Archaeointensity methods
5.2 Experimental methods
Prior to the archaeointensity experiments, the magnetic mineralogy of all materials was determined using a Magnetic measurements variable force translation balance (MM VFTB). Small chips weighing 100-150mg are placed in the instrument and their magnetisation can be measured in fields of up to 750mT. The IRM acquisition, coercivity, hysteresis and behaviour of the saturation magnetisation during heating and cooling (the thermomagnetic curve) were measured for all samples. With weakly magnetic samples it was not possible to obtain IRM acquisition data as the remanent magnetisations were too weak. Susceptibilities were measured on a Bartington MS2 susceptibility bridge.
Essentially the results reported in this study were obtained by two different experimental methods: microwave archaeointensity and Thellier. The apparatus for the microwave experiments is described in Suttie et al. (2010), although many of the results were obtained before the vacuum pump system of sample mounting had been developed. In these case the sample, a 5mm diameter core of approximately 3mm in length was glued to a quartz rod using heat resistant cement (Unifrax Fixwool®). After the development of the vacuum pump system the sample, of the same dimensions, could be directly mounted onto the sample tube, without the need for adhesive. After mounting the sample a computer controlled worm driven carriage lowers the sample into the three Tristan SQUID magnetometers for measurement of the NRM. The sample is then automatically withdrawn and positioned in the centre of the microwave cavity. Microwaves can then be applied at a given power for a set amount of time. The magnetisation is measured and the process repeated. Once the sample’s magnetisation is seen to fall a TRM can be imparted using the triple axis field coils that surround the cavity. This field can be applied in any direction. To minimise the effects of anisotropy, it was applied parallel to the NRM in all of the experiments reported here. High powers were avoided as the large electric fields produced could damage the sample (see chapter 3) and consequently it was not always possible to fully demagnetise the sample. To minimise the effect of any differences in the applied
55 power at each step, it is best if the applied field has a magnitude similar to that of the ancient field, although obviously this can only ever be an approximation.
Conventional Thellier experiments were carried out using a Magnetic Measurements thermal demagnetiser. A solenoid wound around the furnace allows a field to be applied using constant current supply. The field was set by measuring with a fluxgate magnetometer. Samples were prepared as 1” diameter cores. Measurement of the sample moment was done on either an Agico JR6 or an FIT Squid magnetometer. During these experiments the field could only be applied along the axis of the furnace, corresponding to the axis of the cylindrical sample. This makes it necessary to determine the anisotropy of TRM. Anisotropy of TRM was measured after completely demagnetising the samples by applying a field in each of 3 perpendicular directions and cooling the samples down from 470°C, repeating the procedure in anti-parallel directions. After cooling three 3 components of magnetisation gained were measured allowing the tensor components of anisotropy to be estimated. Details of the procedure are given in chapter 13.
The decision to measure anisotropy of TRM after completing the archaeointensity experiment runs the risk that the samples may have altered during the high temperature stages of the experiment. For this reason some workers prefer to determine anisotropy before the sample has been completely demagnetised (Chauvin et al. 2000, Hill et al. 2007), though the six repeated heatings risk damaging the sample before the completion of the experiment. In general the anisotropy measurements did not appear to be very successful. Some degree of the failure may be due to alteration although the holder that was manufactured for the purpose of positioning the samples in the furnace was somewhat crude in design, so the orientation might have been less than perfect. Anisotropy did not in general need to be considered in the microwave archaeointensity experiments because the direction of the applied field with respect to the sample could be precisely controlled (e.g. Le Goff and Gallet 2004).
A further consideration that could not be fully resolved is that of the cooling rate. The TRM gained by a specimen depends slightly on the rate at which it cools down. Essentially, a slower cooling rate allows magnetic grains to equilibrate with the ambient field at a lower temperature than a faster cooling rate. As their spontaneous magnetisation is higher at the lower temperature, there is a shift in the partition function that determines the position of the magnetic equilibrium. An increase in TRM of about 5-6% for every order of magnitude difference in the rate of cooling has been predicted on theoretical grounds for an
56 assemblage of single domain grains (Dodson and McLelland-Brown 1980) and observed experimentally (Fox and Aitken 1980). A thermally insulated furnace, capable of maintaining a temperature of 700°C for several hours, cooled down over a period of about two days, when switched off. After the anisotropy measurements samples were placed in this furnace in an applied field of 50μT and cooled down slowly. After measurement of the TRM, they were heated and cooled in the faster cooling furnace to check that the TRM thus gained was not substantially different from that gained during the main experiment. In general these experiments did not give reproducible results. Attempts to quantify the cooling rate correction appropriate to the microwave archaeointensity experiments are reported in chapters 10 and 11.