Great British Stone : Cumbrian slate

After examining Yorkstone last month, Barry Hunt travels across the Pennines this month to put the case for Cumbrian slate to be included among the Great British Stones.

Cumbria as an administrative region did not come into existence until 1974, while the slates from the area have been quarried since at least the early 17th century and possibly even as long ago as pre-Roman times.

There is a belief that the skills to manufacture and use slate came with the Norman invasion, although there is scant evidence for this. Calder Abbey, founded in 1134 on the coast to the west of the Lake District, is recorded as having been roofed with green slates but history was not kind to the monasteries so now we can only suppose that this involved the nearby resources.

Within the region it was actually the variably black, blue and grey material now most commonly referred to as Burlington slate (or Burlington stone) that came to prominence as the nucleus of the industry. Burlington stone was first quarried in Kirkby-in-Furness, which was formerly the northernmost part of the traditional County of Lancashire and can still be referred to as Kirkby slate.

It was only in the 19th century that quarrying expanded significantly in the nearby old counties of Westmorland and Cumberland, extracting the variously green, white and occasionally slightly pinkish slates that are now probably most readily identified with the region.

There are two geological histories to Cumbrian slate, with those of principally green colour and lighter shades typically laid down in the Ordovician period (495-443 million years ago) and those of blue-grey and black colour laid down in the following Silurian period (443-417 million years ago). Therefore it is best to treat the two rock sequences separately, as there are differences other than just the colours.

Westmorland Green is yet another commonly used term to describe slates from Cumbria and really refers to those that are derived from a geological division known as the Borrowdale Volcanic Group.

The beauty of these slates lies in the fact that volcaniclastic sediments known as tuffs, rather than more typical muds, form the basis of the rock fabric, resulting in a coarse and variable texture.

Volcanoes in the region some 450million years ago blasted out materials that formed a variety of features depending on the distance the ash fell from the source and whether it ended up on the land, in the sea or in lakes and other landforms.

Fine ash tends to migrate many miles from a volcano while large volcanic ‘bombs’ fall close to it and material known as lapilli, with a size range from around 2mm up to 64mm, falls within a few miles.

Whatever size the particles are, a collection of volcanic ash that solidifies into a rock is known as ‘tuff’.

It is the presence of lapilli-sized ash particles that provides everything that is wonderful with these slates, especially when you add in the natural structures from different falls to create bedding and other features, maybe mix the ashes with some muds and sands, then stew everything at a low temperature and pressure to turn it into the metamorphosed rock we know today.

The stone is termed slate due to the presence of the splitting planes that have been created by the metamorphism, overriding the classification of the stone that was originally formed.

When the Westmorland slate is naturally riven the lapilli ash particles produce a coarse finish, which can be exceptionally rough. Sometimes the cleavage planes are also slightly curved. When the stone is cut and polished, the original sedimentary structure of the ash layers can often be seen, especially the variations in the coarseness of the constituent particles. The background green colour is from the growth of chlorite due to the metamorphism, while the speckled white and grey particles of lapilli might include quartz and feldspar crystals.

One common alternative name for the Westmorland Green slate is Lakeland Green. And there are many different local names, depending on where the material is quarried. Today, names such as Bursting Stone, Broughton Moor, Brandy Crag, Elterwater, Honister and Kirkstone are used to describe the different sources.

Other names that have been employed in the past include Buttermere, Cumberland, Cumbria Green and Spout Crag.

Burlington slate is the original Lancashire slate and earliest records of its use date from around 1600, at the end of the reign of Queen Elizabeth I. The Brathay slate quarry is near Ambleside, which is the meeting point of the three old counties of Cumberland, Lancashire and Westmorland, which could be a source of some of the location confusion.

But the most notable quarry is at Kirkby-in-Furness, where the material known today as Burlington stone is derived. The quarry has been worked continuously for 400 years, which is astonishing but still a few hundred years shy of the Delabole Quarry in Cornwall, which holds the crown for longest continuous operation. It is possibly because Burlington slate has been quarried for such a long time that it has entered the national psyche.

Burlington slate is both finer and darker than Westmorland slate, essentially bluish grey or black. The original sediments are about 25million years younger than those that formed the Westmorland slates and they are not necessarily volcanically derived.

The sediments were typically muds and silts laid down in basin environments. The darker colour is due to the presence of pyrite and finely disseminated carbon (usually as graphite that is the remains of long dead life forms).

However, the change of these sediments into slates essentially occurred at the same time as the alteration of the tuffs that became the Westmorland slates, at the end of the great mountain building period known as the Caledonian Orogeny around 400million years ago.

Basically, two continents collided and both these materials were at the centre of that collision. They were subjected to rock-altering forces (although I must confess this is a greatly simplified version of events).

Properties of Cumbrian slates

Several times I have gone on record as saying that the Cumbrian slates are my favourite stone. This is still the case.

It is not just for aesthetic reasons, but for the combination of geological features that provide them with the strength of a good quality roofing slate but with a low potential for delamination, a strong natural surface texture and considerable variety in appearance due to the different bedding of the tuff deposits.

Cumbrian slates are among the very few stones that can be used successfully not just for roofing but also for most masonry purposes, including cladding and paving.

Table 1 (on page 34) lists typical test results for the two main types of Cumbrian slates.

There is relatively low variation between the different Westmorland Green materials. The consistency could be due to the regional scale of the geological processes that originally formed the slates in spite of the potential for significant variation in the original tuffs.

The Burlington slate generally exhibits similar properties to the Westmorland slate, probably as a consequence of the similar metamorphism, although the strength of the Burlington slate can sometimes be significantly higher, probably due to the lack of large granular materials.

The test results are interesting when you compare them with those obtained when testing the material when it has been prepared for use as roofing slate. This is because of the way stone responds when it is tested in much thinner sheets with fewer laminations but a greater surface to volume ratio. Because of these differences, in Table 2 (also on page 34) I have included some of the typical test results for both main types of Cumbrian slate when they are prepared as roofing slates. Notably, the flexural strength reduces while the water absorption increases.

The strength reduction may be due to the influence of the original bedding structures and lines of weakness between cleavage planes becoming more significant. The increase in water absorption might be a consequence of the test methodology and the amount of water remaining attached to the greater surface area.

The tests on roofing slates also suggest that there is a grain or secondary splitting direction present, which may be related to the original sedimentary structures.

But what all these test results confirm is that the Cumbrian slates have good strength and durability characteristics, something that has been confirmed by their successful performance for roofing and masonry purposes over centuries in the UK environment, and more latterly around the world – because Cumbrian slate has achieved an international reputation thanks to marketing by the Burlington Stone company. It might be regarded as one of the very few natural stones from the UK that can claim to have achieved such international status.

Use of Cumbrian slate

Because Cumbrian slate splits easily it has been used for roofing since it was first identified as a building material. It would almost certainly also have been used locally for walling wherever it was easily accessible.

The stone seemingly became the standard material for cills and copings during the 20th century, although its use for other purposes, such as paving, flooring and cladding, is relatively recent. Its use for statuary is more recent still and is the result of bonding panels together securely in order to obtain sound, thick stone.

In all of these uses the stone performs admirably, although there are, as with every stone, some limitations to be aware of in order to reap the full benefits of the material.

Beginning with roofing slate that was the major use of the stone for so many years, there are innumerable buildings around the country for which Cumbrian slate was selected. In the 18th century it was used for the roofs of many major buildings, particularly country estates, and thus its longevity has been well documented.

The history of use and the esteem in which the stone is held allowed it to continue being used for roofing slate after the World War II, despite an aggressive British Standard for roofing slate that essentially decimated the roofing slate industry in Wales.

The low water absorption value that was set could not be achieved by Cumbrian slate but this was simply overlooked. Also, it could have a problem in the concentrated acid test due to the presence of carbonate minerals, so often this test was not carried out on the Cumbrian slate.

It is the carbonate minerals in the stone that are potentially its primary weakness as they allow it to be attacked by acidic environments.

What actually happens is that the natural gradual surface flaking that affects all slate over time is enhanced, so the stone becomes thinner. The indications are that every hundred years or thereabouts anywhere between 2mm and 5mm of surface can be lost.

However, the wide cleavage and rough texture of the stone has resulted in it traditionally being used at thicknesses of 25mm or more. This means that even after 300 years of weathering there is often sufficient material remaining for the slate to continue to perform successfully as roofing.

The European Standard for slate (BS EN 12326-1 and -2) takes into account those slates with carbonate materials and allows them to be used simply by calculating an increase in the thickness of the material required. The European Standard includes limits on dimensions, and the heavy grades of slate can fail these because of their roughness, but good slating practices can deal with the rougher slates and this should not be an insurmountable issue.

The great advantage of slate over most other natural stone is its high flexural strength, which is what allows it to be used in thin sheets.

While the strength of Cumbrian slate is not quite as high as that of good quality Welsh slate, it is still superior to granite, marble and many other high price and high performance materials.

This is why it is also a perfect material for paving, tiling and cladding.

In fact, as the UK began to experiment with thin stone cladding back in the late 1950s and early ’60s, Cumbrian slate panels were employed on many buildings and time has demonstrated how well the cladding has survived. Where there have been failures it has tended to be the fixing design or other factors that caused it.

The advantage of Cumbrian slate over other slates is its lack of cleavage planes, and thus fewer potential locations for delamination to occur. Generally, slate is not recommended for cladding because of potential delamination, but Cumbrian slate is a major exception to this rule.

Cumbrian slate, like the Yorkshire Carboniferous sandstones (Yorkstone), is an almost perfect material for paving, even though it is very different from Yorkstone.

It performs better than other slates due to the more highly siliceous nature of the constituents, so it is more resistant to scratching and abrasion, while the variable nature of the lapilli and ash grains mean wear characteristics can be inconsistent, which maintains a slip resistant surface.

The surface can be naturally riven or a variety of finishes applied. A honed finish shows off the natural variation of the stone to best advantage but this should be reserved for internal flooring.

Cumbrian slate is used for kitchen work surfaces, although there is a degree of limitation to panel sizes as the blocks of stone extracted are relatively small compared with those available in granite and similar. The stone is less resistant to scratching than igneous rocks and if it is polished, regular maintenance will be required to maintain the shine, so a honed or slightly coarser finish could be more appropriate for a kitchen.

The limited size of blocks leads us back to one feature of the Cumbrian slate and the way in which it has been quarried and used. Slabs would be prepared from blocks to the largest possible sizes to minimise waste but there were many different sizes produced. That allowed diminishing courses to be developed on roofs, providing a desirable aesthetic.

Diminishing courses were once normal on most stone roofs but the standardisation of sizes and general loss of roofing skills has meant it is now less common. It is a shame. The yields from many slate resources would be significantly higher if smaller pieces were not simply discarded. Thankfully, the production of random sizes continues with Cumbrian slates and there is nothing quite so magnificent as a well-laid roof with diminishing courses.

Concluding Remarks

Personal preference aside, there is no doubt that Cumbrian slate has stood the test of time both at home and abroad and is a material of exceptional quality in both performance and aesthetics.

The resources are unique because of the unusual combination of volcanic material turned to slate over a relatively limited area. This stone must be treasured. It did not need much assistance to make it a Great British Stone. It just had to be discovered.