Archaeomagnetic Determination of the Past Geomagnetic Intensity Using Ancient Ceramics: Allowance for Anisotropy

Abstract

INTRODUCTIONThe essential basis for determining the intensity of the magnetic field in which pottery was fired is the comparison of the thermoremanent magnetization then acquired, M A , with that acquired due to reheating in a known laboratory field, ML. As long as the intensity of the laboratory field, FL, is of comparable magnitude to the intensity of the ancient field, FA, then FA = (MA/ML)FL.(1)As was established by Thellier (1938) the magnetic grains that acquire a remanent magnetization on cooling through a certain temperature lose it at the same temperature on reheating; this is termed the blocking temperature (TB) for those particular grains. Within a ceramic sample there is usually such a variety of grains that the distribution of blocking temperatures is continuous from the Curie point (680°C for haematite, 575°C for magnetite) downwards. Thus equation (1) can be used to determine values for FA from a succession of laboratory reheatings to increasingly higher temperatures until the Curie point is reached. A given reheating temperature yields the value for FA as recorded by the grains having blocking temperatures in the range up to the reheating temperature. Such stepwise remagnetization is the essential feature of the method developed by Thellier more than two decades ago (Thellier and Thellier 1959) and it is still the method of accepted reliability whether for ancient ceramics or for volcanic lava. This reliability stems from the fact that the method allows detection of the reheating temperature at which mineralogical change interferes with the validity of equation (I), data from higher temperatures then being rejected. Recently an alternative technique for avoiding interference by mineralogical change has been established; this is the 'coercivity monitoring' method developed at the University of Liverpool by Shaw (1974). Its reliability is now well established and its importance for archaeomagnetism has been demonstrated in several applications (Shaw 1979, Gunn and Murray 1979, Games 1980. With the advent of SQUID* cryogenic magnetometers utilization of much smaller samples became feasible, and Walton developed a SQUID magnetometer specifically for paleointensity determinations on 3 mm cores extracted from pottery, using a modified version of the Thellier technique (Walton 1977(Walton , 1979(Walton , 1980. The smaller sample size allows an order of magnitude reduction in the time required for the stepwise reheatings. In the following pages we give an account of the procedures developed for use with this magnetometer that routinely take into account the effects of grain anisotropy (Rogers et al. 1979); such effects had not been * Acronym for Superconducting Quantum Interference Device. 53 54 noted in pottery before the use of this instrument but they are important in paleointensity determinations irrespective of method. ANISOTROPYMost types of pottery exhibit an intrinsic anisotropy in acquiring thermoremanent magnetization (TRM) that is much stronger than any shape anisotropy that arises from the dem...

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“…The PX ranges 3-17% in the bricks and from 10-21% in the potsherds (Table 4). Similar anisotropies for bricks and potsherds have been found elsewhere (Aitken et al, 1981;Veitch et al, 1985;Yang et al, 1993b). The TRM and ARM anisotropies are clearly higher, ranging from 22-44% in bricks and 10-20% in potsherds.…”

“…These values are an order of magnitude smaller than those which would produce significant shape anisotropy effects (Aitken et al, 1987). Using the values in Table 4 and, the rule of thumb by Aitken et al (1981), we estimate the self-demagnetization field B" (= 1.25 x J, where J is the total magnetization), to be <5 pT at maximum, which is <5% of the typical intensity values of this work (Table 1). We conclude that the shape anisotropy is not the cause for the high Ba at -AD 500 although it can cause some scatter in the intensity curves of Figs.…”

“…That archaeological objects have an intrinsic fabric anisotropy has been documented for potteries by Rogers et al (1979), Veitch et al (1985) and Sternberg (1989) and for bricks by Aitken et al (1981) and Tang et at (1991). We first noted the fabric anisotropy effect by observing growth of a perpendicular TRM component in some samples during the Thellier runs (Aitken et al, 1991).…”

“…Several investigators (Rogers et al, 1979;Dunlop and Zinn, 1980;Aitken et al, 1981Aitken et al, , 1987) have documented cases in which sample shape will affect the Ba due to self-demagnetization. The shape anisotropy can affect the measured intensity value in several ways; e.g.…”

Geomagnetic intensity variations for the past 8 kyr: New archaeointensity results from Eastern China

“…The effect of anisotropy of TRM (ATRM) on paleointensity estimation has long been recognized (Aitken et al, 1981(Aitken et al, , 1988Rogers et al, 1979). TRM anisotropy is sometimes observed in geological samples, but occurs frequently in archaeological artifacts because of manufacturing procedures, which makes the anisotropy correction in archaeological study non-trivial.…”