All Welsh slate tends to get lumped together as a single product. But it is not. There are, in fact, five distinct Welsh slate types. In his search for Great British Stones, Barry Hunt this month focuses on one of those Welsh slate types. It comes from the area around Ffestiniog.

Slate grey. It is difficult to get worked up about a stone that has such a dull colour associated with it. But if any material were to be considered the gold standard of roofing, it would have to be the slate from Ffestiniog, North Wales.

The Ffestiniog slate is strong, but not so strong that it becomes brittle. Instead it has excellent and desirable elastic properties and can be split more thinly than most other slates. Potentially, the Ffestiniog slate is the most perfect of all natural slates for roofing purposes.a

Welsh slate has gained an international reputation for quality and durability due to its export across the globe, especially during the Victorian era, the heyday of Welsh roofing slate production.

At that time the Ffestiniog area accounted for one third of the total output from Wales, material being won from at least 27 different locations that, due to the nature of the geology, were mostly mines. The industrial revolution opened up to exploitation the remote and relatively inhospitable mountains by the creation of a narrow-gauge railway system still partly in existence today.

The 20th century was unkind to the Welsh slate industry due to industrial disputes, the two World Wars and increasing competition from other man-made roofing materials, so that by the 21st century there were few slate mines and quarries left working.

In the Ffestiniog area, only the Llechwedd quarry is currently in production, yielding a relatively small amount of finished product, since the Oakley mine, one of the largest slate mines in the world, closed in 2010 due to subsidence.

Consistency out of chaos

In late Cambrian and Ordovician times the area that North Wales now occupies was under the sea and was subsiding.

Sediments that built up in shallow areas would periodically collapse and run down into deeper areas with a chaotic flow known as a turbidity current.

A common rock formed under such conditions is greywacke, which is a mixture of sand, silt and clays, but siltstones, mudstones and claystones were also formed where finer materials took longer to settle.

Over time, thick sedimentary sequences built up that were relatively consistent, despite the chaotic formation.

In some parts of North Wales the sedimentary sequences continued to build up through the later Silurian period.

In the middle of the Devonian, around 375million years ago, the Acadian Orogeny began and lasted for around 50million years. This was a time of tectonic movement, mostly compressive, in the North Wales area. It folded and metamorphosed the sedimentary rocks into the slates we know today.

A series of faults and igneous (magma) intrusions complicate the sequences but essentially several strips, or belts, of slate deposits have been formed running along a north-east to south-west axis, becoming younger in geological age as you travel south through North Wales.

As is always the case with these short articles, the geological history and setting have been greatly simplified, but some basic explanation is necessary to help clarify some of the major problems encountered by the miners and quarrymen.

The Ffestiniog slate deposits occur in a series of five bands, or veins, that run at angles between 30º and 45º to the horizontal. The veins have been given names and, from the lowest to highest (or oldest to youngest), are called Manod Vein, New Vein, Old Vein, Back Vein and North Vein. A variety of non-slate rocks occur between the different veins.

The angle of the slate deposits meant that stripping off the over-burden was often not economically viable unless the deposit ran with the slope of the mountain. Thus mining was the most common form of extraction, although it was awkward due to the steep angle of the deposits.

The deposits range between 30m and 50m in thickness and are separated by varying thicknesses of other materials.

It has been reported that the Romans first used Welsh slate for roofing their fort at Segontium, now Caernarfon, although this is disputed. There is little evidence of any large scale roofing slate production, or any other slate industry, until the 18th century.

Around Ffestiniog there is no evidence for any significant use of the material until the opening of the Diffwys Quarry in the 1760s when Methusalem Jones dug here as a result of a dream.

The slate industry in the area really took off with the completion of the Ffestiniog Railway in 1836. Before that the slate had faced an awkward boat journey down the River Dwyryd and then by sea for the many small operations that had sprung up.

All five veins of material were not necessarily represented at each quarry or mine. Nevertheless, it was not untypical for two or more veins to be worked at the same time, which often led to major problems if the supports were not located at opposing positions. The pillar support system was often poorly thought out and insubstantial, and mine collapses were common. The gradual collapse of whole mountainsides caused many deposits to be abandoned.

History and development

Ffestiniog slate seems to have been used for roofing ever since it was first discovered, with industrial production occurring over the past 200years.

The town of Blaenau Ffestiniog developed rapidly, as this was the true centre of the mining and quarrying operations. The town had a population of around 11,000 during the height of the Victorian operations. According to contemporary records, there were 4,733 people working in the slate industry at its peak in 1898 – almost the same number as the current total population of Blaenau Ffestiniog.

There is considerable information available about the slate industry at the end of the 19th century thanks to a public enquiry being launched following a series of accidents that resulted in 163 fatalities over a 19-year period.

The adverse working conditions had prompted a strike at Llechwedd in 1893 and the public enquiry initiated that same year is considered to have been more than coincidental.

In addition to the fatal accidents, the conditions that prevailed within the slate workings, which were typically dusty, also lead to a high incidence of premature death. Between 1883 and 1892 the average age of a slate miner at death (excluding fatal accidents) was 54 years. However, the 1893 enquiry blamed the high mortality rate on tea-drinking, or, more specifically, the way the miners made their tea by stewing tealeafs in a pot rather than transferring boiled water from a kettle to a teapot!

Slate and the railway

It is difficult to discuss Ffestiniog slate without covering the development of the railway that served the industry.

There are several books on the subject and many other sources of information due to the thirst for knowledge of ferroequinologists (the term ‘trainspotters’ does not do them justice).

Local slate quarry and mine owners recognised the need to replace the slow progress and damage-causing combination of pack-horses, wagons and river boats used to get the slate to seafaring vessels.

After an Act of Parliament in 1832, the railway was planned. It worked on the basis of gravity down to the dockside, with the empty wagons being hauled back up to Ffestiniog by horses.

A narrow gauge of 23.5inches (597mm) was selected for the rails because it matched the gauge used in the quarries and mines that allowed the track to negotiate the many sharp turns of the terrain while accommodating the horses.

The railway network expanded around the rest of the UK on much wider tracks. As a result there were no locomotives readily available for Ffestiniog. There was also the small matter of a law that forbade the carrying of passengers on any narrow gauge rails, depriving operators of a source of income.

These factors slowed the development of the railway until the intervention of Charles Easton Spooner, who finally obtained the first two narrow gauge engines for the railway in 1863. A year later, after the arrival of two more engines, the railway was given permission to run passenger services, the first on a narrow gauge in Britain.

The single track fast became over-loaded with product and in 1869 an Act of Parliament was passed to double the line. However, the operators decided that, rather than resorting to the greater expense of building the second line, they would use more powerful locomotives to speed up the journey on the line.

The fortunes of the Ffestiniog railway ran parallel to those of the Ffestiniog slate industry. Both slowly ran out of steam.

The standard gauge railways reached Blaenau Ffestiniog before the 19th century was out and the slate industry began its decline at the start of the 20th century.

Two world wars and worldwide depression in-between led to the closure of many of the slate mines and quarries and in 1939, at the outbreak of World War II, the railway closed. By then, though, the mines and quarries were already using cheaper road transport.

A slate standard

The first British Standard for slate roofing products was published in 1944. It has been cited by some as a reason for the closure of some of the Welsh slate mines and quarries, although this seems misguided. Most Welsh slate could have met the standard and the decline of the industry producing the roofing had more to do with the changing post-war construction environment and the preponderance of cement roof tiles. These days, the old British Standard is held up as a testament to the quality of Welsh slate, with the new European Standard seen as having accepted poorer quality slates.

Properties of Ffestiniog slates

Ffestiniog slate exhibits an ideal combination of high flexural strength, good elasticity and well developed fissility, properties that are of primary importance for any top quality roofing slate.

These properties also allow slabs to be prepared for use as dimension stone, notably cills, copings, flooring and cladding.

The Ffestiniog slate has also been used occasionally for headstones, while the inert, low water transmission properties of the stone has allowed it to be used as liners for chemical storage and similar purposes. The Craig Ddu quarry has been producing a small amount of dimensional stone lately.

The Llechwedd Quarry is developing adjacent to the former mine that has been preserved and is open to the public as a historical centre. Modern methods of extraction have allowed the quarry, which has been cutting into the Back Vein, to be developed.

The most notable observation is the considerable variation in strength in different directions. This is due to a strong secondary direction of splitting or ‘grain’. The primary splitting direction is the cleavage plane, exploited in the preparation of roofing slates, which are still split by hand. With such a strong grain, the length of the slate has to be oriented parallel to the grain direction for optimum performance. Blocks are examined before they are processed to confirm the direction of the grain. This is done by taking a small piece of slate from the block and striking it with a hammer. The slate readily splits preferentially in the direction of the grain.

Blocks of slate also reveal the original bedding structure, which is identifiable from subtle variations in grain size and proportions, creating laminations or bands that cut at an angle to the cleavage independent of the grain direction. Sedimentary bands with slightly coarser average grain size and potentially higher silica content form subtle ribbons across the cleaved surface.

Microscopic examination of the Back Vein reveals how the cleavage plane has been slightly crumpled in one direction due to a second phase of compression after the original slate formation.

There is also evidence of mineral growth, suggesting the slate has been slightly heated, which may be related to the nearby intrusion of igneous rock.

Chlorite may be responsible for the slate exhibiting a slightly bluish appearance, although the material from the North Vein is particularly dark, almost black.

The occurrence of calcite or other forms of carbonate minerals in this slate is minimal, which is testament to its quality, as an excess of such minerals is often associated with poor performance.

The spacing of the cleavage is in the order of a few microns, which allows the slate to be split into exceptionally thin laminae. It has been reported that a 4mm thick Ffestiniog slate can be split using hand tools into a further 26 laminae. That means each would average just 0.15mm thick.

It has also been reported that a 3m long strip of the slate cleaved to just 1mm thick can be bent as easily as a similar strip of steel might be.

The European Standard test results demonstrate what the considerable history of use has already told us – that at thicknesses of 4mm this slate offers excellent weather resistance.

Concluding Remarks

The history of Ffestiniog slate production is one of hardship. Mountains have literally been moved in the pursuit of a natural material that is one of the most perfect for roofing purposes.

It is too easy to forget the toil and the suffering that has been associated with the production of slate. It is also apparent that the industry in Ffestiniog slowly wrote its own epitaph due to a lack of control and foresight combined with industrial and personal disputes.

The Ffestiniog slate industry deserves a shot in the arm. Welsh natural roofing slate in general continues to be current in architectural design and provides a top quality product that can perform for hundreds of years. And the Ffestiniog slate in particular has to be regarded as one of the very best roofing slates in the world. It is certainly one of the more interesting shades of grey and rightly deserves to be included among the great British stones.

The author: Barry Hunt

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 is also a regular contributor, covering the full spectrum of natural stone use, for Natural Stone Specialist magazine.