Case Study: Magnetic susceptibility

Case Study 9

Magnetic susceptibility: a non-destructive geological technique used in provenancing carved stones  

Nigel A Ruckley

Most igneous and metamorphic rock types contain varying amounts of ferromagnetic minerals such as magnetite. To a lesser extent sedimentary rocks, especially where they have been formed from the decay of igneous and metamorphic material, also contain limited amounts of ferromagnetic material. Measurements of magnetic susceptibility are closely related to the content of magnetite and other iron minerals and can be used in characterizing rock outcrops. By comparing the physical properties of the rock outcrop and its magnetic susceptibility to those of a carved stone it is possible to evaluate the source(s) of the carved stones. This technique was initially used in the provenancing of igneous rocks e.g. Roman granite columns (Williams-Thorpe and Thorpe 1993), greenstone axes (Markham 1997) and Charlemagne’s ‘black stones’ (Peacock 1997).

My attention was brought to this technique by scientists working for the National Museum of Scotland where it was evaluated using a KT-5 Kappameter at the Finlaggan excavation on Islay in the late 1990s. I decided to take this further by purchasing an Exploranium KT-9 Kappameter. Initial magnetic susceptibility evaluation at the Portmahomack excavations in 2000 led to working with the geologist Dr Suzanne Miller (formerly NMS), where this non-destructive geological technique helped to evaluate the carved stones at St Vigeans, Angus. A macroscopic petrological evaluation of the stones and evaluation of thin sections from local quarries allowed us to produce the first detailed geological report on these stones and their sources for HS in 2001 (Miller and Ruckley 2005). A step forward was taken at Whithorn in 2003, where results proved promising as it highlighted the Petrus Stone (No. 2) as being unique (dolerite) amongst the collection of mainly greywackes.

It became clear that sandstones and limestones of the Carboniferous to Permo-Trias age did not provide good subjects for this technique due to their low or lack of any ferromagnetic material. Dalradian metabasites, chlorite schists, slates and some Devonian sandstones have proved to be more promising. Carved Pictish stones, West Highland carved stones and material from quarries on the west coast of Scotland have provided over the years a plethora of data, but data from quarry sources and thin sections is always in short supply.

In 2012 the NMS undertook a trial evaluation of XRF techniques on carved stones at Iona Abbey. The results when linked to previous geological and magnetic susceptibility data were very encouraging. In 2013 a more detailed analysis of all the carved stones at Oronsay Priory was undertaken using these three techniques.(Caldwell et al. 2015). Initial results clearly showed how magnetic susceptibility data complemented the more expensive but more detailed XRF data. The way forward for non-destructive testing of carved stones lies with this triple pronged approach. Material that becomes available during restoration work on carved stones could be used either for geological thin sections or XRF analysis and the results retained for any future study.

A mechanism whereby a register of both published and unpublished geological analysis of carved stones is created would be invaluable for researchers. Perhaps an initial step would be the creation of a list of current researchers with a geological interest.

A KT-9 Kappameter NAR at use on the Isle of Oronsay 2013. Copyright David Caldwell