rhosgadfan five miles away.
Good quality slate such as that suitable for roofing has a low water absorption index of less than 0.4%. The best slate has an index of just 0.002% placing it amongst the least absorbent stone on earth. The poorer quality slate has its origins later in time, about 350 million years ago. The poorer quality slate absorbs water and when it freezes it delaminates and deteriorates. Some of the best preserved ancient gravestones are made from good quality Welsh slate: those made from marble or limestone are typically very prone to weathering.
By the nineteenth century slates were used 'around the world' for writing in nearly every school and were a central part of the Welsh slate industry. In all my lessons when I first started school we used slates to write on and to do our sums (early 1950s). A piece of slate about the same size as a pencil and with a sharp point was used for writing; a damp cloth was used for erasing. The slates were all neatly wooden framed and stored in a wooden box at the front of the classroom near the blackboard. And here I am today writing this on an iPad which is about the same size as that framed slate, saving paper and millions of trees just as the slates did for over a hundred years.
RJPrice