Remanent magnetization of mural paintings from the Bibliotheca Apostolica (Vatican, Rome)

Ž .Journal of Applied Geophysics 41 1999 137–143

Remanent magnetization of mural paintings from the Bibliothecaž /Apostolica Vatican, Rome

G. Chiari a, R. Lanza b,)

a Dipartimento di Scienze Mineralogiche e Petrologiche, UniÕersita di Torino, Italy`b Dipartimento di Scienze della Terra, UniÕersita di Torino, Via Valperga Caluso 35, 101259 Torino, Italy`

Received 29 October 1997; accepted 1 November 1998

Abstract

The remanent magnetization of red colour specimens from five mural paintings in three halls of Bibliotheca ApostolicaŽ .Vaticana Sistus V, Paul V and Alexander VIII, 1580–1690 A.D. was investigated with the usual palaeomagnetic

techniques. All specimens have a strong remanent magnetization carried by haematite, as shown by X-ray powder diffractionanalyses. Secondary magnetization components are either absent or weak and removed by alternating field demagnetizationat peak-field up to 20 mT. The specimens from the Sistus V hall have a very high remanence coercivity, having still80–90% of the initial remanence after demagnetization at 280 mT peak-field. Two murals have remanence direction close tothat of the Earth’s field at the time they were painted. The direction from the mural on the east wall of the Paul V hall,painted around 1610–1615, is in good agreement with the results of the 1640 direct measurement in Rome and that from theAlexander VIII hall, which was repainted around 1815, is close to the 1833 measurement. The directions from the otherthree murals, two in the Sistus V and one in the Paul V halls, are close to each other and to the Earth’s field directionmeasured in Rome in the first half of the 19th century. The three murals might have been restored at this time or theirremanence direction is affected by a systematic misalignement. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Haematite; Mural paintings; Remanent magnetization; Rome

1. Introduction

Ž .In a previous paper, Chiari and Lanza 1997demonstrated that a red coloured mural paintingprepared using haematite as a pigment acquires

Ž .a ‘pictorial’ remanent magnetization PiRMparallel to the Earth’s field, thanks to the factthat the magnetic moment of the haematitegrains orients itself as long as the colour is fluid

) Corresponding author. Fax: q39-011-6707155; E-mail:[email protected]

and maintains its orientation once the paint hasdried. The PiRM analysis of mural paintings ispotentially a new archaeomagnetic method, buta great deal of work is required to improve themethodology. Italy is in an ideal position toevaluate the PiRM experimentally. Dated paint-ings are countless and the first direct measure-ments of the Earth’s magnetic field date back, atleast for Rome, to the 16th Century. It is possi-ble therefore to compare the results obtainedthrough PiRM measurements with experimentaldata over a four-century time span.

0926-9851r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 98 00038-X

( )G. Chiari, R. LanzarJournal of Applied Geophysics 41 1999 137–143138

The Vatican Palace in Rome gives the oppor-tunity to sample in the very same buildingmurals of different ages, making it easier tocompare the results and track the secular varia-tion curve. A sampling was therefore planned inthe gallery of the Bibliotheca Apostolica, whosehalls were painted in different ages. A total offive murals were sampled: sites A, B in the

Ž .Sistus V hall 1585–1590 , sites C, D in theŽ .Paul V hall 1605–1621 and site E in the

Ž .Alexander VIII hall 1689–1691 . The gallery isoriented roughly in a N–S direction; the muralpaintings were sampled on the E and W walls,taking 10 specimens per painting. The speci-mens were oriented with a compass; orienta-tions varied by "18, indicating that no ironobjects were present in the wall. Five directionswere checked with the solar compass and yieldeda value of 18E for the Earth’s field declination.The sampled portion was always either the edgeof a painting or the fake marble decorationbetween two paintings, since sample removal isa destructive process and clearly we could notdamage the actual painting. The sampling tech-nique was as follows:Ø double-faced adhesive tape strips are applied

to a rigid plastic film, of the type used forX-ray radiography;

Ø small disks of 18 mm diameter are cut;Ø the small disk is applied to the wall and the

horizontal direction is traced with a spiritlevel;

Ø the orientation is measured with a compassand clinometer;

Ø the small disk is removed, carrying a bit ofthe painting with it.

2. Magnetic measurements

The specimens were investigated with theusual palaeomagnetic techniques. Remanentmagnetization was measured with a JR-5 spin-ner magnetometer, alternating field demagneti-zation performed with a Molspin demagnetizerup to 100 mT peak-field and in some casescontinued up to 280 mT with a 2G Degausser.Some specimens were given an isothermal re-

Ž .manent magnetization IRM with a Bussi pulsemagnet with a maximum field of 1.5 T.

Ž .The natural remanent magnetization NRMintensity was always very high, but could only

Ž .be given to an order of magnitude 1 Armbecause the volume of the specimens could notbe measured accurately. The amount of paintedlayer actually removed depends on the layer

Ž .Fig. 1. Normalized intensity decay curve during alternating field demagnetization. Symbols: assite D Paul V ; bssite AŽ .Sistus V .

( )G. Chiari, R. LanzarJournal of Applied Geophysics 41 1999 137–143 139

Fig. 2. Zijderveld diagrams for alternating field demagnetization. Same specimens as in Fig. 1; symbols: squaressdeclination; crossessapparent inclination; figuresspeak-field values in mT.

strength and the adhesion to the tape, whichvary from specimen to specimen. Roughlyspeaking, the volume is of the order of a fewmm3. The measured signal is thus one thousand

˚ 3times lower than that of a standard fA10 cmrock-specimen with the same magnetization in-tensity. One pilot specimen for each site was

stepwise demagnetized to a maximum peak-fieldof 280 mT. Results differred in each hall. In the

Žcase of the paintings in the Paul V hall sites C,.D the median destructive field was of the order

Ž .of 20 mT Fig. 1a . At this value, secondarycomponents had been removed and at higherpeak-field values the directions of magnetiza-

Ž . Ž .Fig. 3. Isothermal remanent magnetization IRM acquisition curve for site A Sistus V .


Ž .tion remained stable Fig. 2a , while the inten-sity decreased. At 280 mT peak-field about 15%of the initial NRM was still present. The NRMproperties of the paintings in the Sistus V hallŽ . Ž .sites A, B differred Fig. 1b, Fig. 2b : sec-ondary components were absent and magnetiza-tion presented an exceptional hardness and sta-bility. At a 100 mT peak-field, 65 to 100% ofthe initial NRM was still present and the direc-tion remained practically constant up to the

Fig. 4. Equal-area projection of the ChRM directions.

maximum applicable peak-field of 280 mT. Thepainting sampled in the Alexander VIII hallŽ .site E had intermediate magnetic properties.

These characteristics point to haematite asthe main carrier of magnetization. The IRManalyses corroborated this evidence, since a 1.5T direct field was not enough to saturate the

Ž .specimens Fig. 3 . All specimens were demag-netized at 4–5 steps and the direction of the

Ž .characteristic remanent magnetization ChRMderived by interpolation. The within-site disper-

Ž .sion was small Fig. 4 , as shown by the a95Ž .and Fisher’s precision k values Table 1 , and

the mean directions per painting were all statis-tically significant and very close to each other,with the exception of that from site D.

3. X-ray analyses

Ž .X-ray powder diffraction XRPD was usedto check the mineralogical composition of thepaintings, both for the preparation and the pig-ments. The analyses were carried out using aSiemens D5000 diffractometer equipped withcopper tube and Goble mirrors. Haematite was¨found in all the specimens studied, although indifferent amounts. The preparation of the paint-ings differs in composition in each hall. In the

Ž .Alexander VIII hall Fig. 5a the preparationconsisted mostly of gypsum, while calcite waspresent only in small quantities. The XRPD wascarried out on the same specimens used for themagnetic measurements, which could also in-clude small parts of plaster ripped out togetherwith the paint. Therefore, the small amount ofcalcite might be part of either the preparation

Ž .layer as Bianco di San GioÕanni or the mor-tar. In the Paul V hall calcite was the maincomponent of the preparation and gypsum wasonly present in traces. In this case, the calcitewas likely to be in the preparation layer ratherthan the mortar, since it was the main compo-nent and there was no evidence of pozzolanicfilling which should have been present in the


Table 1Palaeomagnetic results from mural paintings from Bibliotheca Apostolica Vaticana

Site NRM ChRM

n D I a D I a k H95 95 peak

A—Sistus V, west wall 10 344.5 55.9 3.0 345.2 56.4 2.0 578 40–160B—Sistus V, east wall 10 350.9 62.9 2.8 348.7 60.3 1.6 904 40–160C—Paul V, west wall 8 345.5 50.4 25.8 350.8 56.1 7.8 51 10–40D—Paul V, east wall 10 2.6 78.8 7.2 0.8 67.4 2.4 414 20–90E—Alexander VIII, east wall 10 350.2 64.9 7.4 347.3 58.7 3.5 187 20–100

Symbols: NRM, natural remanent magnetization; ChRM, characteristic remanent magnetization; n, number of specimens;D, declination; I, inclination; a , semi-angle of confidence; k, Fisher’s precision; H , alternating field demagnetization95 peak

range.

mortar. The XRPD pattern for the Sistus V hallŽ .Fig. 5b was more complex: in addition toabundant gypsum and some calcite, the prepara-tion included lead white, since the two forms of

Žlead carbonate typical of the biacca cerussite.and hydrocerussite were abundant. The most

interesting result is the presence of two redpigments: in addition to haematite there wasalso minium Pb O . The red pigment minium3 4

was widely used in the antiquity, up to the 16thcentury, and prepared by different processes.

Ž .One process Rinaldi et al., 1986 consisted inŽ . Ž .calcination of Pb CO P OH often with the3 3 2 2

addition of ocra rossa Fe O , i.e., haematite.2 3

The magnetic properties of the paintings fromthis hall may thus be tentatively explained asthe result of a very hard, strong thermal rema-nence of the haematite grains acquired duringthe cooling of the calcinated pigment. The mag-netization of the grains was not later altered bythe painting process, which consisted in a pas-sive orientation of the magnetic moment parallelto the Earth’s field.

4. Discussion and conclusions

Ž .The ChRM directions Table 1 and Fig. 6can be compared with the secular variation curve

Ž .for Rome Cafarella et al., 1992a,b . The site Ddirection falls at the beginning of the curve,while those of the other four sites at its western

edge. Mural D is from the east wall of the PaulV hall, which was painted by G. Ricci around

Ž1610–1615. Its direction Ds18, Is678 with. Ža s2.68 is indistinguishable from that Ds95.28, Is668 of the 1640 direct measurement of

the Earth’s field done by Father KircherŽ .Cafarella et al., 1992a and it can be confi-dently concluded that this mural recorded theEarth’s field at the time it was painted. Asimilar conclusion can be drawn for mural E,since the Alexander VIII hall was repainted

Žaround 1815 by D. Del Frate Michel, 1980–. Ž1982 and its PiRM direction Ds3478, Is598

.with a s3.58 is close to the 1833 direct95Ž .measurement Ds3448, Is628 by Pianciani

Ž .Cafarella et al., 1992a .At this point, there is still the problem that

the directions of three of the five murals differsignificantly from the expected value for thetime of the paintings. A definite reason musthave produced the difference, since the threedirections are all close to each other. A possibleexplanation is that minor restorations and touch-ups were performed after the paintings wereoriginally made. This was actually the case forthe Bibliotheca Apostolica halls as mention is

Žmade in the archives Cicerchia and De Strobel,.1986 of the bills for paying for restoration by

D. Belletti and A. Mazzuoli in the Sistus V andPaul V halls between 1714 and 1720. Unfortu-nately, it is not known which paintings wereactually restored. The Earth’s field declination


. Ž .Fig. 5. The X-ray diffraction pattern for two specimens. a site E in the Alexander VIII hall and b site A in the Sistus VŽ .hall. The strongest lines of haematite, minium and lead white cerussite and hydrocerussite are marked.


Fig. 6. The secular variation curve for Rome and the PiRMresults. The secular variation data are after Cafarella et al.Ž .1992a .

values, measured by Cassini in Rome in 1702and 1730, were Ds3508 and Ds3498 respec-tively, while an inclination Is648 may be de-

Žrived from the Rome curve Cafarella et al.,.1992a . The PiRM declination of the murals A,

B and C agrees with the value measured at thetime of restoration, whereas the inclination isshallower. Not until 1830–1860 were both thedeclination and inclination of the Earth’s fieldsimilar to those of the three murals.

The problem of restoration is particularly im-portant, since sampling was done on the framesand the ornaments between scenes, in order tominimize damage. These parts of the murals aremore likely to be restored at any time to revivethe appearance of the walls, and, being the mosthumble parts, it is quite possible that no trace oftheir restoration is left in the archives. An addi-tional restoration in the first half of the 19thCentury is therefore an attractive hypothesis toexplain the three anomalous directions, but thepossibility of a misalignement of the directionrecorded through the PiRM with respect to the

Earth’s field cannot be excluded. Up to now,few experimental data are available and thereliability of the PiRM direction still requiresfurther investigation.

Acknowledgements

This work was supported by University ofŽTorino and CNR Committee 15 PF Beni Cul-

.turali funds. We are greatly indebted to thePrefect of the Bibliotheca Apostolica Vaticana,Father L. Boyle, for permission to sample thepaintings and to A.M. De Strobel, N. Gabrielliand G. Morello for their invaluable cooperation.The comments by D. Tarling and an anonymousreferee improved the manuscript both in sub-stance and style.

References

Cafarella, L., De Santis, A., Meloni, A., 1992a. Il catalogoGeomagnetico Storico Italiano, Istituto Nazionale diGeofisica, Roma, 160 pp.

Cafarella, L., De Santis, A., Meloni, A., 1992b. Secularvariation in Italy from historical geomagnetic fieldmeasurements. Phys. Earth. Planet. Int. 73, 206–221.

Chiari, G., Lanza, R., 1997. Pictorial remanent magnetiza-tion as an indicator of secular variation of the Earth’smagnetic field. Phys. Earth. Planet. Int. 101, 79–83.

Cicerchia, E., De Strobel, A.M., 1986. Documenti ineditidell’ Archivio Segreto Vaticano sui restauri delle Stanzedi Raffaello e della Cappella Sistina nel Settecento.Boll. Monumenti, Musei e Gallerie Pontificie VI, 105–152.

Michel, O., 1980–1982. Domenico Del Frate, dessinateuret fresquiste meconnu, Colloqui del Sodalizio, 2a serie,´7–8: 141–168.

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, 572pp.

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Remanent magnetization of mural paintings from the Bibliotheca Apostolica (Vatican, Rome)