An Encyclopaedia of Indian Archaeology v1.c10.s2: 5.2 Archaeomagnetic
An Encyclopaedia of Indian Archaeology v1.c10.s2
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5.2 Archaeomagnetic
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Written record of magnetic data from various parts of the world is available only from the last century. But earlier records have been preserved in baked clay objects which have remained in position after cooling down. This property is called thermoremanent magnetism and is induced in iron oxide minerals in the clay by the geomagnetic field as they cool from temperature of 700°C and above. During the process of cooling baked clay acquires a weak but permanent megnetization in the direction of and in intensity proportional to the ambient field.
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For thousands of years clay has been used by man in making pottery, hearths, bricks and kilns. If the baked clay can be dated by reference to archaeological, ¹⁴C or thermoluminescent dating, one can make a record of geomagnetic variations in the past. Once such a secular variation curve is obtained, it can be used to date artefacts by fitting in their remanent magnetism in the known curve. The secular variation curve is different for different places in the world; hence separate magnetic curves have to be obtained for different regions. The necessary technique and criteria for reliable magnetic measurements for this purpose were established by Thellier and Thellier, 1959, and their first results showed that substantial variation had indeed occurred in the past. They found that in the Roman times the field intensity in France was some 60% higher than the present-day value.
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Athavale, 1960, carried out such work on 30 Indian archaeological samples ranging from 350 to 4200 B.P. His data showed a considerable scatter and no marked secular variation could be detected. But he found that the field intensity was 15% smaller in the past. His data did not prove definitive enough for use in any archaeomagnetic dating in India.
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Athavale, R.N., 1966, *in Nature*, 210, pp. 1310-12. Thellier, E., & Thellier, C., 1959, *in Ann. Geophysics*, 15, pp. 285-376.—S. KUSUMGAR