High-quality microwave archaeointensity determinations from an early 18th century AD English brick kiln

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1 Geophys. J. Int. (2005) 161, doi: /j X x High-quality microwave archaeointensity determinations from an early 18th century AD English brick kiln Ll. Casas, 1, J. Shaw, 1 M. Gich 2 and J. A. Share 1 1 Geomagnetism Laboratory, University of Liverpool, Oliver Lodge Laboratories, Oxford Street, Liverpool L69 7ZE, UK 2 Institut de Ciència de Materials de Barcelona, Campus de la UAB, Bellaterra, Catalonia, Spain Accepted 2005 March 8. Received 2005 February 15; in original form 2004 May 20 SUMMARY Samples of bricks from the Dogmersfield Park post-medieval brick kiln (Hampshire, UK), have been used to determine the intensity of the geomagnetic field during the early 18th century AD. Extensive magnetic mineralogical analyses have been carried out revealing weathered zones and multidomain dominated areas within the bricks. Likewise, statistical data treatment has been used to identify mineralogical heterogeneities. Once the most suitable parts for the study were selected, archaeointensity determinations were carried out applying Coe s doublestep variant of the Thellier method. The 14 GHz microwave system used in this study was developed at Liverpool University and the thermal demagnetizer is a commercial instrument. The former uses microwaves instead of heat to demagnetize the sample, the latter uses heat and was used for comparison. Data from the microwave system were of very high quality and uncertainties assigned to single measurements are smaller than the intrinsic variability of the archaeointensity stored within the bricks. Both ptrm and ptrm tail checks rarely fail, and even pass when the sample is almost fully demagnetized. Microwave intensities group around a central value ( 53 µt), slightly higher than the present-day field intensity, and agree well with previous studies and geomagnetic field models. Thermal results exhibit higher and more scattered values; some samples show evidence of laboratory thermal alteration. The different behaviour of samples when subjected to microwave and thermal experiments appears to be linked with the magnetic mineralogy (high-ti titanomagnetite): easy microwave demagnetization without alteration is achieved whereas sample alteration is almost unavoidable during thermal demagnetization. High-Ti titanomagnetite seems to be the ideal mineralogy for microwave experiments instead of pure magnetite, preferred in conventional Thellier experiments. The last part of this paper discusses the usefulness of conventional alteration tests applied to microwave experiments. It appears that for this technique negative alteration checks can originate from poor reproducibility of the absorbed microwave power. Key words: archaeomagnetism, microwaves, palaeointensity. GJI Geomagnetism, rock magnetism and palaeomagnetism 1 INTRODUCTION Palaeointensity determination is commonly performed by Thelliertype (Thellier & Thellier 1959) experiments, which require thermal demagnetization and remagnetization steps in a controlled magnetic field. The experiment can be time-consuming and thermochemical alteration often occurs during laboratory heating (Aitken 1990; Hill et al. 2002a). The microwave palaeointensity technique (Hill & Shaw 1999) avoids heating the sample by directly exciting spin waves in the sample (Walton et al. 1996). This technique has been successfully Now at: Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Napoli, Italy. casasduo@unina.it. applied in several studies of ceramics (Shaw et al. 1996, 1999) and lavas (Hill & Shaw 2000; Hill et al. 2002b). Several alteration tests (Prévot et al. 1985; Riisager & Riisager 2001), quality factors and acceptance criteria (Coe et al. 1978; Selkin & Tauxe 2000) have been established for the conventional Thellier-type experiments as a means of discriminating between good and unacceptable data. Alteration checks can also be applied to the data obtained by the microwave technique. This technique is based on the premise that the microwave (de)magnetization process avoids chemical alteration due to oxidation at elevated temperatures because the samples never become very hot (Walton et al. 1992, 1993), and so alteration checks should not fail. However, it is observed that the checks do not always pass; the reason for this may be the lack of reproducibility of absorbed power by the sample. C 2005 RAS 653

2 654 Ll. Casas et al. The present work deals with an archaeomagnetic intensity study of samples from the Dogmersfield Park post-medieval kiln (Hampshire, UK). The samples have been chosen because their age is well established by documentary and circumstantial evidence together with archaeomagnetic directional analysis (Linford 2003). Due to the particular mineralogy within the samples (highand low-ti titanomagnetite) microwave demagnetization is easily achieved. Moreover, the absorbed power within the sample is very reproducible as revealed by the constant shape and position of the ferrimagnetic resonance peak throughout the experiment. Hence, this study is an ideal opportunity to check the applicability of the conventional alteration tests to microwave experiments as well as provide new archaeointensity data. 2 THE DOGMERSFIELD PARK POST-MEDIEVAL BRICK KILN Archaeological excavations at Dogmersfield Park, in the county of Hampshire (southern England) revealed a post-medieval brick kiln. It is presumed that the kiln was used to manufacture the bricks for the construction of the early 18th century mansion that stands in the park. The kiln is constructed of bricks, and their surfaces show indication of vitrification, evidence that the structure had been exposed to high temperatures. Documentary and archaeological evidence date the last firing in the kiln between AD 1698 and This firing would have produced a thermoremanent magnetization (TRM) within the bricks (Linford 2003). Linford (2003) conducted archaeomagnetic directional analyses of the bricks. From this analysis, the date for the last firing of the kiln was between AD 1700 and 1720 at the 63 per cent confidence level. Starting from this very well dated material, we have carried out an archaeointensity analysis of several bricks using the microwave technique. 3 SAMPLES AND METHODS Three bricks were available for analyses; shallow preliminary investigations were conducted on the three. Simple visual examination reveals two different zones: (i) a fragile red-coloured zone with high porosity and evidence of weathering and (ii) a harder black zone comprising those parts directly exposed to the fire, covered with a vitrified green-coloured glazed surface as a consequence of the high temperatures reached within the kiln. Plate-like samples were cut from the different parts of the bricks and analysed using a Bartington MS2B meter and probe to determine their susceptibility at room temperature and at low and high frequencies, and also the change in susceptibility as they warm up from liquid nitrogen temperatures (77 K to room temperature). For the latter, the samples (with volumes ranging from 1 to 4 cm 3 )were attached to a thermocouple and enclosed in Blu-Tack. Since the system assumes that the sample volume is 10 cm 3 all the readings were corrected and converted into mass specific susceptibility to enable comparison between samples with different densities. Hysteresis properties, isothermal remanent magnetization (IRM) acquisition, remanence coercivity and magnetization versus temperature curves were obtained by using a Magnetic Measurements variable field translation balance (VFTB). Small sample chips (mass around 100 mg) were taken from the different parts of the bricks, placed in a 5 mm diameter cup at the end of the sample holder and packed with quartz wool. Hysteresis parameters were retrieved subtracting the paramagnetic contributors and Curie points were obtained using the graphical tangential method (Grommé et al. 1969). Special attention was given to one of the bricks as it appeared to be the most suitable for microwave archaeointensity analysis. Neighbouring parts of the samples used for microwave analysis were systematically measured and compared with the actual microwave samples once measured in the microwave system to assess the lack of magneto-mineralogical changes due to the microwave irradiation. Samples taken from the different parts of the brick used for microwave archaeointensity analysis were crushed and around 150 mg per sample were packed in small plastic bags and measured with a conventional transmission Mössbauer spectrometer. The system operated in constant acceleration mode, with a 57 Co source in Rh and was used to analyse the Fe valences and the magnetic ordering state of the iron-bearing minerals of the different parts of the brick. Spectra were recorded at room temperature and at 80 K. The calibration was undertaken using a 25 µm thick α-fe foil. Small cylindrical samples (5 mm diameter, 3 mm length) from different parts of the bricks (especially from their black zones) were subjected to a Thellier-type experiment, according to the Coe variant method (Coe 1967) but using microwaves instead of heat to directly generate magnons (Shaw et al. 1999). Samples from one of the bricks exhibited magnetic moment readings one order of magnitude stronger than the others, thus measurements were focused on samples from that brick. The microwave system used operates in the 14 GHz frequency range, and throughout the experiments the frequency was finely tuned to the ferrimagnetic resonant frequency of the sample. In these experiments, the original natural remanent magnetization (NRM) of the sample is measured and gradually removed and replaced by a new microwave thermoremanent magnetization (T M RM). This is achieved by applying microwaves at progressively higher power alternatively in zero (Z) and an applied (A) magnetic field. Remagnetizing the sample in the same direction as the NRM avoids the effects of magnetic anisotropy (Rogers et al. 1979), which can be important for archaeological samples. As well as the conventional Z/A steps, ptrm and ptrm tail checks (Riisager & Riisager 2001) were performed to ensure the absence of alteration and multidomain behaviour within the magnetic remanence carriers. From the various steps performed in zero field, orthogonal vector plots (OVP) were obtained and used to check the directional uniformity of the NRM vector. Additionally, pure demagnetization experiments were also performed for some samples. Cylindrical samples (9 mm diameter) from the black zones of the three bricks were subjected to the conventional Coe variant of the Thellier experiment (Coe 1967). Alternate heating steps of thermal demagnetization and remagnetization in 50 µt applied field were performed in an air atmosphere using a Magnetic Measurements thermal demagnetizer. The samples were placed into glass tubes to minimize oxidation during heating. As in the microwave experiments two sets of ptrm and ptrm tail checks were also carried out. However, because of the experimental set-up, in these conventional experiments the magnetic field was not applied in the same direction as the NRM but perpendicular to the base of the bricks. Magnetic moments were measured using a Molspin spinner magnetometer. 4 MAGNETIC MINERALOGY In agreement with the visual differences, magnetic properties differ from the red (R) and black zones. Moreover, magnetic measurements obtained by using the VFTB reveal two different magnetic areas within the black zone (B1 and B2). Spatially, the B1 area is in the opposite corner with respect to the red zone, whereas B2 is between both (Fig. 1). Each zone has a characteristic magnetic

3 Microwave archaeointensity determinations 655 Figure 1. A brick from the Dogmersfield Park kiln with an indication of the different magnetic mineralogical parts. property pattern with only two samples with anomalous values within the B1 zone, labelled hereafter AN1 and AN2 (see Table 1). Curie points, which were obtained from M s versus T plots, allow an easy distinction between B1 and B2 areas (Fig. 2). In B1, a low Curie temperature phase (T C 330 C) dominates the signal and a higher T C phase is also present. Zone B2 is dominated by high T C ( 570 C) minerals, and only in some cases is a lower T C phase present. The ferrimagnetic mineralogy of the R zone is similar to B2. As in common sedimentary materials (Smith 1999) and burnt clays (Jordanova et al. 2001, 2003), the magnetic phases present in all parts of the brick are probably members of the magnetite ulvospinel solid solution series. In this series the increase in Ti content result in a lower T C. Cooling curves suggest the type of thermal alteration reactions that occurred in the samples during the measurement of thermomagnetic curves; titanomagnetite into haematite for the R-type samples and high-ti titanomagnetite into low-ti titanomagnetite for the B1 samples. The absolute values of M rs and M s are quite variable from one sample to another, ranging from a few tens to several hundred Am 2 kg 1 indicating that the ferrimagnetic mineral concentration is not homogeneously distributed. However, within the red area the values tend to be lower than in the black. For a general view of typical hysteresis loops for each brick zone see Fig. 3. The M rs /M s and H cr /H c ratios indicate fine particles commonly ascribed to the pseudo-single-domain regime (PSD) (Day et al. 1977), or mixtures of single-domain (SD) and multidomain (MD) states (Dunlop 2002). However, a measurement from a B1-type sample exhibits an anomalously low M rs /M s ratio close to the accepted values that reveal MD behaviour. Except for the anomalous B1-type sample, the higher H cr /H c and lower M rs /M s ratios found in the R zone indicate a higher proportion of MD particles, and thus a larger particle size compared with the B areas. H c values indicate that B1 and R zones are magnetically softer than the B2 zone, as the latter has similar mineralogy to R zones; this suggests again a higher MD content in the R zones. Wasp-waisted hysteresis loops, especially noticeable from R-type samples, suggest more than one magnetic mineral population or a broad range of particle sizes. Also in R-type samples, the fact that loops do not close at high fields gives evidence of the presence of haematite (Fig. 3c). The χ hf slope points to a lower paramagnetic content in the R-type samples. The temperature dependence of susceptibility provides more data with which to distinguish the different parts within the bricks (Fig. 4). The B1 areas exhibit strong increases in susceptibility from 77 K to room temperature as expected for high-ti titanomagnetite (Moskowitz et al. 1998), whereas the B2 areas show less temperature dependence but with a characteristic peak around 130 K (the Verwey transition). It is well known that this peak is the fingerprint for magnetite or low-ti titanomagnetite in a MD state (Moskowitz et al. 1998). Unfortunately the absence of a peak in B1-type samples does not imply the absence of MD high-ti titanomagnetite. High-Ti titanomagnetite would keep the SD state at sizes larger than those of low-ti titanomagnetite (Day et al. 1977; Moskowitz 1980) and it has even been suggested that the MD state is suppressed for such compositions (Radhakrishnamurty et al. 1980). The interpretation of susceptibility plots for R-type samples is not so clear. The susceptibility is not temperature dependent at low temperature, but at around 100 K susceptibility starts to increase, becoming strongly temperature-dependent; this dependence should not be attributed to high-ti titanomagnetite, as this composition is not seen in the Curie curves. However, the change at 100 K could be related to an ilmeno-haematite phase (Kontny & de Wall 2000). Magnetic susceptibility measurements at room temperature were performed at two different frequencies (470 and 4700 Hz); the values are within the same order of magnitude for all the samples. It is worth noting that the lowest and highest mass specific susceptibility was found for R-type samples (see standard deviations in Table2)revealing high magnetic heterogeneities in this zone of the bricks. The superparamagnetic content is evaluated by means of the frequency-dependent susceptibility (χ FD ), (Dearing et al. 1996) which indicates a relatively high superparamagnetic content for R- type samples. Mössbauer measurements on R-type samples show a magnetic sextet and a Fe 3+ non-magnetic doublet. The hyperfine parameters of the sextet correspond to haematite, especially conclusive is the negative sign of the quadrupolar shift ( 0.18 mm s 1 ) and the hyperfine field (50.0 T). The lack of Fe 2+ is further evidence of the oxidation that has occurred in this part of the bricks. In contrast to R-type samples, in B-type samples the amount of Fe 2+ is at least as important as the amount of Fe 3+ ; the actual ratio varies from sample Table 1. Hysteresis and thermomagnetic parameters: M rs, saturation remanence; M s, saturation magnetization; H c, coercivity; H cr, coercivity of remanence; T C, Curie temperature and χ hf, paramagnetic slope averaged parameters for the two black (B1 and B2) and the red-coloured (R) areas within the bricks. Sample M rs /M s H c H cr H cr /H c T c1 T c2 χ hf type (mt) (mt) (K) (K) (10 6 m 3 kg 1 ) B ± 0.07 a 18 ± 9 b 51 ± ± ± ± ± 2.5 B ± ± 7 86± ± c 574 ± ± 3.8 R 0.20 ± ± 1 35± ± ± ± 1.2 a A sample from this area gave an anomalous M rs /M s value of This sample is labelled as AN1. b A sample from this area gave an anomalous high H c value of 50. This sample is labelled as AN2. c This transition temperature is not always clearly seen.

4 656 Ll. Casas et al. Figure 2. Characteristic thermomagnetic curves for B1-, B2- and R-type samples. Figure 3. Characteristic hysteresis curves for B1-, B2- and R-type samples. This increase indicates the presence of superparamagnetic grains within the samples. It is clear that the magnetic sextet seen for B- type samples has Fe 2+ and Fe 3+ contributors as both Fe 2+ and Fe 3+ doublets decrease its area when the spectrum is recorded at 80 K. 5 ARCHAEOINTENSITY ANALYSIS Figure 4. Low-temperature susceptibility curves for several B1-, B2- and R-type samples. Table 2. Susceptibility measurements at room temperature: χ LF, specific mass susceptibility at low frequency (470 Hz); χ HF, specific mass susceptibility at high frequency (4700 Hz); χ FD, frequency-dependent susceptibility. Sample χ LF χ HF χ FD type (10 6 m 3 kg 1 ) (10 6 m 3 kg 1 ) (per cent) B ± ± ± 0.9 B2 7.5 ± ± ± 1.0 R 9.6 ± ± ± 0.4 to sample. At room temperature, the magnetic sextet (48.9 T) represents a small fraction ( 20 per cent) of the total spectrum area, indicating that the iron-bearing remanence carriers constitute only a small portion of all the iron-bearing components. Spectra recorded at 80 K show similar features to the ones at room temperature; however, the fraction of the magnetic sextet increases to around 10 per cent for R-type samples and 14 per cent for B-type samples (Fig. 5). 5.1 Microwave analysis Magnetic mineralogy data, weathering evidence on the red zone and orientation of the bricks with respect to the firing place suggest the most suitable part of the bricks to be used for archaeointensity. In addition, the composition of B2 could be the result of alteration at intermediate temperatures ( 300 C) and evidence of MD behaviour has been found in this part of the bricks. Thus the B1 part, despite being more susceptible to thermal alteration, holds the highest fraction of SD and PSD particles and can be readily demagnetized using the microwave system. However, it has to be remembered that only a small fraction of the magnetic grains are the NRM carriers (Radhakrishnamurty 1989). Low magnetic moment readings were found on samples from two of the three bricks resulting in noisy NRM directions. From the remaining brick, all except two samples from the B zone showed stable NRM directions. The samples with unstable NRM directions are the previously labelled AN2 and another without distinctive magnetic properties, hereafter named AN3. Maximum angular deviations (MAD; Kirschvink 1980) show lower values for the B1 area (excluding the AN2 and AN3 anomalous samples). That is especially clear when the first NRM measurement, which often includes a viscous remanent magnetization, is excluded from the calculation (see Table 3). Conversely, R-type samples were virtually impossible to demagnetize by using the maximum microwave power available. To determine the archaeointensity, the NRM lost was plotted against the T M RM gained (both normalized to initial NRM), along with the ptrm and tail checks as in conventional Arai diagrams (Yu & Dunlop 2003). Representative plots are shown in Fig. 6 with the corresponding OVPs. The slope of the diagrams indicates the ratio between the palaeointensity and the applied field in the

5 Microwave archaeointensity determinations 657 Figure 5. Characteristic Mössbauer spectra for B- and R-type samples recorded at 300 and 80 K. An indication of the relative areas of magnetic sextets and Fe 3+ and Fe 2+ doublets is given. Table 3. Mean maximum angular deviations (MAD) from orthogonal vector plots. Zone MAD ( ) MAD a ( ) B1 1.8 ± ± 0.3 B2 2.5 ± ± 0.5 a MAD calculation excluding the first NRM measurement. laboratory. The intensity results are shown in Table 4 along with the associated Coe quality parameters (Coe et al. 1978). From Table 4 it can be seen that acceptance criteria (Selkin & Tauxe 2000; Riisager & Riisager 2001; Carvallo et al. 2004) are easily met. For example in this study f factors are generally >0.8 and according to Biggin & Thomas (2003) f need only be greater than 0.5. Acceptable quality factors (q) must be more than unity (Selkin & Tauxe 2000) and high-quality results have q values of several tens (Hill & Shaw 2000). Apart from one anomalous sample, all the samples in this study yielded q values greater than 10 and in some cases over 100. The ptrm check acceptance criteria (Selkin & Tauxe 2000) are also generally met; samples B1b, B2d, AB3b (all with a single failed ptrm check) and B2e (with three failed ptrm checks) are the exceptions. Reasons for these failed checks are discussed in the next section. The ptrm tail checks pass for all samples, although the validity of this check has been questioned when the applied field is parallel to the original NRM (Yu & Dunlop 2003). Combining the results from B1-type and B2-type samples the averaged archaeointensity is found to be 52.8 ± 2.5 µt. Presuming that results from B1-type samples reproduce more faithfully the archaeointensity we could exclude B2-type samples from the average, giving 51.9 ± 1.9 µt asthe final result. It is noticeable that all samples classified as anomalous (AN) have adequate quality factors and some of them extremely good ones. Moreover, all but one passed the ptrm checks. However the archaeointensities calculated from anomalous samples deviate significantly from the mentioned averaged values (see Table 4). 5.2 Thellier analysis Since the introduction of the microwave technique, several comparisons with the conventional Thellier method have suggested that T M RM and TRM are equivalent (Shaw et al. 1999; Hill et al. 2002a). Four B1-type samples were measured: B1Ta and B1Tb, from the brick intensively investigated using the microwave system and B1Tc and B1Td from the remaining two bricks. From the Arai plots shown in Fig. 7 it is clearly seen that B1Ta and B1Tb samples do not pass the last ptrm check, which was performed at 300 C. It has been already commented that Curie curves recorded on B1-type samples reveal thermal alteration (see Fig. 2). The minimum temperature to initiate the alteration was investigated measuring the irreversibility of Curie curves. The alteration does not occur at a given temperature. It seems that it is a gradual process that actually starts from temperatures as low as 200 C. Although the alteration rate is strongly sample dependent (see Fig. 8), all measured samples showed clear alteration at 400 C. The conventional thermal Thellier intensity results are shown in Table 5. In contrast to the former results, the dispersion of values is quite high. Besides the alteration processes, part of this dispersion can be attributed to the anisotropy of the remanence. Thermoremanence acquisition ellipsoids were determined for several samples and the main axis was found to be subperpendicular to the brick faces. The percentages of anisotropy vary from 6 per cent to 10 per cent, implying possible correction factors ranging from 1.1 to 0.9. However, the field was applied perpendicular to the base of the bricks (only 15 away from the NRM direction). Samples whose ptrm check fail exhibit higher archaeointensities. Results from B1Tc and B1Td samples could be considered as acceptable. The result from B1Tc lies in the range of values obtained by the microwave technique

6 658 Ll. Casas et al. Figure 6. Representative plots of normalized NRM remaining against T M RM gained with OVPs for B1-type, B2-type and anomalous samples, obtained from the microwave equipment. The applied field was 50 µt inall cases. Table 4. Results from microwave archaeointensity analysis: N, number of data points; f, g, q are quality factors defined by Coe et al. (1978); H, archaeointensity estimate. Uncertainty is calculated after Coe et al. (1978). Sample N f g q H (µt) Uncertainty B1a B1b B1c B1d B1e B1f B1g B2a B2b B2c B2d B2e AN AN2a AN2b AN3a AN3b and is in keeping with higher quality factor values with respect to sample B1Td. 6 RELIABILITY OF THE RESULTS The consistency of the overall results obtained can be assessed by comparison with available data from archaeomagnetic studies, direct measurements of the Earth s magnetic field and models derived from these direct observations. There are not many published archaeointensities from English sites. Two articles report archaeointensities from a similar period to the Dogmersfield Park bricks: Games & Baker (1981) published data from material dated as AD 1645 ± 10, with an overall archaeointensity of 51.5 ± 2 µt from Rainford (Merseyside) and Games & Davey (1985) report 50 ±2 µt for AD samples at Exeter, but only 40 ± 1 µt for AD samples from Liverpool. All these archaeointensities were obtained using Shaw s ARM technique (Shaw 1974) and Chauvin et al. (2000) consign low reliability to them. Chauvin et al. (2000) published archaeointensities from a kiln located near Durtal (Pays de la Loire, France) with an estimated age between AD 1650 and From this site, the calculated archaeointensity adjusted to the latitude of Paris is 51.8 ± 4.2 µt. There is good agreement with the microwave archaeointensity from Dogmersfield Park, which can also be adjusted to Paris, assuming a dipole geomagnetic field. The mean

7 Microwave archaeointensity determinations 659 Figure 7. Plots of normalized NRM remaining against TRM gained with OVPs obtained from the Thellier experiment. The applied field was 50 µt in all cases. Table 5. Results from conventional Thellier archaeointensity analysis: N, number of data points; f, g, q are quality factors defined by Coe et al. (1978); H, archaeointensity estimate. Error is calculated after Coe et al. (1978). Sample N f g q H (µt) Error B1Ta B1Tb B1Tc B1Td Figure 8. Evolution of the irreversibility between the heating and the cooling curves (measured at 100 C) when progressively heating a set of B1-type samples. Dogmersfield relocated archaeointensity is 52.1 ± 2.5 µt (or 51.2 ± 1.9 µt using only B1-type samples). Direct geomagnetic measurements started during the 17th century, but were focused on declination and inclination observations. The first reliable field intensity records from the London area started in 1846 in the Greenwich Observatory (at almost the same latitude as Dogmersfield Park); they yielded a value of 47.8 µt (Barraclough et al. 2000). Models based on historical record yield an intensity value of µt (Bloxham & Jackson 1992) and µt (Jackson et al. 2000) at the Dogmersfield site coordinates (longitude 0.9 W, latitude 51.3 N). The above comparisons confirm that the microwave results from B1 areas (high-ti titanomagnetite) and from B2 (although with lower quality factors) give a good estimate of the Earth s magnetic field intensity. However, conventional Thellier experiments on samples from the B1 area give much more scattered results. Commonly, because of low thermal stability, high-ti titanomagnetite is regarded as unsuitable for palaeointensity analysis (Tarling 1983). The lack of thermal alteration in microwave experiments allows the use of samples with such magnetic mineralogy. Moreover, high-ti titanomagnetite appears to be easier to demagnetize and produces higher-quality archaeointensity estimates. Also, the high- Ti titanomagnetite critical size for SD behaviour is larger than for magnetite, thus being more likely to meet the SD requirement for palaeointensity determinations. The susceptibility to thermal alteration of high-ti titanomagnetite-bearing samples is confirmed by the thermal (de)magnetization experiments (from the conventional Thellier technique) and the usefulness of ptrm checks has been confirmed for these experiments. However, such utility is not so apparent for the microwave experiments. Some experiments give anomalous archaeointensity estimates but with positive ptrm checks (samples AN1, AN2a, AN3a and AN3b). On the other hand, there are some samples that give consistent archaeointensity estimations but some of their ptrm checks fail (samples B1b, B2d, B2e or even AB3b). There is no evidence that ptrm checks do not detect alteration during the microwave experiments. The anomalous results from almost all AN-type samples are probably related to heterogeneities within the brick. For several reasons, these parts of the brick may not have been able to store correctly the archaeointensity of the last firing. The detection of their anomalous character has been

8 660 Ll. Casas et al. identified by different magnetic behaviour (samples AN1 and AN2) and unstable NRM directions (samples AN2 and AN3). The smaller size of the samples used for microwave experiments could imply a higher incidence of heterogeneity problems that would be averaged out if larger samples were used. However, a statistical analysis of sufficient data will always reveal such heterogeneities. It is clear that for microwave experiments, excluding alteration, other mechanisms could result in a failure of ptrm checks. Such a mechanism, as pointed out in the introduction to this paper, is probably a simple lack of reproducibility of absorbed microwave power by the sample. It is observed that for some samples, during the several microwave steps, the shape and position of the resonance peak changes. This problem does not always occur and it seems to be related to the nature of the sample, for instance B2-type samples appear to be more inclined to experience resonance peak changes than B1-type samples throughout the microwave experiment. 7 CONCLUSIONS Bricks from the Dogmersfield Park post-medieval kiln have accurately recorded the geomagnetic field of the early 18th century, with an intensity of around 53 µt. This result is consistent with available data from other archaeomagnetic studies and models based upon historical geomagnetic records. The microwave technique has proven to be a very useful technique with which to obtain high-quality archaeomagnetic estimates. However, a magnetic mineralogical analysis is always recommended to locate suitable sampling zones of bricks. High quality factors and positive alteration checks on individual samples are important, but statistical data treatment of a representative population of samples is necessary to identify heterogeneities that result in anomalous individual archaeomagnetic estimates. In contrast with conventional Thellier methods, where magnetite is the ideal mineralogy, the microwave technique appears to work better with samples dominated by high-ti titanomagnetite. This will allow the analysis of samples that would normally alter when subjected to conventional thermal Thellier experiments. Negative alteration checks can originate from poor reproducibility of the absorbed microwave power. The source of peak resonance changes should be a subject of further study. At present, it can only be stated that changes in resonance depend upon the nature of the sample. Consequently, the usefulness of the microwave technique remains moderately sample dependent. ACKNOWLEDGMENTS P. Linford (English Heritage) is recognized for providing the samples. We express our gratitude to M. Hill and M. Gratton for useful comments and revision of the text. R. Holme is acknowledged for comments and help on magnetic field models. The paper benefited from constructive reviews by E. Petrovsky and L. Tauxe. Financial support from AARCH Research Training Network (UE) and NERC is acknowledged. REFERENCES Aitken, M.J., Science-based Dating in Archaeology, Longman, London. Barraclough, D.R., Carrigan, J.G. & Malin, S.R.C., Observed geomagnetic field intensity in London since 1820, Geophys. J. Int., 141, Biggin, A.J. & Thomas, D.N., The application of acceptance criteria to results of Thellier palaeointensity experiments performed on samples with pseudo-single-domain-like characteristics, Phys. Earth planet. Inter., 138, Bloxham, J. & Jackson, A., Time-dependent mapping of the magnetic field at the core mantle boundary, J. geophys. 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