In the fifth of our series of blogs about the origins of natural stone, we are looking at where slate comes from. Slate is a fine-grained, homogeneous metamorphic rock, which is foliated (in geology: consisting of layers, layered). It comes mainly in a great variety of grey but can also be purple, green or cyan. It is mainly quarried in Spain and Brazil but is also quite commonly found in the USA and in Britain and Asia. Slate tiles have been used for roofing, flooring and paving for many centuries and are still one of the most attractive and durable materials to choose for your home decoration and renovation.
Formation and quality of slate
Naturally slate is formed by the alteration of a shale-type sedimentary rock that consists of clay or volcanic ash through regional metamorphism – mainly under low-temperature and pressure. It is the finest grained foliated metamorphic rock and its foliation is referred to as slate cleavage. When cut with the respective tools in the quarry along the foliation slate splits into nice flat sheets, which can easily be further processed into tiles of various usage. Slate has a mineral composition and consists mainly of quartz and muscovite along with chlorite (the abundance of which causes its green colour) or hematite (which turns slate into purple or more reddish shades) and other minerals. Darker slate colours are mainly due to the presence of iron sulfide in the rock. It is interesting that some slate tiles will fade in colour when exposed to the natural weather conditions, while there is some slate that doesn't change its colour and is referred to as "unfading".
Because of its durability and natural beauty slate is preferred for many usages in construction – from roofs to facades and pavement. It has a very low water absorption index, which makes it practically waterproof and very resistant to frost and freezing. Slate is preferred by professional builders both for roofing and for other construction works because it is extremely durable and will last centuries without or with very little maintenance. Slate is also fire resistant and energy efficient, hence perfect for roofs. It was, however, very expensive and therefore not all people were able to afford having it. It was also used for making walls in the past, which besides being strong also look quite attractive.
A slate roof of a tannery in St. Fagans, National Museum Wales (Cardiff, UK)
A roof window, covered with slate tiles.
Another example of a slate-covered roof with a window.
A wall made of slate tiles.
Slate tile quarries and supply
Quarrying or extraction of slate basically consists of removing layers or stone slabs form an identified geological deposit. The main slate producer in Europe and worldwide is Spain. Brazil is the second largest slate producer in the world. In the USA the main slate deposits are in New York, Pennsylvania and Vermont, while in the UK it is mainly found in Wales, Cornwall and Cumbria. In Asia, China is the leading slate producer and exporter both of raw material and finished products.
There are differences in the quarrying techniques depending on the place of origin but the overall process is the same. It starts with locating or creating breaks in the stone, then removing it, transporting it with the proper heavy vehicles and storing it where it can undergo further processing. The first step for any new quarry is first to expose the face of the deposit by clearing any vegetation or excessive earth on top of it. Afterwards a small explosive charge can be used for the slate extraction or the stone can be cut into by means of diamond belt saw. Due to its prominent cleavage slate slabs can be detached using steel wedges. The pieces vary in size and weight and can be carried away from the quarry by means of front-end loaders, dump trucks and other heavy machinery. After the extraction, slate is subject to additional processing depending on its purpose. This may involve initial cutting and splitting, applying a finish and additional cutting or shaping.
The first step is cutting and splitting the slate into slabs and tiles which can be done with a circular blade saw, diamond wire saw or splitter. Once the stone has acquired the desired shape it can be split into tiles either manually with a chisel and hammer or by means of automatic or semi-automatic splitting machines. Thus, the natural tiles, without any further processing, can be used for roofing for example. For other purposes the stone can be additionally treated to have a polished, honed or thermal-treated finish and shaped into a custom size.
It is interesting to note that many slate quarries, which are no longer in exploitation have been turned into attractive tourist sites, visited by a great number of hikers. Most of them are located in Wales, Scotland and Ireland, where you can also visit some museums, railway stations and other historic places related to slate quarrying.
The Ballachulish slate quarry, which is now an attractive tourist destination in Scotland.
Bryn Hafod-y-Wern Slate Quarry in Wales, operational in the 18th and 19th centuries.
The Cilgwyn Quarry, the oldest in Wales, was one of the most important producers of slate in the 18th century.
Slate debris at a disused quarry north of Bwlch-Llan, Ceredigion, Mid Wales
Entrance of the National Slate Museum at Llanberis, North Wales.
The Talyllyn Railway is a 7.25 miles (11.67 km) narrow-gauge railway in Wales, which carried slate from the quarries at Bryn Eglwys to Tywyn.
Usage of slate tiles
As already mentioned slate has been used extensively in building for quite a long time – for roofs, walls, etc. However, the stone was utilized for other purposes as well. In the 18th and 19th century slate was used in schools for blackboards and individual writing slate, for which chalk or slate pencils were used. A curious fact is that slate is used in the construction of pool tables as well. Since it doesn't deform as other materials used for the construction of pool tables it is a very good choice. Manufacturers use a polished slab with fine finish, which is 1 inch (2,5 cm) thick and cover it with fell. Due to its weight, however, the delivery of such pool tables is quite expensive.
Nowadays, slate tiles are quite popular both for interior and exterior projects. They are an excellent choice for a rustic kitchen floor or countertop, ideal for a mudroom or entryway and perfect for your patio or garden alleys. Slate tiles are also suitable for bathroom or wet room walls and can even be turned into a beautiful accent as a backsplash in the kitchen.
Individual writing slate.
Slate tiles used for a kitchen floor.
Exterior black slate paving.
Slate tiles used in an exterior garden project.
Have a look at the variety of slate tiles that we offer here.