Slate Mines

Slate is a collective term for rocks that have excellent cleavage along closely spaced parallel surfaces, so-called schistosity surfaces. This primarily concerns rocks that have emerged from sedimentary rocks through metamorphism or tectonic forces. There are also sedimentary rocks that are very fissile along their primary bedding planes even without metamorphism and are called slate. In modern petrography, however, slate is only used for tectonically stressed or metamorphic rocks.

photography — Ardoisières de Vénosc. Public Domain.

On showcaves.com we refer to slate that was extracted in mines. A scientific definition of the term is much less interesting than the economical use of the rock.

Slate is primarily used for roofs. To do this, it must be able to be split so thinly that the weight can be borne by the roof. It must also be waterproof. And finally, it should last for many decades without weathering. In other words, slate has no pore space and is therefore both waterproof and resistant to frost heave.

Other uses for slate are wall panels, so-called façade slate, for which essentially the same applies. Recently, slate has been increasingly used as floor tiles, table tops, window sills, kitchen worktops and settings for stoves and fireplaces. Industrially, it has been used as a wall material for acid-proof tanks. Billiard tables are also usually made of slate. There is also its use as slate boards for writing, but these have nowadays been completely replaced by coated wooden boards.

Mining

Slate has been extracted from mines since Roman times. The reason for this is simple: if the slate is on the surface for a long time and is exposed to the weather, it weathers. Then it is no longer waterproof and also no longer durable. If it is mined in quarries, the upper metres have to be removed to reach the unweathered slate. Nevertheless, even exposed blocks weather quite quickly and can no longer be split well after a short time. Slate quarries are more or less only found in Britain, where winters are usually frost-free and weathering is therefore low.

Slate is quarried in large blocks, depending on the geological conditions. In the past, this was done with picks, with hammer and chisel, and with riving knives. In some cases, special, very weak explosive charges were also used. These blocks were broken down into transportable sizes with wedges and impact hammers. Transport to the surface was done by mine train cart and mine elevator, as is common in mines.

Mining in the 21st century is done using modern equipment and machinery. The blocks are raster-sawed in the mine with a diamond saw, and the blocks are loaded onto wheel loaders. Onward transport is still done by mine train and mine elevator. But there are also alternatives. The Margareta mine had driven a ramp in the shape of a spiral down to the mine, which allowed two-way traffic. In this way, the stones were transported from the quarry to the processing plant by large trucks without reloading. The empty trucks return to the mine via the same ramp.

Processing

The actual processing takes place above ground. Slate mines therefore have a very high proportion of personnel above ground. Here, the slates are first split so that they have a thickness of about 5 mm, which is ideal for roofing slabs. Then they are hewn or cut into the desired shape. Cutting is done with special scissor-like machines where one cutting blade is fixed, and one is moved with a long lever. For roof shingles, they are usually made square with rounded corners and a hole in one corner. Through this hole they can be fastened to the roof truss with a nail. The roof is covered by overlapping the sheets.

For the final finishing of roof slates, manual labour is still required. The prepared blocks are brought to their final shape on a piecework basis. This high level of manual labour is probably one of the main reasons why slate mines in Germany, which were market leaders only ten years ago, have now closed down. However, the depletion of deposits is also partly responsible.

Show Mines

The use of roofing slate seems to have a long tradition in certain parts of Germany, Luxembourg, Belgium, and France, as well as in Great Britain. Of course, it was mainly used where it could be quarried. The best slate is probably found in the Eifel and Ardennes, although the Australian Mintaro Slate claims to be the best slate in the world.

With the industrial revolution, however, there were also transport possibilities, especially with the railway. And so slate was used in areas where it was not mined, and areas with slate deposits could mine for export. In many cases this led to greater sales and better profitability, but it could also lead to cheaper competition.

In the 20th century, there was a clear trend towards fewer and fewer, larger and larger producers. The large number of small-scale mines operated by craftsmen was not competitive. At the end of the 20th century, there were only a few slate mines in Central Europe that were more or less market leaders.

However, due to the high costs of the necessary manual labour, slate mining was again relocated, mainly to Spain. The use of slate is probably greatest in France, followed by Benelux and Germany, as well as Great Britain. Today, however, the slate mostly comes from Spain, which hardly uses any slate itself and only mines it for export to Central Europe. Often, however, the mines there are operated by the mining companies that have given up their own mining in Central Europe.

A very significant effect of this development is also that in the meantime a great many slate mines have been converted into show mines. There are now two "slate routes", tourist routes that can be travelled by car, along which several slate mines are located. In France and the Benelux countries this is the Route de l'Ardoise (Slate Route), in the Eifel the Moselschiefer-Straße (Mosel Slate Road), and in eastern Germany the Thüringisch-Fränkische Schieferstraße (Thuringian-Franconian Slate Route). We have pages for two, the Moselschiefer-Straße concentrates on uses of slate and does not include any mine, so we recommend the respective Wikipedia page.