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Stone : Welsh Slate

12 September 2011
Welsh slate is not only used for roofing.

Slate is so good as a roofing material that it has become a generic term for roof tiles. And the producers of Welsh slate have some powerful arguments for claiming theirs is the best in the world. But slate is not only ideal for roofing, it also makes excellent paving, walling, architectural stonework and interior decoration as worktops and tiles. No wonder it is still an important part of the North Wales economy.

Slate from North Wales has been exported all over the world because it is nature’s ideal roofing material for almost any climate – and it remains ideal roofing material even when it is only 5-10mm thick, which means you get a lot of roof covered for the volume of material used.

According to the producers of it, Welsh slate is the best slate in the World – and they make a good case for their claim. It is impermeable (porosity is less than 0.2%), it is unaffected by UV light or normal heating, freezing and thermal cycling. It is unaffected by atmospheric pollution, sea air and sea spray, vegetable growth, rot or insect attack. It is non combustible, giving it a AA fire rating. Thermal expansion is 8.5–11 x 10-6mm/ºC and thermal conductivity is approximately 2.0W/m2K.

For many of the mines and quarries of North Wales in the days before the internal combustion engine made remote places more accessible, another advantage was that they were near the sea, making it relatively easy to ship slate out to other markets in the UK and further afield.

As long ago as Tudor times slate was the main export of North Wales. Then it was going primarily to England. It was the Victorians who started taking it to the furthest flung corners of the British Empire.

At its height, at the end of the 19th century, Wales was producing about 500,000 tons of finished roofing slate a year. Today it produces considerably less than a tenth of that, but the slate industry is still a significant private sector contributor to the economy of North Wales.

At its zenith, Wales was producing and exporting five times more slate than the UK imported from Spain last year – and Spain is still the prime source of the UK’s imported slate, although China is catching up.

In the 19th century the industry employed thousands of people extracting and splitting the stone. One valley overlooked by various quarries including Cwt-y-Bugail, which is still worked today, saw its population rise from 500 at the start of the 19th century to 18,000 by the end of it as a result of the expansion of the slate industry.

Today, the industry employs about 550 full-time equivalent people directly and supports another 500 jobs in associated enterprises, according to the Slate Appraisal Report by Map Analysis commissioned by Gwynedd Council. Most of the roofing produced in Wales is still split by hand in the traditional way because although mechanisation has been tried it has not succeeded in improving efficiency.

Roofing remains an import product for the Welsh slate industry but it is not only roofing that slate is used for. The same qualities that make it perfect for roofing also make it ideal as walling, paving and flooring, worktops and tiles.

It is produced as outdoor furniture, memorials, window cills and other architectural details, and a whole range of souvenirs for the tourists who now form an important part of the Welsh economy.

In fact, the tourist trade is a significant part of the mix for some of the slate companies. J W Greaves & Sons have converted part of their 2,000-acre site into a tourist attraction with train rides round part of the Llechwedd slate caverns at Blaenau Ffestiniog that they have worked since 1836.

The visitors can go underground to experience what slate mining was like in the past, but the active slate extraction takes place by opencasting.

Greaves were also pioneers of hydroelectricity generation and still have turbines and switch gear dating back to 1904. They currently operate two plants, one 200kW generator at the top of their quarry and a 400kW generator at the bottom.

Managing Director Andrew Roberts loves it when it rains because it brings in the tourists and increases electricity generation.

Inigo Jones have also encompassed the tourist trade and invite visitors to tour their workshops to see how slate products are made. Proprietor John Lloyd also has a stake in the tourist attraction of King Arthur’s Labyrinth at Corris in Powys that has been created in a former slate mine.

Inigo Jones are celebrating their 150th anniversary this year – which offers a clue that their name does not originate from the 16th–17th century British architect of the same name. In fact, the company takes its name from one of its founders.

As part of this year’s anniversary celebrations a commemorative plaque was unveiled at the company’s premises in March by Alun Ffred Jones, the Welsh Minister for Heritage.

It was particularly relevant that the Heritage Minister was performing the ceremony because at the same time Wales was launching a bid to gain World Heritage Site status for its slate industry in Gwynedd, putting it in the same league as the Taj Mahal and the Great Wall of China.

It is one of 11 bids from the UK to become a World Heritage Site, a designation that recognises outstanding universal value to culture, history and / or science.

The slate bid includes Penrhyn and Blaenau Ffestiniog quarries; the Port Penrhyn and Penrhyn Castle rail system; an early hydro-power station; the main university building at Bangor, reflecting the quarrymen’s financial contribution and zeal for education; and the National Slate Museum at Llanberis, created with the help of a £1.6million Lottery grant.

The bid has been submitted to the UK Government’s Department for Culture, Media & Sport in the first step of a process that could take years to complete. If it is successful, the World Heritage status is finally bestowed by the United Nations Educational, Scientific and Cultural Organisation (UNESCO).

Using slate for purposes other than roofing is not new. In fact, not all slate cleaves easily into roofing, which is why there are huge heaps of slate waste littering the mountain sides around production sites. Finding seams of stone in the mines and quarries that will cleave is part of the quarrymen’s skill.

Finished product has been as little as 1-2% of the stone won from the ground, although the exemption of slate from the Aggregates Tax means more of what was previously considered waste is now crushed for aggregate.

The product that led to the growth of John Lloyd’s Inigo Jones company in the 20th century was enamelled slate.

It was produced by covering the stone in a tar varnish and then decorating it, sometimes to look like marble or wood, and adding hand-painted pictures to panels (see the photograph of the fireplace on the previous pages). The slate was enamelled by ‘cooking’ it at 250ºC for 48 hours.

As the use of electricity grew exponentially, so did sales of Inigo Jones’ enamelled slate because, being a poor conductor of electricity, it was used on the switch gear for electrical installations. It protected operators from electric shocks and would not catch alight. One of the most notable uses of Wincilate’s products was for the electrical panels on the luxury ocean liner, the Queen Mary.

John Lloyd also heads the Wincilate company at the Aberllefenni slate mine at Machinllyth in Powys. The mine is not currently in production but slate from the tips is being crushed for fill and reclaimed to make name plates and plaques. Wincilate also now buy slate from Penrhyn and Ffestiniog quarries for the production of large memorials and architectural jobs and are currently exploring the idea of redeveloping the hydroelectricity plant that once powered the factory.

There remain six slate quarries in North Wales still in production and about 60 processing companies working the material. The biggest and best known of the quarries is Welsh//Slate’s Penrhyn, which accounts for about 90% of slate production in Wales.

The slate blocks are sawn from the face these days, which reduces waste, and the product range goes far beyond roofing. As part of £1.5million investment, handling has been completely automated and a tile line has been installed for the production of flooring. Alan Smith, who heads Welsh//Slate, says: “There are opportunities to grow the business and we are growing it – we’re learning.”

The Welsh Slate Museum is on the opposite side of the valley to Penrhyn in the Dinorwic quarry, which rivalled Penrhyn as one of the largest slate operations of the principality 40 years ago but is no longer active.

Welsh//Slate also operate another of the best known slate quarries, Cwt-y-Bugail. Under Welsh//Slate it has reversed its emphasis so that now it is an architectural stone business that also sells some roofing.

The demand for the dark blue-grey roofing from the quarry (largely from the heritage sector) is higher than production but Welsh//Slate prefer to supply walling, cills, worktops and floors because the return is better, especially for worktops and floors.

When Welsh Slate Ltd took over the slate business of Alfred McAlpine in December 2007 and Alan Smith was recruited to run it, he intended to close Cwt-y-Bugail and concentrate on roofing from Penrhyn. But he was surprised at the level of demand for slate products other than roofing and production at Cwt-y-Bugail has continued.

It has been worked opencast for many years and as the extraction works down into the mountain the tunnels of the mines from which previous generations took the slate are continually exposed (see the picture above left).

There are many miles of old slate mines through the mountains but a major disaster in 1890, when a roof collapsed, changed the preferred method of operation from mining to opencasting, which continues to the present.

Further over to the East of North Wales, in Denbighshire, is the blue-black Berwyn Slate. This is slate from the geological age of the Silurian, younger than the Cambrian and Ordovician slate from further West. It is not easily riven, so most of what is produced from it is walling, cills, hard landscaping and floors.

Berwyn bought Guglielmi machinery five years ago to produce ashlar for a large cladding job and have now built a new workshop in which to set it up to produce wall and floor tiles. As the slate is not readily riven, it will be sawn and given a brushed finish.

Managing Director Andrew Bickford says the move will increase production to the point where they hope to be able to achieve distribution through tile shops as well as continuing to carry out bespoke production for individual projects.

 

The geology of Welsh slate

The slate-belt of North Wales began its long journey to becoming the materials we know today in the Iapetus Ocean, which ceased to exist about 450million years ago. Further South in Wales, as well as in Cornwall, Brittany and Spain, the same thing happened about 150million years later.
The stone began as muddy sediments washed into the sea during the Cambrian (545-495 million years ago [mya]) and Ordovician (495-443mya).
But slate is not a sedimentary stone. It is metamorphic, altered by heat and pressure into the harder, impervious stone with a well-developed rock cleavage – that is, a preferred direction of splitting – which we know it as.
The build up of  an increasingly thick sedimentary pile on the seabed would not have been sufficient on its own to create the temperature and pressure increases necessary to achieve the metamorphosis. The crucial factor was the lateral forces provided by the colliding of the World’s tectonic plates that support the continents.
This not only directly squeezed the sediments but also forced them deep below the surface to where pressures and temperatures are higher. The compression rotated the clay particles so that their flat, flaky faces were perpendicular to the direction of stress. The clay re-crystallised with the same alignment and the effect is that the whole rock splits easily in a direction of 90º to the original squeezing stress, which, of course, does not need to bear any relationship to the original bedding planes.
 

 

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