Category Archives: Ballachulish slate

Traditional roofing

A waterproof material to cover buildings is an universal requirement, not least in a wet climate such as Britain’s. In the past the choice of material usually depended on local availability. Hence, in the absence of suitable stone, thatch, either of straw or reeds, was commonly used throughout the country, while clay tiles were used  in areas with  suitable clay deposits, such as the southeast of England, . Flagstone was used locally in many parts of Britain, such as Horsham stone in Sussex or Caithness flagstone in the north of Scotland.    Slate, which is generally found in mountainous parts of Britain,  was used  as a building material in the North Wales,  parts of Scotland and the Lake District.  This use of locally sourced stone led to regional vernacular architecture reflecting the local geology.

Unlike flagstone which was rarely transported far from its source, the distribution of slate gradually increased in the 19th century, spreading out from its source, often in remote areas, along historic trade routes.  Improved transport systems coinciding with rapid urban growth resulted in slate being transported to all the major cities in Britain and Ireland, eventually becoming the principal roofing material.  The industry expanded rapidly to accommodate this demand, producing over 650,000  tonnes per annum in 1898. In spite of the rationalisation of the industry, and more recently globalisation, it is still possible to recognise the historical trading links. For example, because of the ease of transport across the Irish Sea, Cambrian slate from North Wales was used widely throughout Ireland and is still the preferred slate in that country. Similarly Cumbrian slates were transported northwest along the coast into Ayrshire and are still used extensively in the SW of Scotland.  Scotland too had a significant slate industry, producing from four different areas of which Ballachulish is the best known.

Production started to decline soon after 1900 and had already dropped to 111,000 tonnes in 1918. The industry partly recovered in the 1920s, to 297,000 tonnes in 1929, but by then manufactured clay tiles had become a major competitor and were taking an increasing proportion of the roofing market. This decline in production continued to the end of the 20th century reaching a low of 25,000 tonnes in 1993 followed by a modest recovery.  Quarries in Wales and England continued to close although production never ceased completely. However, in Scotland the largest quarry at Ballachulish closed in 1955 and the last remaining Scottish quarries closed in the 1960s.

 
 
 
 

 

Ballachulish Slate

East Laroch, largest of the Ballachulish quarries.

 

Ballachulish slate from the East Laroch and Khartoum quarries is grey-black with a slight sheen. It is coarse-grained, giving the slate a gritty texture. One of the most distinctive characteristics of this type of slate is the strong mineral lineation clearly visible on the surface.  Pyrite grains are common and are usually widely dispersed throughout the slate. The smaller grains are subeuhedral, i.e. they have recognizable but imperfect crystal faces, while the larger grains are anhedral, having irregular faces. In addition there are large clusters of pyrite grains concentrated in quartz veins running through the blocks of slate. The slate is very durable due to the higher than average metamorphic grade and the coarseness of the quartz grains. Pyrite grains when present in an euhedral form, are very resistant to weathering.

 

Not all Ballachulish slate is of the same high quality, in some quarries the pyrite crystals have been altered to a less stable mineral pyrrhotite which is prone to leaching and often fall out leaving a hole.

 

Ballachulish slate containing pyrrhotite are prone to weathering

Macduff slate

Many buildings in the area are still roofed with Macduff slate over a hundred years after production ceased.

Macduff Slate

Slate was extracted from the so called Slate Hills  of Kirkney, Corskie, Foudland, Tillymorgan and others in the NE of Scotland. Of these the most important quarries were on the Hill of Foudland and the name Foudland is sometime used as a generic trem for slates from the area.  Production started in the 1700s and reached a peak of almost 2 million slates in the mid 19th century. Most of the quarries closed during the second half of the century as the development of railways enabled slate from other parts of  Great Britain to be sold competitively in the area.

One of several quarries on the Hill of Kirkney. All of the quarries are very overgrown.

All of the quarries are located within the Macduff Slate Formation.  This Formation outcrops over a large part of the NE of Scotland, from Macduff on the coast, from where it gets its name, to Huntly 50km  to the south.   However slate has only been quarried as a roofing material in a range of hills just south of Huntly.  This is due to the proximity of an igneous intrusion which, due to increased temperatures at the time of emplacement, hardened the surrounding rock. As a result of this hardening, the slate rock forms the high ground, relative to the softer slate to the north.

Macduff slate have a rough gritty texture of a coarse-grained material, rich in quartz. It is possible to see small grains of quartz on the surface. The slates are generally blue-grey in colour often with a purple hue.  Unlike Ballachulish and Easdale slate, there is no pyrite present. Instead, the iron ore mineral  is an oxide, haematite, which gives the slates a purple colour. The most distinctive property of Macduff slate is “spotting”: small dark specks approximately 0.5 mm in size evenly distributed throughout the slate. These dark spots are mainly chlorite with mica intergrowths along the cleavage.

Macduff slate is still found on the roofs of buildings in the area over a hundred years after production had ceased; a testimony to the durability of the material. The Scottish Stone Liaison Group, in an attempt  to find new sources of Scottish slate, selected the Hill of Foudland as one of two locations for the extraction and testing of new slate. In 2003  blocks of rock were extracted  and split into slates from one of the Lower Quarries on the Hill  ( NJ608337), the first new Macduff slate in over a century. In 2005 this exercise was followed by the extraction of two cores, over 40m in length, from the floor of the quarry in order  to assess the resources of slate in the vicinity. (The results of this exercise are recorded in “Macduff Slate; Extraction and testing of slate from the Hill of Foudland, Aberdeenshire.” published by Historic Scotland in 2008.