Magnetic susceptibility thickness corrections for small artefacts and comments on the effects of 'background' materials

Correction factors for magnetic susceptibility measurements on thin (<c.50m m thick), artefacts have been determined experimentally for a KT-5 Exploran ium G.S. instrument, using prepared blocks of Whin Sill dolerite. The corre ction factor is large (>1.4) for samples less than 10mm thick, and reduces to 1.01 for samples of 50mm thickness. Measurements on thin samples can als o be affected by the backing or substrate material on which they are measur ed. 'Background' material, for example, soil or plaster, can contribute sig nificantly to recorded measurements on artefacts, particularly for thin art efacts with low susceptibilities.