Magnetic mapping and dating of prehistoric and medieval iron‐working sites in northwest Wales

Abstract

As part of a long-term research project on prehistoric and medieval iron-working sites in northwest Wales, a technique has been developed for processing magnetic survey data to improve the presentation and recognition of the high-amplitude dipolar signals that are characteristic of ironsmelting furnaces. This technique has now been used successfully on some 37 British iron-working sites. High-resolution surveys, on a 10-cm grid, have been made both by caesium magnetometer and fluxgate gradiometer over four prehistoric furnaces and one medieval furnace giving detailed maps of their magnetic signals. Mathematical models of these maps, using multiple dipoles, has given estimates of the directions of total magnetization of the furnaces. Three of the furnaces were also surveyed at subsequent stages of excavation and after the removal of the furnace, giving the background signal. The background signals can then be subtracted from the survey data to give clean residual maps. These are easier to model, giving more reliable results, and they give useful information on the contribution of the different furnace materials to the overall magnetic signals. The corrections required for the influence of induced magnetism and for viscous remanent magnetism are examined. The results are compared with the British archaeomagnetic curve, and with the archaeomagnetic date estimates from three of the furnaces, to assess the value of the modelling technique in giving usable estimates of the date of last firing of the furnaces.

Using mineral magnetism to characterise ironworking and to detect its evidence in peat bogs

“…Magnetic susceptibility and fluxgate gradiometer surveys have been used at Crawcwellt and Llwyn Du to map the ironworking debris and to identify furnaces, smithing hearths and other features (Crew, 1999(Crew, , 2002. Archaeomagnetic measurements have also been made to date the last firing of furnaces and dipolar signals from high-resolution fluxgate gradiometer surveys have been modelled to recover the direction of total magnetization of the furnaces as a potential dating tool (Crew, 2002).…”

Geophysics in support of industrial archaeology in a challenging environment: Shade iron furnace, Pennsylvania, USA

“…Of these, magnetic, electrical resistivity and ground-penetrating radar (GPR) surveys are commonly most successful (Wynn, 1986;Sharma, 1997;Kvamme, 2000). Significantly for the present study, it is apparent that archaeogeophysical case studies in general, and those of iron workings in particular, have generally been poorly documented in North America (Jones, 2001) compared with Europe (Presslinger et al, 1986;Smekalova et al, 1986;Vernon et al, 1998;Crew, 2002), for instance. The reasons for this discrepancy are multifold, but we specifically observe that archaeological sites in North America are particularly regularly located in remote areas in woodland or other unfavourable terrain.…”

The use of magnetometry and pole-dipole resistivity for locating Nabataean Hawar archeological site in the SW-Jordan

“…Iron oxides are commonly found in the clay deposits that were used for the construction of these architectural features. If materials that are rich in iron oxides are heated above their Curie temperature and then allowed to cool in the ambient earth's magnetic field, they have the potential to acquire a considerable magnetic anomalies through thermoremanent magnetization as a result of the conversion of iron oxides to a more magnetic state (reduction of hematite to magnetite) that takes place in an organic environment (e.g., Tite and Mullins 1971;Clark et al 1988;Bevan 1994;Gibson 1986;Crowther and Barker 1995;Batayneh et al 2001;Crew 2002;Herbich 2003;Hus et al 2003;Batayneh et al 2007;Kovacheva et al 2004;Weston 2004). Thus, a sufficient contrast in material properties between the target and surrounding materials (dry loess sand) can be expected.…”