Archaeomagnetic results from mural paintings and pyroclastic rocks in Pompeii and Herculaneum

Physics of the Earth and Planetary Interiors 118 Ž2000. 227–240 www.elsevier.comrlocaterpepi

Archaeomagnetic results from mural paintings and pyroclastic rocks in Pompeii and Herculaneum E. Zanella

a,)

, L. Gurioli b, G. Chiari c , A. Ciarallo d , R. Cioni b, E. De Carolis d , R. Lanza a

a

c

Dipartimento di Scienze della Terra, UniÕersita` di Torino, Via Valperga Caluso 35, 10125 Torino, Italy b Dipartimento di Scienze della Terra, UniÕersita` di Pisa, Via Santa Maria 53, 56126 Pisa, Italy Dipartimento di Scienze Mineralogiche e Petrologiche, UniÕersita` di Torino, Via Valperga Caluso 35, 10125 Torino, Italy d Soprintendeza Archeologica di Pompei, Via Villa dei Misteri 2, 80045 Pompei, Italy Received 28 May 1999; received in revised form 29 September 1999; accepted 21 October 1999

Abstract This work investigates the magnetic remanence associated with red pigments from murals at Pompeii and compares their directions to those of the pyroclastic rocks from the Vesuvius AD 79 eruption. The remanence of the murals is shown, using X-ray analyses, to be carried by haematite. Murals in Thermae Stabianae, known to have been painted a few years before AD 79, yield an archaeomagnetic direction Ž D s 1.28, I s 58.08; a 95 s 5.58. indistinguishable from that of a nearby kiln Ž D s 358.08, I s 59.18; a 95 s 1.78. ŽEvans and Mareschal, 1989. probably last used immediately prior to the eruption. The directions are also consistent with those of fine-grained pyroclastic rocks from the eruption Ž D s 351.28, I s 57.98; a 95 s 3.48. and lithic and tile fragments embedded within them Ž D s 358.58, I s 60.48; a 95 s 8.58.. Other paintings of the 1st century AD yield similar directions, with a lower statistical definition. This study shows that murals can retain their remanent magnetization for centuries and demonstrates the viability in principle of pictorial remanence as an archaeomagnetic tool. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Archaeomagnetism; Mural paintings; Pompeii; Pyroclastics; Vesuvius

1. Introduction In AD 79, Vesuvius suddenly awoke and buried the Roman towns of Herculaneum, Oplontis and Pompeii ŽFig. 1.. This famous eruption is described by the eyewitness account of Pliny the Younger in the form of two letters sent to the historian Tacitus about the death of his uncle Pliny the Elder. The ) Corresponding author. Tel.: q39-11-670-7193; fax: q39-011670-7155; e-mail: [email protected]

southern slopes of Vesuvius are studded with remains that preserve many traces of the destruction caused by the pyroclastic flows. Excavations in this area since 1709 have disclosed a rich and detailed record of the Roman civilization. Historical dating of the eruption — August 24th–25th AD 79 — and archaeological information, together with detailed volcanological studies, can be used to check the accuracy of archaeomagnetic methods. Since the pioneering work of Thellier Ž1938., much has been done to derive the palaeosecular

0031-9201r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 Ž 9 9 . 0 0 1 4 6 - 6

228

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

Fig. 1. Geographic sketch map Žcontour interval in m. of the Somma–Vesuvius region. The grey area represents the fan of the main fallout of the AD 79 eruption after Sigurdsson et al. Ž1985..

variation ŽPSV. of the Earth’s field from the magnetic record of baked clays, such as kilns, bricks and pottery. PSV curves spanning several millenia are now available for various regions ŽTarling, 1989; Bucur, 1994; Daly and Le Goff, 1996; Kovacheva, 1997.. A new archaeomagnetic technique ŽChiari and Lanza, 1997, 1999. concerns the remanence carried by powdered haematite used as a pigment to prepare red colours for murals. It has been shown that such red paint acquires a remanent magnetization aligned with the direction of the ambient magnetic field, since the haematite grains act as tiny magnets suspended in the liquid colour and are free to orient before the paint dries. Murals thus retain a record of the Earth’s magnetic field at the time they were painted. A number of mural paintings survived the AD 79 eruption at Pompeii. By being buried by thick pyroclastic deposits they were, in effect, kept safe from the ravages of time. Pompeii is therefore an ideal place to compare archaeomagnetic, ‘‘pictorial’’ remanent magnetization ŽPiRM. results with those of thermal remanent magnetization ŽTRM. from pyroclastic rocks. For this investigation, samples were collected from the murals in four buildings at Pom-

peii and the pyroclastic rocks in two sites at the Herculaneum excavations.

2. Palaeomagnetism of the pyroclastic rocks 2.1. Volcanological setting and sampling The stratigraphy of the AD 79 eruption has been studied by several authors ŽLirer et al., 1973; Sheridan et al., 1981; Sigurdsson et al., 1982, 1985; Carey and Sigurdsson, 1987.. Cioni et al. Ž1992; 1996. divided the deposits into 8 Eruption Units ŽEU., each inserted in the chronological scheme proposed by Sigurdsson et al. Ž1982; 1985. and correlated to a phase with a well-defined eruptive mechanism ŽFig. 2., namely an opening phreatomagmatic phase ŽEU1., a Plinian magmatic phase ŽEU2 and EU3. and a final phreatomagmatic phase ŽEU4 to EU8.. At Herculaneum, only 7 km to the west of Vesuvius ŽFig. 1., the sequence is dominated by pyroclastic flow deposits, with poorly sorted pumice and lithics as well as artifact fragments dispersed in a matrix of coarse and fine ash ŽFig. 2.. These deposits were emplaced

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

229

Fig. 2. Schematic section of the AD 79 eruption and stratigraphy of the ‘‘Parete a mare’’ section at Herculaneum. Main phases of eruption and eruptive units ŽEU. after Cioni et al. Ž1992., timing after Sigurdsson et al. Ž1985. and Cioni et al. Ž1992.; stratigraphic subdivision and nomenclature according to Sigurdsson et al. Ž1985. are shown to the right. The dots represent the sampled layers; the vertical bar in the bottom left corner gives the scale Ž1 m.. In the inset, map of the excavations and location of the sampling sites.

at high temperature and their remanent magnetization can be expected to have faithfully recorded the direction of the ambient geomagnetic field. The pyroclastic sequence at Pompeii, on the other hand, consists mainly of fallout pumice, whose remanent magnetization may be affected by transport and deposition dynamics ŽThomas and Sparks, 1992.. Sampling was therefore planned in the Herculaneum excavations. A fines-poor, discontinuous, up to 10cm-thick bed made of pumice lapilli and lithic fragments ŽEU4bl of Cioni et al., 1996. represents a

clear discontinuity which marks the transition from magmatic to phreatomagmatic products. The samples were collected from three layers located astride this reference bed and whose matrix consists of pumice, glass shards, crystals and minor lithic fragments: Ø the top of the thickest pumice flow deposit of the magmatic phase ŽEU3pf2b., pumice-rich, coarsegrained facies; Ø the bottom of the first lithic-rich pyroclastic flow deposit ŽEU4., lithic-rich, coarse-grained facies; Ø the fine-grained layer resting on the EU4bl bed.

230

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

Fig. 3. Zijderveld diagrams for thermal and AF demagnetization of fine-grained pyroclastics Ža, b. and thermal demagnetization of a tile fragment Žc.. Symbols: diamondss declination; dots s apparent inclination; numberss temperature Ž8C. or peak-field ŽmT. value.

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

231

The sampling sites are located along the excavation wall, one just in front of the Suburban Thermae Ž Parete a mare section. and the other near the recently excavated Villa of the Papyri ŽFig. 2.. The samples were cored with a battery-powered drill, oriented by magnetic and sun compass and then cut to standard cylindrical specimens Ž w s 25 mm, h s 22 mm. in the laboratory. Care was taken to avoid large clasts that might have strongly biased the magnetization direction. Cognate xenoliths Žlavas, tuffs, cumulites. and fragments of roof tiles were collected from the same deposits at Villa of the Papyri and from the EU4 deposits in the Terzigno quarry 7 km to the southeast of Vesuvius ŽFig. 1.. 2.2. Magnetic measurements Natural remanent magnetization ŽNRM. and susceptibility of all specimens from the three layers were first measured using a JR-4 spinner magnetometer and KLY-2 bridge. Isothermal remanence magnetization ŽIRM. and susceptibility vs. temperature analyses were performed on at least one specimen for each facies from both sites, using a pulse magnet with a maximum field strength of 1.5 T and a Bartington MS2WrF furnace, respectively. The Curie point was around 520–5408C, the IRM saturation was reached at 0.1 T field, and the remanent coercive force was approximately 40 mT. These results were alike in the three facies and indicated titanomagnetite as the most likely magnetic carrier. The only difference concerned the susceptibility and NRM intensity, which in the fine-grained matrix were one order of magnitude higher Ž10 = 10y3 SI and 1 Arm. than in the other two coarser facies Ž1 = 10y3 SI and 0.1 Arm.. Stepwise thermal and alternating field ŽAF. demagnetization up to 6008C and 100 mT, respectively, showed negligible secondary magnetization components ŽFig. 3a, b.. The characteristic remanent magnetization ŽChRM. direction was easily identified by the linear decay towards the origin of the Zijderveld diagrams. Stepwise AF demagnetization of six pilot specimens showed that a peak-field of 20 mT was suitable for unambiguous determination of the ChRM direction ŽFig. 3b., and this value was used to demagnetize the remainder of the specimens. The directions are shown in Fig. 4a. The site mean values are listed in Table 1, together

Fig. 4. Equal-area projection of ChRM directions of fine-grained pyroclastics Ža. and low-temperature component directions of lithic fragments Žb.. Star s mean value with a 95 ellipse of confidence.

with the findings of Hoye Ž1981. from an unspecified volcanic level at Herculaneum. The mean directions of the three sites show a very low dispersion Ž k ) 212, a 95 - 2.58. and are very close. A similar direction should also have been recorded as secondary magnetization in the lithic fragments

232

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

Table 1 Palaeomagnetic data from Herculaneum and archaeomagnetic data from Pompeii Symbols: n s number of specimens; D, I s declination, inclination; k s Fisher’s precision parameter; a 95 s semiangle of confidence; Demagnetization: minimum and maximum peak-field or temperature using either AF or thermal technique.

a 95

Site

n

D

I

k

Herculaneum — pyroclastics Ž1. Villa of the Papyri Ž2. Parete a mare Ž3. Hoye, 1981

14 12 20

353.6 348.3 351.9

59.9 56.8 57.0

258 423 212

2.5 2.1 2.2

AF 20 mT AF 20 mT AF

9 6

358.5 359.2

60.4 56.8

38 251

8.5 4.2

Th 20–3608C Th 20–4008C

Pompeii — mural paintings Ž6. Thermae Stabianae Ž7. House of C. Iulius Polybius Ž8. House of Fabius Rufus Ž9. House of the dancing faun

18 5 8 5

1.2 346.6 354.1 352.9

58.0 52.6 51.8 54.7

40 28 36 18

5.5 14.7 9.3 18.5

AF 20–60 mT AF 20–60 mT AF 20–60 mT AF 20–60 mT

Pompeii — kiln Ž10. Evans and Mareschal, 1989

9

358.0

59.1

952

1.7

Herculaneum and Terzigno— lithic fragments Ž4. Villa of the Papyri and Terzigno quarry Ž5. Kent et al., 1981

embedded in the pyroclastic deposits and hence reheated up to the flow emplacement temperature. Various palaeomagnetic studies ŽHoblitt and Kellog, 1979, Kent et al., 1981; McClelland and Druitt, 1989; McClelland and Thomas, 1990; Tarling and Downey, 1990. have shown that the primary magnetization of such fragments is partially removed, because heating erases the remanence whose blocking temperature is lower than that of the pyroclastic flow. The fragments thus carry a two-component magnetization. The high-temperature component is the primary remanence; however, since the fragments were torn from their original position and chaotically embedded within the pyroclastic deposit, the primary directions are randomly dispersed. The secondary, low-temperature directions are similar to each other and to that of the fine-grained matrix, since this component was acquired during the in situ cooling. Several oriented samples of lithic and roof tile fragments were collected from EU4 unit at Villa of the Papyri and in a Õilla rustica near Terzigno. Stepwise thermal demagnetization showed that the remanence of 9 out of 14 measured fragments consists of two components, clearly defined in the Zijderveld diagrams ŽFig. 3c.. First, the low-temperature magnetization was removed, then the high-tem-

Demagnetization

Th

perature magnetization withstood heating up to the Curie point. The low-temperature directions obtained by interpolating the first steps of the demagnetization curves are reported in Fig. 4b. Table 1 reports their mean value, which is close to the directions of fine-grained deposits, and that from other six lithic fragments ŽKent et al., 1981.. 3. Archaeomagnetism of mural paintings 3.1. Archaeological setting and sampling The many paintings in private houses and public buildings brought to light during the excavation of the towns buried by the Vesuvius in AD 79 have provided an exhaustive panorama of Roman tastes from the 2nd century BC to the moment of the eruption. The wall paintings are conventionally divided into four so-called ‘‘Pompeian Styles’’ ŽBarbet, 1985; Guzzo et al., 1997. as follows. Ži. The First Style was in vogue in the 2nd century BC. Stucco paintings imitated the external wall masonry in opus quadratum and were embellished with representations of projecting architectural features, such as slender columns and architraves. Žii. The Second Style popular during the 1st century BC was inspired by the stage backdrops of

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

the Roman–Hellenistic theatre. A trompe-l’oeil recession of the wall was achieved by the representation of colonnades, entablatures and ceilings superposed in height and depth. Human figures, masks and small pictures with various subjects were added during the course of the century. Žiii. The Third Style prevailed from the last decades of the 1st century BC to the first half of the 1st century AD. Walls were divided into a base, a middle section and a frieze. The middle section was divided into large rectangular panels, with a figurative mask in the central panel and small vignettes in those at the sides. Živ. The Fourth Style began in the middle of the 1st century AD and was cut short by the eruption. It

233

was marked by the introduction of elaborate, fanciful architectural features between the panels in the middle section. Figurative paintings were smaller and almost become part of the architecture. Detailed descriptions of ancient painting techniques and classification of the types of colours used in mural decorations can be found in Teophrastus, Vitruvius and Pliny. Reds in a great variety of shades are the most numerous colours found at Pompeii ŽAugusti, 1967.. The most characteristic colour, now known as Pompeian red, consists of cinnabar ŽHgS. and corresponds to what Vitruvius ŽVII, 8, 9. and Pliny ŽXXXIII, 7. call minium. The red pigments with magnetic characteristics are formed of haematite ŽFe 2 O 3 . and are known by the generic name rubri-

Fig. 5. Map of the Pompeii excavations, sampled buildings and location of samples at Thermae Stabianae. Numbers: 1 s House of C. Iulius Polybius; 2 s Thermae Stabianae; 3 s House of the dancing faun; 4 s House of Fabius Rufus.

234

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

cae ŽAugusti, 1967; Rinaldi et al., 1986.. This colour was generally used abundantly at Pompeii for the backgrounds of wall paintings. About one hundred red colour samples were taken from four buildings ŽFig. 5.: Thermae Stabianae ŽThird and Fourth Style murals., House of C. Iulius Polybius and House of Fabius Rufus ŽThird Style. and House of the dancing faun ŽFirst and perhaps Third Style.. Evidence of restoration work after the AD 62 earthquake was found in the Thermae Stabianae Žscaffoldings in the gymnasium. and the House of C. Iulius Polybius Žpots with colours and lime ready for use.. The technique for sampling mural paintings was described earlier ŽChiari and Lanza, 1997, 1999.. A double-faced adhesive tape is applied to a 18-mm disk cut from radiographic film. The disk is applied to the painting, oriented and then

ripped out, carrying a bit of the painting with it. Whenever possible, walls with different orientation are sampled, just as geological units are sampled at different outcrops. 3.2. Analyses of red pigments Nine samples were analyzed by X-ray powder diffraction ŽXRPD. to determine the type and quality of the pigments and the painting technique. Use was made of a SIEMENS D5000 diffractometer, equipped with Gobel ¨ mirrors on the primary beam, which act also as monochromator for the Cu K a radiation ˚ . used. The only red pigment found Ž l s 1.54178 A consisted of haematite. This is in agreement with the Ž1996., who analyzed findings of Varone and Bearat ´ not only fragments of Pompeii paintings but also dry

Fig. 6. X-ray powder diffraction spectrum. Vertical lines: haematite Ždouble triangle., calcite Žsquare., gypsum Ždiamond. and weddellite Žtriangle.. The two reflections Ž104. and Ž110. of haematite are also shown Žsee text for further explanation..

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

pigments ready for use at the moment of the eruption. No ferromagnetic pigments were found in other colours, except in sample PO7, from House of C. Iulius Polybius, which also included goethite. This sample, in fact, was a tempera painting with two layers: a yellow one, obtained with a yellow ochre, consistent with the presence of goethite, and a superposed red layer made of haematite. The XRPD pattern also gives information on the Ž1993. analyzed the variety of haematite used. Bearat ´ red pigments of a 3rd century AD Gallo–Roman wall painting and showed that three types of haematite can be distinguished on the basis of the intensity ratio and sharpness of the reflections Ž104. and Ž110.. The first variety is more common in mural paintings and corresponds to the natural, well-crystallized haematite, finely crushed in order to make the pigment. The ratio IŽ104.rIŽ110. is greater Ž1996. than one and the lines are sharp. Bearat ´ suggests that this high-quality pigment is the sinopis described by Vitruvius ŽVII. and Pliny ŽXXXV.. The second variety has much broader lines, indicating a poorly crystallized phase, with the intensity ratio still ) 1. It may correspond to red ochre or rubrica. The third variety has broad lines and an intensity ratio

235

- 1. It is derived from goethite heated at temperatures between 2508C and 7008C and corresponds to disordered haematite. In all samples analyzed for the present study, the intensity ratio was ) 1 ŽFig. 6., with a broadening of the peaks comparable to those of the other minerals. This suggests that the pigment used was the best-quality, crushed natural haematite. The other minerals identified by the XRPD analysis provide further information. The rip-out technique ŽChiari and Lanza, 1997, 1999. may result in samples which contain some material from the very superficial part of the support, together with the painted layer. The analysis of this material can shed light on the painting technique. For example, the only minerals present in the samples from House of the dancing faun are calcite and haematite. This suggests that the a fresco technique was used and that the intonaco was composed of lime and crushed calcite, or limestone as aggregate. The use of sifted marble as aggregate, described by Vitruvius and Pliny, gives a highly brilliant surface. This kind of Ž1996., was apparently mortar, according to Bearat ´ restricted to special rooms with high-quality paintings. The paintings of the House of C. Iulius Polybius, beside haematite, show dominant calcite with

Fig. 7. Isothermal remanent magnetization ŽIRM. acquisition curve for painting samples from House of C. Iulius Polybius ŽPO8-haematite and goethite. and Thermae Stabianae ŽTS42-haematite.. IRM intensity reported as magnetic moment ŽA m2 ..

236

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

small amounts of quartz. They were probably painted a fresco on an intonaco made of lime and a carbonatic aggregate containing a small amount of quartz. In fact, the quartz quantity present with respect to calcite is not enough to constitute the aggregate. The samples from Thermae Stabiane and House of Fabius Rufus show abundant gypsum mixed with calcite and a small amount of quartz. This indicates the use of a lime-gypsum mortar for the final layer, which would be compatible with the tempera technique. Furthermore, the small amount of Ca oxalate in the form of weddellite in the paintings at the House of Fabius Rufus gives some information about their age. Ca oxalate is derived from alteration of the protein used as binder in the tempera technique. Several decades are needed to form an oxalate patina: 140 years according to Mariottini et al. Ž1989. and about 80 years according to Chiari Ž1996.. These paintings were therefore at least 50–100 years old when Vesuvius erupted in AD 79. 3.3. Magnetic measurements The NRM of the colour samples was measured using a JR-5 spinner magnetometer. Its intensity could not be precisely determined, because the sampling technique does not control the amount of paint carried off from the wall, which depends on the layer strength and adhesion to the tape, and varies according to the painting technique and from one point of the wall to another. Intensity is therefore reported as magnetic moment M ŽA m2 .. At Pompeii, the amount of paint was often very small and the magnetic moment was accordingly very low. Samples with moment less than 10y9 A m2 were not investigated further, because the signal measured throughout the demagnetization process would have fallen to the instrument noise level. IRM measurements were done on at least two specimens per painting. Saturation was approached but not achieved at the maximum available field of 1.5 T ŽFig. 7. and remanent coercive force was 250–300 mT. The results were typical for haematite and corroborated those from the XRPD analyses. It is worth noting that sample PO8, from House of C. Iulius Polybius, is the only one still far from saturation at the field value of 1.5 T ŽFig. 7.. This trend

Fig. 8. Zijderveld diagrams for AF demagnetization of mural paintings. Symbols: diamondssdeclination; dotssapparent inclination; figuress peak-field value ŽmT.. Intensity reported as magnetic moment ŽA m2 ..

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

points to the occurrence of goethite, as found by the XRPD analysis in its twin-specimen PO7. No thermal experiment was possible, since it would have resulted in the destruction of the specimens. The 50 most strongly magnetized samples were AF demagnetized in 7 to 10 steps up to 100 mT. Secondary magnetization components were removed at peak-field values ranging from 10 to 40 mT ŽFig. 8.. A stable ChRM was isolated in 36 samples, usually in the range 20 to 60 mT, and its direction was computed by interpolation. The ChRM directions are shown in Fig. 9 and their site mean

237

values listed in Table 1, which also reports the direction obtained by Evans and Mareschal Ž1989. from the kiln in the oil-lamp workshop close to the Porta Nocera ŽFig. 5.. The results from Thermae Stabianae Žnumber of specimens n s 18; D s 1.28, I s 58.08; semiangle of confidence a 95 s 5.58. are statistically well defined and close to those from the kiln Ž D s 358.08, I s 59.18; a 95 s 1.78., whereas the dispersion at House of Fabius Rufus, House of C. Iulius Polybius and House of the dancing faun is higher than usual in archaeomagnetic studies. In our case, the size of the statistical parameters results to

Fig. 9. Equal-area projection of ChRM directions of mural paintings. Star s mean value with a 95 ellipse of confidence.

238

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

some extent from the small number of samples. The main reason, however, is probably the fact that the volume of a specimen taken from a mural painting is a few cubic millimetres and hence three or four orders of magnitude smaller than usual for baked clays, and the smaller the volume the less are inhomogeneities averaged out.

4. Discussion The archaeomagnetic and palaeomagnetic directions from this investigation and the literature ŽHoye, 1981; Kent et al., 1981; Evans and Mareschal, 1989. are reported in Table 1 and shown in Fig. 10. The remanent magnetizations of the pyroclastic deposits, the kiln and the paintings from Thermae Stabianae share nearly the same age, namely that of the eruption. Indeed the kiln had its last firing prior to the eruption and the paintings were restored in the years between the earthquake ŽAD 62. and the eruption ŽAD 79.. The directions from the lithic fragments, the Thermae Stabianae paintings and the kiln ŽFig. 10, nos. 4, 5, 6 and 10. are almost identical and indistinguishable at the 95% confidence level. Their mean value is D s 359.28, I s 58.68 Žnumber of sites N s 4; Fisher’s precision parameter k s 2264; a 95 s 1.98.. On the other hand, the directions from the fine-grained pyroclastics ŽFig. 10, nos. 1, 2 and 3. are systematically shifted a few degrees toward the west and their mean value is D s 351.28, I s 57.98 Ž N s 3; k s 1295; a 95 s 3.48.. This difference might reflect some small bias introduced by the

Fig. 10. Equal-area projection of archaeomagnetic directions from Pompeii Ždots. and palaeomagnetic directions from Herculaneum Ždiamonds.. Numbers refer to Table 1.

deposition process. Kent et al. Ž1981. estimated the emplacement temperature of the Herculaneum pyroclastics as less of 4008C. As the unblocking temperatures of the fine-grained pyroclastics extend to 5508C ŽKent et al., 1981; this study, Fig. 3., some of the ferromagnetic grains acquired their remanence before deposition and came to rest as already magnetized particles. This detrital component may have been affected by the deposition mechanism and hence resulted in the remanence bias. Most of the other paintings Ž House of C. Iulius Polybius, House of Fabius Rufus and House of the dancing faun. are referred to the Third Style and are therefore a few decades older than the eruption. The older age at House of Fabius Rufus was also substantiated by the finding of the Ca oxalate. The directions of these paintings differ from those coeval to the eruption and might thus be interpreted in terms of PSV. The quality of the data, however, is not enough to support this hypothesis, because the number of samples that could be measured was low and the dispersion of the directions rather high.

5. Conclusion This investigation has shown that the haematitebearing, red coloured Pompeii murals have a remanent magnetization whose direction is consistent with that of the Earth’s field at the time they were painted. One site yielded a well-defined archaeomagnetic direction, which was statistically indistinguishable from those derived from a coeval kiln ŽEvans and Mareschal, 1989. and the lithic fragments embedded in the pyroclastic deposits from the AD 79 Vesuvius eruption ŽKent et al., 1981; this study.. The directions from the other sites were also close to the reference direction, but their statistical definition was lower, mainly because the very low magnetic moment strongly reduced the number of measurable specimens. The Pompeii murals have retained their remanent magnetization for nearly two thousand years. Red-coloured paintings may thus be included among the materials suitable for archaeomagnetic investigations. They have both advantages and drawbacks: they are easier to date than baked clay and historical lava flows, but the precision of the results strongly depends on the quality of the pigment and

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

the actual volume of the specimens. When the strength of the colour layer is weak, as often in the tempera paintings, the rip-out technique gives regularly shaped disks, whose PiRM intensity may be as high as 0.1–1 Arm if the pigment consists of wellcrystallized haematite. In this case, the precision of the results is similar to that achieved with usual archaeomagnetic methods ŽChiari and Lanza, 1999.. In the case of the Pompeii paintings, precision was lower, mainly because the colour layer strength was high and the specimens volume very small. Improvements to the sampling technique are therefore essential to further development of the PiRM method.

Acknowledgements This research was supported by funds from University of Torino and the Italian National Research Council ŽGrant No. 98.00323.CT05 and GNV97.00099.PF62 to R. Santacroce.. One of us ŽG.C.. acknowledges the support from CNR-P.F. Beni Culturali. We are indebted to Prof. P.G. Guzzo, Head of the Soprintendenza Archeologica di Pompei, for permission to sample; to D. Tarling, N. Abrahamsen and R. Sternberg for improvements to early versions of the manuscript, and to the journal referees J. Love and M.E. Evans for valuable comments and suggestions.

References Augusti, S., 1967. I colori Pompeiani, Studi e Documentazioni I. Ministero della Pubblica Istruzione, Direzione Generale delle Antichita` e Belle Arti, De Luca Editore, Roma, 162 pp. Barbet, A., 1985. La peinture murale romaine. Les styles decora´ tifs pompeiens. Picard, Paris, 285 pp. ´ Bearat, H., 1993. Analyses mineralogiques sur les peintures ´ ´ alterees ´ ´ de la Villa Gallo–romaine de Vallon. Revue d’Archeometrie 17, 65–74. ´ Bearat, H., 1996. Chemical and mineralogical analyses of Gallo– ´ Roman wall painting from Dietikon, Switzerland. Archaeometry 38, 81–95. Bucur, I., 1994. The direction of the terrestrial magnetic field in France during the last 21 centuries. Recent progress. Phys. Earth Planet. Inter. 87, 95–110. Carey, S., Sigurdsson, H., 1987. Temporal variation in column high and magma discharge rate during the AD 79 eruption of Vesuvius. Geol. Soc. Am. Bull. 99, 303–314. Chiari, G., 1996. Calcium oxalate: sporadic findings of some

239

interest. In: Realini, M., Toniolo, L. ŽEds.., Proceedings of II International Symposium ‘‘The Oxalate Films in the Conservation of Works of Art’’, Milano, Italia, pp. 47–58. Chiari, G., Lanza, R., 1997. Pictorial remanent magnetization as an indicator of secular variation of the Earth’s magnetic field. Phys. Earth Planet. Inter. 101, 79–83. Chiari, G., Lanza, R., 1999. Remanent magnetization of mural paintings from the Bibliotheca Apostolica ŽVatican Rome.. J. Appl. Geophys. 41, 137–143. Cioni, R., Marianelli, P., Sbrana, A., 1992. Dynamics of the AD 79 eruption: stratigraphic, sedimentologic and geochemical data on the succession of Somma–Vesuvius southern and eastern sector. Acta Vulc. 2, 109–124. Cioni, R., Sbrana, A., Gurioli, L., 1996. The deposits of AD 79 eruption. In: Santacroce, R., Rosi, M., Sbrana, A., Cioni, R., Civetta, L. ŽEds.., Vesuvius Decade Volcano Workshop Handbook, International CEV-CMVD Workshop on Vesuvius, September 1996, CNR, Italy. Daly, L., Le Goff, M., 1996. An updated and homogenous world secular variation data base: 1. Smoothing of the archaeomagnetic results. Phys. Earth Planet. Inter. 93, 159–190. Evans, M.E., Mareschal, M., 1989. Secular variation and magnetic dating of fired structures in southern Italy. In: Maniatis, Y. ŽEd.., Archaeometry, Proceedings of the 25th International Symposium, Elsevier, pp. 59–68. Guzzo, P.G. et al., 1997. Pompeii. Picta Fragmenta. Umberto Allemandi and C., Torino-Londra, 170 pp. Hoblitt, R.P., Kellog, K.S., 1979. Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques. Geol. Soc. Am. Bull. 90, 633–642. Hoye, G.S., 1981. Archaeomagnetic secular variation record of Mount Vesuvius. Nature 291, 216–218. Kent, D.V., Ninkovich, D., Pescatore, T., Sparks, S.R.J., 1981. Palaeomagnetic determination of emplacement temperature of Vesuvius AD 79 pyroclastic deposits. Nature 290, 393–396. Kovacheva, M., 1997. Archaeomagnetic database from Bulgaria: the last 8000 years. Phys. Earth Planet. Inter. 102, 145–151. Lirer, L., Pescatore, T., Booth, B., Walker, G.P.L., 1973. Two Plinian pumice-fall deposits from Somma–Vesuvius, Italy. Geol. Soc. Am. Bull. 84, 759–772. Mariottini, M., Laurenzi Tabasso, M., Bianchetti, P., 1989. Indagini sulla possibilita` di formazione degli Ossalati di Calcio sulle superfici lapidee esposte all’aperto. Proceedings of the meeting ‘‘The Oxalate Films: Origin and Significance in the Conservation of Works of Art’’, Milano, Italia, October 1989, pp. 53–73. McClelland, E.A., Druitt, T.H., 1989. Palaeomagnetic estimates of emplacement temperatures of pyroclastic deposits on Santorini, Greece. Bull. Volcanol. 51, 16–27. McClelland, E.A., Thomas, R., 1990. A palaeomagnetic study of Minoan age tephra from Thera. In: Hardy, D. ŽEd.., Thera and the Aegean World III. The Theran Foundation, London, pp. 129–138. Rinaldi, S., Quartullo, G., Milaneschi, A., Pietropaoli, R., Occorsio, S., Costantini Scala, F., Minunno, G., Virno, C., 1986. La fabbrica dei colori, Il Bagatto, Roma, 572 pp.

240

E. Zanella et al.r Physics of the Earth and Planetary Interiors 118 (2000) 227–240

Sheridan, M.F., Barberi, F., Rosi, M., Santacroce, R., 1981. A model for Plinian eruptions of Vesuvius. Nature 289, 282–285. Sigurdsson, H., Cashdollar, S., Sparks, R.S.J., 1982. The eruption of Vesuvius in AD 79: reconstruction from historical and volcanological evidence. Am. J. Archaeol. 86, 39–51. Sigurdsson, H., Carey, S., Cornell, W., Pescatore, T., 1985. The eruption of Vesuvius in AD 79. Nat. Geogr. Res. 1, 332–387. Tarling, D.H., 1989. Geomagnetic secular variation in Britain during the last 2000 years. In: Lowes, F.J., Collinson, D.W., Parry, J.H., Runcorn, S.K., Tozer D.C., Soward, A. ŽEds.., Geomagnetism and Palaeomagnetism, Kluwer Academic Publishing, Dordrecht, pp. 55–62. Tarling, D.H., Downey, W.S., 1990. Archaeomagnetic results from Late Minoan destruction levels on Crete and the

‘‘Minoan’’ tephra on Thera. In: Hardy, D. ŽEd.., Thera and the Aegean World III. The Theran Foundation, London, pp. 146–159. Thellier, E., 1938. Sur l’aimantation des terres cuites et ses applications geophysiques. Ann. Inst. Phys. Globe 16, 157– ´ 302. Thomas, R.M.E., Sparks, R.S.J., 1992. Cooling of tephra during fallout from eruption columns. Bull. Volcanol. 54, 542–553. Varone, A., Bearat, H., 1996. Pittori romani al lavoro. Materiali, ´ strumenti, tecniche: evidenze archeologiche e dati analitici di un recente scavo Pompeiano lungo Via dell’Abbondanza ŽReg. IX Ins. 12.. Proceedings of International Workshop on Roman Wall Painting, Friburg, pp. 199–214.