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.

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