Barry Hunt continues his examination of building stones by turning his attention to slate. Although it is considered synonymous with roofing, it is supremely versatile and is used for just about every aspect of construction, from walls, worktops and floors to paving.
Slate is possibly the greatest gift of the Earth to human construction in stone. The ability of slate to be readily split manually into thin sheets, capable of resisting breaking under considerable loading, has made it highly desirable.
Also, most slates are derived from some of the most weather resistant minerals, imparting the necessary properties that allow it to be used as the first line of defence in many buildings – the roof.
It is somewhat incredible to think, therefore, that the amazing properties of slate were not fully realized in the UK until after the Romans had departed these shores.
There is some debate over the first use of slate for roofing, although (naturally) Wales claims it. The Romans built a fort at Segontium (Caernarfon) that originally had roofs covered with the standard Roman clay tile. However, it is believed that these were replaced at a later date with slate.
It does appear that some floors were made of slate slabs around the fourth century, although a similar use of slate at the Roman fort in Caer Llugwy possibly occurred two centuries earlier.
It is likely that transport problems in Roman times prevented greater exploitation of slate, and its use was extremely localized.
According to some authorities, the French claim slating in the Ardennes started in at least 466AD.
The earliest known contemporary documentation of roofing slate was the use of 800,000 blue slates transported by ship from Dartmouth and Totnes to Winchester between 1171 and 1187.
A century later, records show that slate was used for roofing the eight towers of Conwy Castle (1283-87).
These early documented examples appear to be exceptional and it is only in the last 300 to 400 years that a tangible UK slate industry developed.
Delabole Quarry in Cornwall was established in 1600 and is still being worked today, while the Welsh are known to have shipped 100,000 slates to Ireland in the late 1500s, increasing that by an order of magnitude over the next hundred years.
With the Industrial Revolution and the building of railways, slate production eventually hit a peak in the UK of 500,000tonnes a year during the 1870s, 90% coming from 670 quarries in Wales.
By the end of the 20th century, just nine Welsh slate quarries remained, producing less than one-tenth of the peak amount.
What’s in a name?
Slate is defined in a geological sense as a fine grained, low-grade or regionally metamorphosed mudrock with a well developed penetrative cleavage – which is not the same as a bedding plane and should not be confused with it. Slate does not split along the bedding plane.
The cleavage is a foliation in which
sub-microscopic phyllosilicate minerals are in well developed parallel alignment so that the rock splits into platy sheets.
As ever, not all materials classified as slate for construction purposes are true geological slates. In roofing terms, a slate is essentially any material that may be split and dressed for roofing purposes, which includes what are now termed stone slates such as the Collyweston limestone used extensively in the East Midlands. Some thinly laminated York sandstones are also traditionally used for roofing purposes in the North. Such stone slates are not considered to be within the scope of this article, although some phyllites and even schists have been classified as slate due to their splitting properties.
At the other end of the spectrum there is considerable debate regarding when some metamorphosed mudstones, marls and shales become slates (in the construction sense).
The actual cost of slate production is mostly dictated by the amount of waste, which can be huge – as much as 99% for some high quality products but typically above 90%. Therefore a good slate deposit with low wastage can lead to a dramatic reduction in costs, something that both Spanish and Chinese resources have benefited from in recent years. The low price has often led to them being considered as inferior products. Some of them from some sources have failed for various reasons, but to brand them all as inferior is erroneous.
Another source of wastage has been the dominance of a few standard sizes when it comes to roofing slate, the most common being 500mm by 250mm (often referred to as ‘20-by-10s’, from their Imperial measurements).
Rather than maximizing the size of slate obtained from an extracted block, or simply throwing away smaller blocks, in past times a wide range of sizes were prepared which allowed the slater to create diminishing courses, which is a true art form.
The Ballachulish and Macduff quarries in Scotland reported recovery rates of between 15% and 20% in the early part of the 20th century, which reflected the willingness to use even the smallest ‘Peggies’.
‘Peggie’ is just one of the many names employed to describe slate sizes. The most common format is to use female aristocratic names but the naming of slates varied between quarries and often led to confusion, which is probably why such terms are no longer generally used.
Slate producing regions
By their nature, different slates can be difficult to tell apart unless they have a distinctive colour or defining feature.
There is a tendency to think of slate as dark grey and some of it is. But it comes in many different versions – one from the UK that many would recognise is the purple slate of Penrhyn, although you might also be fooled into thinking the Trinity Slate from Burgoyne’s Cove in Newfoundland or the Unfading Purple from Vermont are from Penrhyn.
The reason why these slates are similar goes back to when the muds were being laid down. They were deposited in huge volumes in the Iapetus Ocean and eventually transformed into slate as this ocean closed during the Caledonian orogeny.
These Caledonian slate deposits run through what is now Scotland, Wales, Ireland, Spain, Newfoundland, Quebec, Vermont, Maine, Pennsylvania and other North-Eastern US states.
Testament to the similarity of slates on either side of the Atlantic comes from how the American slate industry was founded and developed by Welsh immigrants who immediately recognized the similarity of the American slates to their own.
Dotted around many US cemeteries close to slate quarries are many slate tombstones bearing Welsh inscriptions, as crisp today as when they were carved, just as are the inscriptions on slate headstones in the UK.
China probably has the greatest resource of slate in the world, with deposits stretching across at least six of its large provinces: Hebei, Hubei, Jiangxi, Shaanxi, Sichuan and Yunnan.
Approximately 50% of current Chinese slate production actually comes from Hebei, which includes Beijing, where Man Tou Gou slate reputedly has been used for more than 500 years.
The reason there is not even more Chinese slate in world markets is that many deposits are served badly by roads, railways and rivers, and it may take a week to travel within China to some of the quarries.
Slate in the UK
The UK is blessed with a fantastic variety of slates that, without doubt, include some of the highest quality found anywhere in the world.
Figure 1 (above right) shows a simple map of the principal slate resources in the UK, which includes not just my favourite slate but my favourite stone, Westmoreland from Cumbria.
The slates from Cumbria have a special aesthetic quality which places them in a class of their own and makes them one of the world’s most exclusive stone materials.
The Cumbrian slates were born from sediments into which volcanic ashes were mixed, imparting a more granular texture that creates a rougher, less planar cleavage that is a little more widely spaced than in most slates.
The slates are not quite comparable to their Welsh counterparts but, by using them more thickly, there are many examples in Cumbria and other parts of the country, including London, where they are still performing as they were intended to after many centuries of exposure to the high levels of pollution that have existed since the start of the Industrial Revolution.
- Barry Hunt continues his analysis of slate in next month’s issue of Natural Stone Specialist when he examines the qualities of slate that make it so ideal as a natural covering for roofs.
Barry Hunt is a chartered geologist, a surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation and Specialist Surveyor. He is also a Member of the Chartered Institute of Building and a Fellow of the Royal Microscopical Society.
Barry has served on a number of professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB), which provides advice on all stone construction issues. He is one of the authors of the SFGB team that has published codes of practice for the installation of stone floors and internal stone finishes.
Barry gained 14 years’ experience working as a consultant for two renowned civil engineering materials consultancies before, in 2001, establishing his own consultancy, IBIS, specialising in the investigation of construction materials.
The specialist knowledge and services provided by Barry have allowed him to be instrumental in the resolution of problems ranging from blast damaged claddings in London’s West End to advice on the quarrying and extraction of stone from abroad for import to the UK. Other areas of experience include the investigation of all types of building finishes, specialist advice on remedial treatments and the preparation of advice for potential and actual use in litigation or arbitration.
Having worked for consultancies that both undertook in-house laboratory investigation to UKAS requirements, Barry is also able to conduct or oversee a wide range of on-site and laboratory techniques and ensure they are carried out to traceable standards.
One speciality in all investigations is Barry’s hands-on approach. Being trained in industrial roped access (abseiling) allows him to get close to the problems with external building envelopes quickly, efficiently and cost-effectively.
Throughout his working life Barry has published findings from the many investigations he has undertaken. He has also authored chapters for two books on building stone and is currently engaged in other book projects in this field. His most important contribution is considered to be the chapter on the repair and maintenance of stone in the landmark Geological Society publication Stone.
Barry has also written a regular column covering the full spectrum of natural stone use for Natural Stone Specialist magazine.