The slate industry is the industry related to the extraction and processing of slate. Slate is either quarried from a slate quarry or reached by tunneling in a slate mine. Common uses for slate include as a roofing material, a flooring material, gravestones and memorial tablets, and for electrical insulation. Slate mines are found around the world and the major slate mining region in the United Kingdom is Wales: in Cornwall there are a number of slate quarries (famously the village of Delabole) and in the Lake District there are numerous slate mines and quarries.
In the remainder of Continental Europe and the Americas Portugal, Italy, Germany, Brazil, the east coast of Newfoundland, the Slate Valley of Vermont and New York, and Virginia are important producing regions. The Slate Valley area, centering on a town called Granville in the state of New York is one of the places in the world where colored slate (i.e. slate which is not grey or blue) is obtained. (A fuller account is given in the article Slate: section Slate extraction.)
Slate Industry in Spain
The slate deposits in this region of northern Spain are over 500 million years old, having formed during the Palaeozoic period. The colour and texture of the slate produced is largely dependent upon the tectonic environment, the source of the sedimentary material from which the slate is comprised, and the chemical and physical conditions prevalent during the sedimentation process. The region has been subjected to periods of volcanism and magmatic activity, leading to a unique geological development in the region.
An important use of Spanish slate is as a roofing material. It is particularly suitable for this purpose as it has a low water absorption index of less than 0.4%, making it very resistant to frost damage and breakage due to freezing. Tiles produced from Spanish slate are usually hung using a unique hook fixing method, which reduces the appearance of weak points on the tile since no holes are drilled, and allows narrower tiles to be used to create roofing features such as valleys and domes. Hook fixing is especially prevalent in areas subject to severe climatic conditions, since there is a greater resistance to wind uplift as the lower edge of the slate is secured.
Slate industry in Wales
Slate has been mined in north Wales for several centuries — this was recently confirmed by the discovery in the Menai Strait of the wreck of a 16th century wooden ship carrying finished slates.
Large-scale commercial slate mining in North Wales began with the opening of the Cae Braich y Cafn quarry, later to become the Penrhyn Quarry near Bethesda in the Ogwen Valley in 1782. Welsh output was far ahead of other areas and by 1882, 92% of Britain's production was from Wales (451,000 t): the quarries at Penrhyn and Dinorwic produced half of this between them.
The men worked the slate in partnerships of four, six or eight and these were known as "Bargain Gangs". "Bargains" were let by the "Bargain Letter" when a price for a certain area of rock was agreed. Adjustments were made according to the quality of the slate and the proportion of "bad" rock. The first Monday of every month was "Bargain Letting Day" when these agreements were made between men and management. Half the partners worked the quarry face and the others were in the dressing sheds producing the finished slates. In the Glyndyfrdwy mines at Moel Fferna each bargain worked a horizontal stretch of 10 by 15 yards. Duchesses, Marchionesses, Countesses, Viscountesses, Ladies, Small Ladies, Doubles and Randoms were all sizes of slates produced.
Rubblers helped to keep the chambers free from waste: one ton of saleable slate could produce up to 30 tons of waste. It is the mountainous heaps of this very same waste that is perhaps the first thing to strike someone visiting the old regions nowadays. The men had to pay for their ropes and chains, for tools and for services such as sharpening and repairing. Subs (advances) were paid every week, everything being settled up on the "Day of the Big Pay". If conditions had not been good, the men could end up owing the management money. At Moel Fferna a team could produce up to 35 tons of finished slate a week. In 1877 they received about 7 shillings a ton for this. After paying wages for the manager, clerks and 'trammers' the company could make a clear profit of twice this amount. This system was not finally abolished until after the Second World War.
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. In some rare instances, such as Moel Fferna, there is no trace of surface workings and the workings were entiely underground.
Chambers were usually driven from the bottom, by means of a "roofing shaft" which was then continued across the width of the chamber: the chamber would then be worked downwards. Slate was freed from the rockface by blasting in shot holes hammered (and later drilled) into the rock.
Slate would be recovered from the chamber in the form of a large slab, which would be taken by truck to the mill where it would be split and cut into standard-sized roofing slates.
Slate mines were usually worked in chambers which followed the slate vein, connected via a series of horizontal "Floors" (or "Levels"). The chambers varied in size between mines and were divided by "pillars" or walls which supported the roof. The floors were connected by underground "Inclines" which used wedge-shaped trolleys to move trucks between levels.
In some mines, where slate was worked away below the main haulage floor, the route was maintained through the construction of a wooden bridge across the chamber, often supported from chains attached to the roof above. These bridges could be as much as 100 feet/30 m above the floor below.
- Penrhyn Quarry, Bethesda. The largest slate producing quarry in the world. Bought by Alfred McAlpine plc in 1964.
- Dinorwic Quarry, Llanberis.
- Cilgwyn quarry, Nantlle Valley. Dating from the 12th century it is thought to be the oldest in Wales.
In the Blaenau Ffestiniog area, most of the workings were underground as the slate veins are steeply angled and open cast workings would require the removal of a massive amount of rock to gain access to the slate. The larger mines in the Ffestiniog area include:
- Llechwedd quarry – now open to the public as a "tourist mine". Most of the underground workings destroyed by quarrying. Llechwedd Slate Caverns
- Manod – used by the National Gallery, London to store artworks in World War II
- Oakeley – now partially untopped as an opencast working by Alfred McAlpine plc
There were also a number of slate mines in the Llangollen area which produced a much darker "black" slate:
- Moel Fferna
Another cluster of mines were found in mid Wales centered on Corris. These all worked a pair of slate veins that ran across the Cambrian mountain range from Bryn Eglwys in the west through Corris and Aberllefenni in the Dulas Valley to the mines around Dinas Mawddwy in the east.
Most underground slate mines in north Wales were closed by the 1960s although some open-cast quarries have remained open, including the Penrhyn Quarry and the untopping work at Oakeley in Blaenau Ffestiniog. Work also continues at Berwyn near Llangollen. The final large-scale underground working to close was Maenofferen Quarry (which is owned by the Llechwedd tourist mine) in 1999 although opencast quarrying continues at this location.
Many of the mines are now in a state of considerable decay and those that are accessible should not be entered as they are on private property and contain many hidden dangers. The lower levels of many mines are now flooded and collapses are commonplace, for example the hillside above the Rhosydd workings has many pits where the roofs of the chambers below have collapsed.
Other slate producing areas in Great Britain
The most significant is that of Cornwall and Devon where the Delabole Quarry is thought to be the largest single quarry in the island. Many of these are no longer worked owing to lower costs of extraction in the larger British workings. The quarrying of slate in Cornwall is known to have been carried out from the late mediaeval period and there was a considerable export trade from some of the quarries near the coasts in the 19th century.
During the last 500 years, much slate extraction has taken place in the Lake District at both surface quarries and underground mines. The major workings are:
- Broughton Moor
- Kirkby Moor (Burlington Slate Quarries)
- Old Man Complex (Coniston); Cove Quarries (south of Coniston Old Man)
- Elterwater Quarries
- Hodge Close
- Honister Slate Mine (including Yew Crag and Dubs)
- Kentmere Workings
- Kirkby Moor (Burlington Slate Quarries)
- Petts, Kirkstone
- Little Langdale Quarries
- Skiddaw Slate
- Common Wood, Ulpha
Slate was also quarried in Scotland.
Slate industry in North America
The slate industry in the United States is concentrated in the Taconic Mountains region of Vermont and New York, known as the Slate Valley (the district of Granville, New York is well known for its slate). Slate was first quarried in 1839 at Fair Haven, Vermont. An influx of immigrants from the North Wales slate quarrying communities saw a boom in slate production that peaked in the latter half of the 19th century. The slate of the region comes in a variety of colors, notably green, gray, black and red. Some production continued in 2003 with 23 operating full-time mines employing 348 people.
Additionally, one of the oldest quarries in America continues to quarry slate in Buckingham County, Virginia. Their trademark Buckingham Slate has been continually quarried since the 18th century and has a distinct, unfading blue/black color and Mica sheen. Buckingham Slate is used on many Federal buildings in the Washington, D.C. area.
Large scale slate quarrying also took place around the town of Monson, Maine where an extensive series of quarries flourished from the 1860s onwards. A small scale quarrying and dressing operation continues in Monson into the 21st century.
Slate industry in Brazil
95% of the slate extraction in Brazil comes from Minas Gerais. Slate from this region is formed differently from traditional slate areas such as Galicia. Such products are sedimentary rocks that have split along their original bedding plane, whereas true slate has been subjected to metamorphism and does not split along bedding, but rather along planes associated with the realignment of minerals during metamorphism. This realignment, known as ‘schistosity’, bears no relationship to the original horizontal bedding planes .
The independent Fundación Centro Tecnológico de la Pizarra’s report into the ’Technical properties of Bambui Slate from the State of Minas Gerais (Brazil) to ascertain its compliance with the Standard EN12326’ describes how certain products originating from Brazil on sale in the UK, are not entitled to bear the CE mark. Because such Brazilian products display higher water absorption indexes than those from other areas such as Galicia, this makes them less suitable for use as roofing tiles since the study showed a significant loss of strength when subject to thawing and freezing.
- European Association of Mining Industries website retrieved on 26/01/2010
- Galician and Spanish Slate website “Hook Fixing” . Retrieved on 26/01/2010
- Lindsay p. 314
- "English, Scottish, Irish and Australian slate". Retrieved 2009-05-20.
- "Slate". Vermont Mining. Retrieved 2011-04-06.
- Fundación Centro Tecnológico de la Pizarra’s report into the ’Technical properties of Bambui Slate from the State of Minas Gerais (Brazil) to ascertain its compliance with the Standard EN12326’ Brazilian Slate Report, retrieved on 04/02/2010
- Cris Ebbs (1998). Underground Clwyd: A Pictorial Expedition into the Nether Regions of Northeast Wales. Gordon Emery. ISBN 1-872265-93-6.
- Gordon Emery (1996). Curious Clwyd: A Second Photographic Album of Oddities from Northeast Wales. Gordon Emery. ISBN ISBN 1-872265-97-9.
- Jean Lindsay (1974). A history of the North Wales Slate Industry. David and Charles. ISBN 0-7153-6264-X.
- British Isles
- History of the Welsh slate industry
- Slatesite — bilingual site focusing on Welsh slate
- Aerial photographs of slate quarries in Wales
- John T F Turner - A Familiar Description of the Old Delabole Slate Quarries, 1865
- Outside the British Isles