Category Archives: Identification of slate

Identification of slates

The Watt Library, the Watt Hall and the Mclean  Museum and Art Gallery, Greenock

The Watt Library, the Watt Hall and the Mclean Museum and Art Gallery is a group of buildings located in the block between Union Street, Kelly Street and Watt Street in the centre of Greenock (Figure 1). The first building to be built was the Watt Library, built in 1832 and extended in 1837. The Watt Hall and the Mclean Museum were built in 1876. The Art Gallery is a modern building, built in 1955.

Figure 1: Aerial view of the buildings. This photo was provided by Collective Architecture.

The Watt library

 

 

 

 

 

 

 

Figure 2: Watt Library from Union Street.

The Watt Library 

The slates were originally blue-grey in colour which have darkened on the weathered surfaces. The dimensions of the slates are variable which indicates that they are Scottish. The presence of cubic crystals of pyrite and the crinkling surface called crenulation cleavage are characteristic of slate from Ballachulish or one of the Slate Islands. To distinguish between these two sources, it is necessary that XRF trace element analysis be carried out to determine the concentration of barium and other key elements used to distinguish between these sources. Some of the original slates have been replaced with Welsh purple.

 

 

Figure 3: The slates on the Watt Library were identified as either Ballachulish or from one of the Slate Islands on the west of Scotland.  Some of the original slates have been replaced with Welsh purple. 

The Library Extension

The slates on the Library extension were not identified.

 

 

 

 

 

 

 

 

Figure 4: The north-west extensions to the library.

The Watt Hall and the McLean Museum

Figure 5: Watt Hall and McLean Museum from Kelly Street

 

The slates on Watt Hall have a purple tint and are all the one size, namely 400x200mm. The uniform dimensions suggests that the slates are not Scottish and the purple colour suggests that they are Welsh from one of the Cambrian group of quarries. There was only limited access to the roofs of the Museum, however those slates that were inspected were similar to those on the Watt Hall. As both buildings were built at the same time, it is not surprising that the slates are from the same source.

Figure 6: The slates on the  the Watt Hall were identified as Welsh, from of the Cambrian group of quarries

 

 

 

 

 

 

 

Identification of slates of Royal Cottage

Case Study

There is no single procedure used to identify used slates, but rather a combination of methods ranging from local knowledge to more scientific analyses . For a general discussion on the methods used see Identifcation of used slates  The identification of the slates of the Royal Cottage are just one example of the application of some of these methods.

View of the intake of Glasgow’s main water supply

The front of the Royal Cottage

Royal Cottage is situated on the south shore of Loch Katrine at Stronachlachar approximately 20km north-west of Aberfoyle.   Loch Katrine became the primary water source for the city of Glasgow and the surrounding area when in the middle of the19th century an aqueduct was built to transport water to the city. Construction of the aqueduct started under the supervision of James Watt and Thomas Telford and was completed in 1859 (www.incallander.co.uk accessed 08/01/2010). The Royal Cottage was built to accommodate Queen Victoria on the occasion of the inauguration of the aqueduct. However, she never actually stayed in the Cottage as the windows were shattered during the 21 gun salute. 

 In November 2009 the Scottish Stone Liaison Group requested that slates from the roof of the  Cottage be tested in order to establish their provenance.     Using XRD and trace element analyses (as described below), it was found that the slates were from one of Aberfoyle Group of quarries

The slates of the Royal Cottage

Methodology: The slates vary widely in thickness, width and length from which it was inferred that they are Scottish.  However the absence of pyrite crystals or crenulation of the surface excludes Ballachulish and Easdale slates. For these reasons, and substantiated by their grey-green colour, the source of the slates was identified as one of the Highland Boundary Group of quarries.   This Group consists of a series of quarries  located at intervals just north of the Highland Boundary line from Arran to Dunkeld (Scottish Slate Quarries 2000 J A Walsh, Publishers Historic Scotland). However, of these the most likely quarries are Luss and Aberfoyle which are the closest geographically and are known to have produced slates in the 19th  century.  

To distinguish between  slates from the different Highland Boundary quarries,  X-Ray Diffraction  (XRD) scans of the Royal Cottage slates were compared with those in the database. One  of the characteristics of the  XRD scans used in distinguishing between different Highland Boundary slates is the shape of the white mica peak (or peaks) at 9° 2 theta angle.  In some slates there is a double peak at this location due to the presence of the sodium-rich mica, brammulite in addition to the normal potassium-rich mineral     In Bute  slates this secondary peak is well-defined, in Lusss slates less so, while in Aberfoyle slates it appears as a small lip on the usual white mica peak.  It was found that the Royal Cottage samples lacked the double peak of Luss and Bute slates, matching instead the shape of the peak of Aberfoyle slates in the database.  This observation was substantiated by comparing selected  trace element concentrations of the Royal Cottage samples with those in the database.  It was therefore decided that the Royal Cottage slates were from the Aberfoyle group of quarries.

 

Identification of used slates

Slates come from many different sources and it is often possible to identify the provenance based on local knowledge and obvious characteristics such as colour , the type of cleavage and the effects of weathering.   However, in order to uniquely identify the source, it is necessary to look at its properties in greater depth.  Unfortunately, identification of the principal minerals present is not a useful tool, as the same three minerals; quartz, chlorite and white mica, make up over 80% of the composition of most slates. It is the type of accessory mineral present (making up between 5 and 15% of the total) which is a better indicator of provenance.  For example, carbonate is present as dolomite in Ballachulish slate but as calcite in  Cumbrian and Highland Boundary slates. Similarly the type of iron ore mineral present is also characteristic of an area; it is present as a sulphide in Welsh grey and Ballachulish slates but as an oxide in Welsh purple and Macduff slates. 

 Trace elements are also a powerful tool in distinguishing between similar slates. As the name suggests these are elements which are present in very small amounts, some of which are characteristic of an area.  For example the  concentration of barium in Easdale slate is 1000ppm, 120% the norm for slates, while that in Ballachulish is 500ppm or 60%of the norm, making it possible to distinguish between two very similar Scottish slates.

Historical records are also useful in identifying a slate.  When no match in the database was found for a sample  from the Orkney Islands, it was necessary to look at possible sources outside of Britain.  It was known that Norwegian slates had been  exported extensively into Scotland until the 1940s.  A sample of Norwegian slate from Alta in the north of the country was obtained and found to mathc the unknown Orkney sample in all the tests performed.

 There is not a single pathway to identifying a slate, but by using a combination of historical records and individual tests, it is usually possible to identify the group of quarries from which it came.  However this may  not be so easy in the future, with ever increasing amounts of imported slate being used, unless good records are kept.