Slate is a fine-grained, foliated, homogenous metamorphic rock. This is created from a Shale or Mudstone sedimentary rock. This sedimentary material forms a shale which is typically composed of clay, volcanic ash, quartz and mica, with a small percentage of other minerals.
Colour variations
The colour of slate is determined by the amount and type of iron and organic material present in the rock. Various shades of grey, blue, plum and green can be found in North Wales. Due to the natural variations of this material, the veins of colour often run very close together or even intertwine. This impacts the colour when quarrying, as the slate can be a mix of colours.
History of the slate industry
The slate industry in Wales began during the Roman period when slate was used to roof the fort at Segontium. The industry grew at a steady rate until the 18th Century and then increased rapidly until the 19th Century. The two largest slate quarries in the world are based in Wales; Penrhyn and Dinorwig.
The slate quarried in North Wales is from Ordovician times and is between 435 and 500 years old. The working methods have changed considerably over the years; early workings tended to be in surface pits, but as the work progressed downwards, it became necessary to work underground. This was often accompanied by the driving of one or more adits to gain direct access to a level.
Chambers were usually driven from the bottom, by means of a "roofing shaft", and this was then continued across the width of the chamber. Slate was freed from the rock face by blasting in shot holes hammered into the rock. Slate would be recovered from the chamber in the form of a large slab, and this was taken by truck to the mill where it would be split and cut into the standard-sized roofing slates. The offcuts were then used for slate chippings.
Current work methods are less physical; our slate chippings sold at Decorative Aggregates are a by-product from roofing tiles and dimensional slate production. The recycled slate from these productions has been piled and previously thought of as waste material. The slate waste is taken to be crushed and graded to form slate chippings in a mulch, at sizes 20mm, 40mm and 50-100mm. Our paddle stones and Snowdonia Polished Slate go through a tumbler to smooth the edges and create their signature polished look.
