Sandwich structures in the Etruscan-Padan type pottery

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Applied Clay Science 27 (2004) 119–128

Sandwich structures in the Etruscan-Padan type pottery

L. Nodaria,*, L. Maritanb, C. Mazzolib,c, U. Russoa

aDipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita di Padova, Via Loredan 4, I-35131 Padova, ItalybDipartimento di Mineralogia e Petrologia, Universita di Padova, C.so Garibaldi 37, I-35137 Padova, Italy

c Istituto di Geoscienze e Georisorse, CNR-Padova, C.so Garibaldi 37, I-35137 Padova, Italy

Received 25 April 2003; received in revised form 14 January 2004; accepted 31 March 2004

Available online 17 June 2004

Abstract

The sandwich (black core) structure in the production of Etruscan-Padan type pottery was investigated. Petrographic, X-ray

diffraction (XRD) and Mossbauer data showed that colour changes from core to margin are related to differences in both Fe

oxidation state and abundance of maghemite, hercynite and hematite. The occurrence of maghemite and hercynite and the

higher quantity of Fe(II) in the cores suggest poor oxygen diffusion in the potsherds during firing. This is congruent with the

occurrence of high fractions of paramagnetic Fe in octahedral sites, located in the amorphous phase probably derived from

chlorite breakdown, and with the wide range of grain-size distribution of oxidic particles, indicated by the slowing down of

superparamagnetic relaxation as temperatures are lowered.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sandwich structures; X-ray diffraction; Mossbauer spectroscopy; Iron oxidation state; Iron age pottery; Veneto region

1. Introduction on raw materials used and on firing conditions, e.g.,

In the study of ancient potsherds, colour is an

important feature to be determined, as it can provide

information on technological aspects of pottery man-

ufacture, as well as on the aesthetic preferences of a

specific production culture. The latter aspect is indeed

relevant when we consider the variety of painted

decorations and covering slips, which often over-

comes mere technical aspects such as to make the

pottery impermeable. Analysis of a freshly cut cross

section of the ceramic paste can provide information

0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.clay.2004.03.003

* Corresponding author. Fax: +39-49-8275161.

E-mail address: [email protected] (L. Nodari).

maximum temperature and firing atmosphere (Krei-

meyer, 1987; Stienstra, 1986).

In the present work, pottery characterised by sand-

wich (black core) structure was examined. Potsherds

were provided by the Italian National Museums of

Altino (AL11; AL17), Este (E22; E25) and Adria

(AD1; AD11) (Veneto region, northeastern Italy)

which preserve collections of findings from local

archaeological sites (Fig. 1). The potsherds belong

to the Etruscan-Padan type class, a ceramic production

that spread over the Veneto region from the end of the

V century B.C.

This type of pottery includes fine ceramic vessels

of open and closed shapes, characterised by pinkish to

reddish paste and reddish to brownish, sometimes

Fig. 1. Geographic location of the Veneto centres where the ceramic samples are from.

L. Nodari et al. / Applied Clay Science 27 (2004) 119–128120

banded, decorated external surface (Gamba and Gam-

bacurta, 1987; Rossi, 2000).

On the basis of geochemical and petrographic

study, Maritan (2002, 2004) demonstrated that Etrus-

can-Padan type pottery is of local production and that

potsherds from the towns of Altino, Este and Adria

can be distinguished from each other by their chem-

ical composition and firing temperature. Analysis of

the fabric reveals that the colours of the paste change

from core to margin in most of the samples, display-

ing a sandwich structure with a black core (Maritan,

2002, 2004).

According to Picon (1973) and Rye (1981), this

type of structure is due to firing in reducing con-

Table 1

Pottery paste colour as a function of redox conditions during heating

and cooling and of occurrence of organic matter (modified after

Picon, 1973; Rye, 1981)

Pottery colour Heating Cooling Organic matter

Beige-red with reducing oxidising absent/present

black core oxidising oxidising rich

Grey-black reducing reducing absent/present

Beige-red oxidising oxidising absent/scarce

reducing oxidising absent

ditions with an oxidising cooling stage, or to firing

organic matter-rich clays in oxidising conditions

(Table 1). The dark colour of the core is normally

attributed either to a high Fe2 +/Fe3 + ratio, in par-

ticular to the presence of magnetite (Fe3O4) or

wustite (FeO) in the paste (Letsch and Noll, 1983;

Harrel and Russel, 1967), or to the presence of

unburned carbon particles (Letsch and Noll, 1983).

The reddish and beige colour of the margins is

generally attributed to hematite (a-Fe2O3) and Fe–

Ca silicates, respectively (Kreimeyer, 1987; Molera

et al., 1998).

The main aim of the present work was to determine

which mineral phases controlled paste colour and its

variations from core to margin in the sandwich struc-

ture, by combining petrography, X-ray diffraction and

Mossbauer data. They reflect the oxidation state of the

various layers, providing information on environmen-

tal conditions and changes during firing.

2. Experimental methods

The potsherds were analysed by several different

methods, summarised below. The colours were mac-

roscopically defined by comparison with Munsell

Table 2

Colour of cores (c) and margins (m) of sandwich structures,

according to Munsell (1975) and NCS (1997) classifications

Sample Portion Colour Munsell NCS

AL11 m light red 2.5YR6/6 S3030-Y60R

c dark grey 10YR4/1 S6005-Y50R

AL17 m light red 2.5YR6/6 S2050-Y50R

c grey 5Y3/1 S7502-Y

E22 m light brown 7.5YR6/4 S3020-Y40R

c very dark grey N5 S5500-N

E25 m pink 7.5YR7/4 S2020-Y40R

c very dark grey N5 S5500-N

AD1 m red 10R5/8 S3560-Y20R

c dark grey 2.5YR4/0 S6010-Y70R

AD11 m light reddish brown 5YR6/4 S3020-Y50R

c very dark grey 7.5YR3/0 S7010-Y50R

L. Nodari et al. / Applied Clay Science 27 (2004) 119–128 121

(1975) and NCS (1997) charts on freshly cross-cut

surfaces of the sherds, and petrographic features were

determined on standard (30 Am) thin sections under a

polarised transmission microscope.

Chemical compositions for both major and trace

elements were determined on the bulk by X-ray

fluorescence (XRF) spectroscopy using a Philips

PW 2400 spectrometer equipped with an Rh tube, at

the Dipartimento di Mineralogia e Petrologia, Univer-

sity of Padova. Samples were prepared as beads by

dilution 1:10 of a calcined powder with Li2B4O7.

Geological standards were used for calibration. The

FeO/FeOtot ratio was determined by titration with

potassium permanganate (on bulk samples) and by

Mossbauer spectroscopy (separately on cores and

margins), considering the latter more reliably indica-

tive of the redox conditions of each of the two

portions.

The coloured margins were then mechanically

separated from the black cores by means of a glass

chisel, to avoid Fe contamination. Both margins

and cores were ground in an agate mortar under

acetone, to avoid oxidation, and separately analysed

by X-ray diffraction (XRD) and Mossbauer spec-

troscopy. XRD analyses were carried out by a

Philips PW 3710 BASED Diffractometer with

CuKa radiation, at the Istituto di Geoscienze e

Georisorse (CNR-Padova). Mossbauer spectra were

recorded on a conventional constant-acceleration

spectrometer, which used a room temperature Rh

matrix 57Co source, at the Dipartimento di Chimica

Inorganica, Metallorganica e Analitica (University

of Padova). Spectra were collected both at room

temperature (RT) and at 80 K. When necessary,

spectra were also collected at 21 K, using a closed-

circuit cryostat installed at the National Institute for

Materials Physics in Bucharest (Romania). In this

kind of cryostat, the natural line width is increased

by about 15%, due to pump vibrations. The hyper-

fine parameter isomer shift (IS), quadrupole split-

ting (QS), full line width at half maximum (W),

expressed in mm/s, and the internal magnetic field

(Hint), expressed in Tesla, were obtained by a

standard least-squares minimisation technique. The

spectra were fitted to Lorentzian line shapes using

a minimum number of sextets and doublets. Isomer

shift is quoted relative to metallic iron at room

temperature.

3. Results

The analysed potsherds are 5 to 8 mm thick, in

which two-thirds of the body is generally made up of

a black core and one-third of coloured inner and outer

margins, although their relative thicknesses may vary

significantly among samples and even within a single

fragment. The change in colour from core to margin

is often sharp, although shaded boundaries were

observed in some samples. The cores are generally

very dark grey to grey, and the margins are red,

brown or pink. Colour codes are listed in Table 2,

following the classification of Munsell (1975) and

NCS (1997).

Analysis of microstructural features in thin sections

revealed that cores and margins are very similar, as

they display identical porosity and similar type and

distribution of inclusions, when the change in colour

of the groundmass is excluded.

The geochemical data of Table 3 show that the

potsherds are chemically heterogeneous. In particular,

Fe2O3tot and CaO, which may determine changes in

relative abundance of colouring mineral phases, vary

from 5.02 to 8.61 wt.% and from 1.19 to 6.11 wt.%,

respectively. We refer to Maritan (2004) for a com-

plete discussion on the chemistry of the Etruscan-

Padan type pottery and on its implication for prove-

nance studies. As concerns the oxidation state of iron,

the reduction index (RI = FeO/FeOtot) proposed by

Maggetti and Galetti (1981), calculated by titration

on bulk samples (i.e., core plus margins), was quite

variable among the samples, from 0.16 to 0.66. This is

Table 4

Semiquantitative mineral composition for cores (c) and margins (m)

Sample Qtz Pl Kfs Ill Cc Dol Di Gh Hem Mgh

AL11 m �� c ��

AL17 m �� c ��

E22 m �� c ��

E25 m �� c ��

AD1 m �� c ��

AD11 m �� c ��

Mineral abbreviations: (Qtz) quartz; (Pl) plagioclase; (Kfs) K-

feldspar; (Ill) illite; (Cc) calcite; (Dol) dolomite; (Di) diopside; (Gh)

gehlenite; (Hem) hematite; (Mgh) maghemite. Quantities: very

abundant (�� ); abundant (�� ); frequent (�� ); scarce

(� ).

Table 3

Geochemical composition of samples and relative reduction index

(RI) for bulk samples (titration data) and for margins and cores

separately (Mossbauer data)

AL11 AL17 E22 E25 AD1 AD11

SiO2 52.90 60.48 56.72 63.65 62.03 61.82

TiO2 0.99 0.94 0.82 0.89 0.83 0.83

Al2O3 27.54 22.38 22.64 16.92 18.69 19.40

Fe2O3tot 8.61 8.18 8.52 5.02 7.02 6.83

MnO 0.08 0.06 0.16 0.09 0.09 0.08

MgO 2.62 1.35 2.23 1.65 3.94 3.77

CaO 1.39 1.19 2.22 6.11 2.36 2.32

Na2O 0.54 1.06 0.83 1.85 1.34 1.09

K2O 4.02 3.32 4.29 3.40 3.34 3.52

P2O5 0.39 0.31 1.30 0.66 0.25 0.41

Tot 99.08 99.27 99.73 100.24 99.89 100.07

FeO 1.25 1.19 1.63 1.61 4.19 1.66

RI bulk 0.16 0.16 0.21 0.36 0.66 0.27

RI margin 0.00 0.00 0.04 0.00 0.00 0.00

RI core 0.16 0.11 0.41 0.43 0.79 0.74


in agreement with the data of Maggetti et al. (1988) on

sandwich structure pottery from Sissach-Bruhl. As the

apparent, RI value calculated in such a way is greatly

affected by differences in the volumetric ratio between

core and margin, it does not give any reliable infor-

mation on firing conditions. For this reason, we

preferred to calculate the RI for cores and margins

separately, by using Mossbauer data. Results are listed

in Table 3. They indicate that margins are completely

oxidised, as only Fe(III) was detected, if we exclude

sample E22 which bears a small amounts of Fe(II);

cores display different degrees of oxidation, with RI in

the range 0.11–0.79, very close to or higher than that

estimated on bulk samples.

The mineral assemblage was determined by XRD

analysis. Varying proportions of quartz, plagioclase,

K-feldspar and illite (Table 4) were identified in all

samples. Gehlenite and/or diopside was only detected

in a few fragments (AD1, AL17). When these mineral

phases were absent, calcite and dolomite were ob-

served elsewhere (AD11, AL11, E22, E25). Regard-

ing Fe oxide phases, hematite was recognised in all

margins, with the exception of sample E25, where

hematite is apparently absent or at least its concentra-

tion is below detection limit. It was always associated

with another Fe oxide mineral phase, displaying a

peak at 35.64j of 2h in the XRD pattern, attributed to

the near end-member maghemite of the Fe spinel

series. As diopside and K-feldspar display minor

reflections at similar 2h angles, the attribution to

maghemite was made by subtracting from the inten-

sity of the peak the contribution of diopside and K-

feldspar, calculated on the basis of the relative inten-

sity with that of the major peaks, assuming random

distribution of particles in the powder. As regards the

distribution of Fe oxide, cores generally contain only

maghemite (Table 4).

The magnetite–maghemite series (i.e., kenomag-

netite series) has a general formula Fe2 +1� y(Fe3 +

Fe3 +1 + 2/3y[ ]y/3)O4, where y may vary continuously

from 0 (magnetite Fe3O4) to 1 (maghemite Fe2.67O4 =

g-Fe2O3), according to the substitution 3Fe+ 2!2Fe+ 3 + vacancy [ ]. Synthetic end-member magnetite

and maghemite have rather similar diffraction pat-

terns, complicating their distinction when they occur

in low concentrations in multiphase systems. Since

maghemite often derives from the oxidation of orig-

inal magnetite (Deer et al., 1992), determination of the

composition of this spinel is essential to obtain

information on the oxidising vs. reducing conditions

of firing.

The composition of spinel within the magnetite–

maghemite series can be more reliably determined by

Mossbauer spectroscopy, which provides the Fe va-

lence, site geometry and nature of hyperfine interac-

tions (Greenwood and Gibb, 1971).

At room temperature, the spectra of all samples

show doublets due to paramagnetic Fe(III) and Fe(II)


and sextets attributable to Fe(III) contained in mag-

netic phases. It is evident from the area values (Table

5) that the percentage of Fe(II) is greater in the cores

than in the margins, and that the dimensions and

nature of the oxidic particles are different in the two

portions. These differences are discussed below.

On the basis of isomer shift values, Fe(III) occu-

pies only octahedral sites, whereas Fe(II) occupies

both octahedral and tetrahedral sites. These signals are

generated by Fe in both silicates and the amorphous

phase deriving from decomposition of phyllosilicates.

The magnetic signals of all samples were charac-

terised by a large line width, at both room temperature

(RT) and low temperature (LT), due to poor crystal-

linity and/or degree of high Al-substitution. Thus, a

hyperfine field distribution was first used to obtain

information on the presence of different oxides with

similar magnetic fields. The hyperfine parameters

obtained from the distribution of hyperfine magnetic

field (H) were then used in subsequent fitting to

crystalline subspectra. In this way, reasonable hyper-

fine values of H were obtained for most of the spectra

(Table 5). Several spectra for which the signal-to-

noise ratio was particularly low did not supply valu-

able results concerning the attribution of oxide phases

(AL17(m) LT, AL17(c) LT, AD1(m)).

The hyperfine parameters obtained for the magnet-

ic sextets in the spectra of the margins are typical of

Al-substituted or poorly crystallised hematite and

maghemite (Da Costa et al., 1998; Wagner et al.,

1992) (Table 5). Those of the cores also indicate the

presence of other magnetic species, although they are

not well defined due to the low quality of the spectra.

One important difference among the sherds from the

three different archaeological sites refers to the dimen-

sions of the Fe oxide particles, in both cores and

margins. It is well-known that superparamagnetic

relaxation is a function of temperature and grain size

of oxide particles, so that their grain size can be

estimated from low-temperature spectra. The Altino

sherds display a sextet even at RT, the area of which

increases further at 80 K, suggesting a wide grain-size

distribution. At RT, the sherds from Este and Adria

show a magnetic pattern and a superparamagnetic

doublet, which develops into a sextet at LT, indicating

a much smaller grain size for the Fe oxides.

Samples AL11 and AL17 are characterised by the

absence of Fe(II) sites in the margins and the appear-

ance of a magnetic subspectrum at RT. In fact, in the

margin of sample AL11, 23% of total iron occupies a

site which is already magnetic at RT (Fig. 2a). The

internal field of 49.6 T and the values of isomeric shift

(IS) and quadrupole splitting (QS) (0.35 and � 0.25

mm/s, respectively) suggest the presence of an Al-

substituted hematite (Greenwood and Gibb, 1971).

The remaining 77% gives rise to a paramagnetic

doublet with a large line width, attributable to octa-

hedral Fe(III). On decreasing the temperature, the

doublet area decreases in favour of that of the sextet

(Fig. 2b), proving the superparamagnetic behaviour of

this oxide. The broad line width supports the contem-

porary presence of hematite and superparamagnetic

hematite and/or maghemite. The hypothesis of super-

paramagnetic maghemite is also supported by XRD

data. In the core of sample AL11, 16% of total iron is

attributed to Fe(II) in an octahedral site, whereas

Fe(III) is distributed between a paramagnetic and a

magnetic site, the population of which increases at 80

K. The small QS value supports the attribution to an

Al-substituted superparamagnetic maghemite (Da

Costa et al., 1998).

In the margins of sample AL17, 74% of total Fe is

located in a paramagnetic octahedral site, while the

remaining 26% occupies a magnetic site, attributed to

Al-substituted hematite. On cooling the system down

to 80 K, the area of the magnetic sextet increases by

about 25%, at the expense of the paramagnetic signal,

and can be fitted by two sextets related to hematite

and a superparamagnetic oxide. In the core of sample

AL17, RT measurements indicate that Fe is distributed

over three different sites: 89% in two Fe(III) octahe-

dral sites, and the remaining 11% in a Fe(II) octahe-

dral site. The presence of small oxidic particles is

confirmed by LT measurement, and the new magnetic

pattern, with a very large line width, is attributable to

Al-substituted oxides.

In sample E25 from Este, a well-defined magnetic

pattern appears only at 21 K. It is known from the

literature (Da Costa et al., 1995) that, for small Al-

substituted maghemite particles with dimensions be-

tween 2 and 5 nm, T0 (defined as the temperature

above which only the doublet exists) is in the range

100–140 K. Therefore, the occurrence of the sextet at

21 K indicates that T0 is between 21 and 80 K and that

the oxidic particles are sized about or lower than 2 nm.

At RT, the margins of sample E25 show two paramag-

https://www.researchgate.net/publication/237571262_Influence_of_Nonstoichiometry_and_the_Presence_of_Maghemite_on_the_Mssbauer_Spectrum_of_Magnetite?el=1_x_8&enrichId=rgreq-96f16d6c-0946-450f-964a-abe923f56dca&enrichSource=Y292ZXJQYWdlOzIyMzE4Njk0NDtBUzoxMDIyNjIwMjIwODI1NjFAMTQwMTM5MjU3NDA4MQ==

Table 5

Mossbauer parameters for room temperature (RT) and low temperature (LT) measurements

RT LT

Sample IS (mm/s) QS (mm/s) H (T) W (mm/s) A (%) Attributions T (K) IS (mm/s) QS (mm/s) H (T) W (mm/s) A (%) Attributions

AL11(m) 0.30F 0.02 0.80F 0.01 0.47F 0.02 77F 1 Fe(III) M 80 0.43F 0.01 0.98F 0.02 1.07F 0.03 52F 1 Fe(III) M

0.35F 0.03 � 0.25F 0.05 49.6F 0.2 0.77F 0.06 23F 1 Hem 0.46F 0.02 � 0.06F 0.03 51.7F 0.1 1.15F 0.07 48F 1 Hem

AL11(c) 0.30F 0.04 0.85F 0.05 0.59F 0.08 62F 1 Fe(III) M 80 0.42F 0.02 0.89F 0.04 0.78F 0.04 58F 1 Fe(III) M

1.10F 0.05 2.23F 0.03 0.55F 0.03 16F 1 Fe(II) M 1.24F 0.05 2.40F 0.20 0.58F 0.08 13F 1 Fe(II) M

0.35F 0.03 � 0.02F 0.06 49.3F 0.2 1.1F 0.1 22F 1 Mgh 0.47F 0.04 � 0.02F 0.08 51.9F 0.3 1.1F 0.20 30F 1 Mgh

AL17(m) 0.34F 0.01 0.84F 0.01 0.66F 0.02 74F 1 Fe(III) M 80 0.45F 0.01 0.91F 0.02 0.74F 0.03 49F 1 Fe(III) M

0.35F 0.02 � 0.19F 0.04 50.7F 0.3 0.63F 0.08 26F 1 Hem 0.43F 0.08 � 0.08F 0.06 49.4F 0.9 1.5F 0.30 26F 1 Oxid

0.47F 0.01 � 0.01F 0.06 52.8F 0.2 0.7F 0.10 25F 1 Hem

AL17(c) 0.37F 0.01 0.58F 0.02 0.33F 0.04 41F1 Fe(III) M 80 0.46F 0.01 0.76F 0.01 0.52F 0.02 53F 1 Fe(III) M

0.37F 0.09 1.0F 0.10 0.51F 0.04 48F 1 Fe(III) M 1.11F 0.05 2.6F 0.10 0.9F 0.20 13F 1 Fe(II) M

1.0F 0.40 2.20F 0.02 0.5F 0.10 11F1 Fe(II) M 0.4F 0.10 � 0.07F 0.06 48.4F 0.6 1.8F 0.40 33F 1 Oxid

E22(m) 0.31F 0.01 1.05F 0.03 0.80F 0.01 72F 1 Fe(III) M 21 0.52F 0.01 1.16F 0.01 0.85F 0.01 36F 1 Fe(III) M


0.38F 0.01 � 0.14F 0.02 48.9F 0.1 1.07F 0.05 24F 1 Hem 0.42F 0.02 0.04F 0.05 45.7F 0.8 1.5F 0.10 22F 1 Oxid

0.47F 0.01 � 0.08F 0.01 52.1F 0.4 0.67F 0.02 31F1 Hem

E22(c) 0.37F 0.02 1.08F 0.04 0.78F 0.03 59F 1 Fe(III) M 21 0.45F 0.01 1.22F 0.01 1.00F 0.01 41F1 Fe(III) M


0.9F 0.10 1.8F 0.30 0.8F 0.20 14F 1 Fe(II) T 0.4F 0.10 � 0.3F 0.10 46.4F 0.3 1.2F 0.10 17F 1 Oxid

0.47F 0.01 � 0.03F 0.01 51.6F 0.1 0.57F 0.01 17F 1 Mgh

E25(m) 0.37F 0.02 0.98F 0.01 0.62F 0.02 63F 1 Fe(III) M 21 0.47F 0.01 1.26F 0.01 0.98F 0.01 68F 1 Fe(III) M

0.36F 0.01 1.49F 0.02 0.66F 0.04 37F 1 Fe(III) M 0.4F 0.10 � 0.2F 0.40 51.5F 0.3 1.8F 0.10 32F 1 Oxid

E25(c) 0.38F 0.01 1.18F 0.03 0.71F 0.05 57F 1 Fe(III) M 21 0.53F 0.04 1.11F 0.08 1.1F 0.03 37F 1 Fe(III) M


0.44F 0.03 � 0.3F 0.10 51.5F 0.2 2.0F 0.10 27F 1 Oxid

AD1(m) 0.32F 0.01 0.92F 0.01 0.62F 0.01 64F 1 Fe(III) M 21 0.42F 0.01 0 1.89F 0.02 44F 1 Relaxing

component

0.30F 0.01 � 0.02F 0.01 51.0F 0.3 0.46F 0.02 36F 1 Hem 0.52F 0.01 � 0.05F 0.01 53.0F 0.1 1.76F 0.02 50F 1 Hem

AD1(c) 0.41F 0.05 0.70F 0.08 0.63F 0.09 21F1 Fe(III) M 0.45F 0.06 0.06F 0.01 48.2F 0.4 0.55F 0.02 6F 1 Oxid

1.10F 0.03 2.04F 0.04 0.66F 0.05 52F 1 Herc 21 1.24F 0.01 2.59F 0.01 0.92F 0.01 91F1 Herc

1.16F 0.01 2.53F 0.03 0.39F 0.06 27F 1 Herc 0.49F 0.02 0.55F 0.03 0.66F 0.02 9F 1 Fe(III) M

AD11(m) 0.32F 0.03 0.73F 0.07 0.4F 0.2 23F 1 Fe(III) M 21 0.48F 0.01 1.32F 0.01 1.01F 0.01 54F 1 Fe(III) M

0.35F 0.02 1.3F 0.10 0.70F 0.06 77F 1 Fe(III) M 0.49F 0.02 � 0.13F 0.07 52.5F 0.1 0.76F 0.05 20F 1 Hem

0.48F 0.04 � 0.19F 0.02 44.0F 0.2 1.9F 0.4 26F 1 Oxid

AD11(c) 0.48F 0.02 0.85F 0.10 0.70F 0.20 26F 1 Fe(III) M 21 Relaxing components

1.09F 0.03 2.02F 0.10 0.68F 0.10 47F 1 Herc

1.14F 0.04 2.51F 0.10 0.46F 0.10 27F 1 Herc

IS: isomeric shift; QS: quadrupole splitting (DEQ) for nonmagnetic or quadrupole shift (2e) for magnetic subspectra; H: hyperfine magnetic field; W: full width at half height; A:

relative area; M: octahedral site; T: tetrahedral site; Hem: hematite; Mgh: Al-substituted maghemite; Herc: hercynite, Oxid: poorly crystallised oxides; (m) margins, (c) core. All

values are relative to metallic a-iron.

L.Nodariet

al./Applied

ClayScien

ce27(2004)119–128

124

Fig. 2. Room and low temperature Mossbauer spectra of sherds AL11(m) (a: RT; b: 80 K), AD1(m) (c: RT; d: 21 K), E22(m) (e: RT; f: 21 K).

E22(c) (g: RT; h: 21 K).


netic Fe(III) doublets in octahedral coordination. On

cooling the system to 21 K, a sextet with a broad line

width appears with the complete disappearance of one

of the two paramagnetic doublets. The RT spectrum of

the core of sample E25 shows two different paramag-

netic absorptions, one related to octahedral Fe(III) (IS:

0.38 mm/s; QS: 1.18 mm/s) and the other to octahedral

Fe(II) (IS: 1.04 mm/s; QS: 2.17 mm/s). At LT, a very

broad sextet appears. Although this is not attributable

to a specific oxidic mineral phase, it is compatible with

the occurrence of maghemite, as suggested by XRD

data. The RT spectrum of margins of sample E22

displays central absorption with a high velocity com-

ponent, due to two octahedral site, one of Fe(III) and

one of Fe(II), and a broad sextet (Figs. 2e and 2c),

representing 24% of the total area, attributed to Al-

substituted or to a poorly crystallised hematite. On

cooling the system to 21 K, the total magnetic pattern

area increases from 24% to 53%, and one of the

doublets completely disappears (Figs. 2f and 2d). At


RT, the core of sample E22 shows a spectrum similar

to that of the case of E25 (Fig. 2a). The absorption was

fitted using three doublets: an Fe(III) octahedral site

(IS: 0.37 mm/s; QS: 1.08 mm/s), an Fe(II) tetrahedral

site (IS: 0.90 mm/s; QS: 1.80 mm/s) and an Fe(II)

octahedral site (IS: 1.16 mm/s; QS: 2.24 mm/s). At 21

K, a magnetic pattern characterised by a very large line

width becomes evident (Fig. 2b). This pattern was

fitted using two sextets: the first, with a higher hyper-

fine field (HF) (51.6 T), is compatible with the

presence of Al-substituted maghemite; the second,

with a very broad line width, suggests the presence

of a poorly crystallised oxide and/or highly Al-substi-

tuted maghemite.

The RT spectrum of the margin of AD1 presents

absorption due to two octahedral Fe(III) sites and a

sextet due to Al-substituted hematite (Fig. 2c). On

cooling to 21 K (Fig. 2d), the system presents one

residual signal related to octahedral Fe(III) and two

magnetic sextets, one attributable to hematite and the

other to a poorly crystallised oxide. At RT, the

spectrum of the core presents a strong doublet fitted

to an octahedral Fe(III) component and to two Fe(II)

sites, one attributed to hercynite (Stevens et al., 1998).

At 21 K, the two Fe(II) doublets collapse into a single

doublet, perhaps as a consequence of crystal structure

contraction; the Fe(III) site seems to be unaffected.

At RT, the margins of sample AD11 present two

octahedral Fe(III) sites. On cooling to 21 K, two

sextets appear: the first with a hyperfine magnetic

field (Hint.) value of 52.5 T, indicating Al-substituted

hematite; and the second, with a lower Hint. and a

broad line width, suggesting Al-substituted oxides.

The core of sample AD11 also shows a strong

absorption due to Fe(II) and a shoulder in the low-

velocity component, which reveals a small amount of

Fe(III). The spectrum was fitted with two doublets for

octahedral Fe(II), with parameters typical of hercyn-

ite, and one doublet due to octahedral Fe(III). On

cooling the system to 21 K, the spectrum becomes

complex, due to slow relaxing species, and was not

fitted.

4. Conclusions

A key point in the characterisation of pottery is to

assess firing conditions. This task is particularly hard

in the core of sandwich structure pottery, since differ-

ent combinations of redox conditions and bulk chem-

ical composition of the raw materials can produce

such a structure during firing. In this work, the

oxidation state of both core and margins was deter-

mined by combining XRD and Mossbauer data.

At RT, Mossbauer spectra of potsherd margins are

characterised by the typical sextet of hematite, if we

exclude sample E25, where hematite is absent, and

sample AL11, where hematite is identified at LT. An

additional magnetic component was also identified at

LT and attributed to Al-substituted oxides.

At LT, maghemite was recognised in the cores of

samples AL11 and E22. The occurrence of super-

paramagnetic phenomena at LT indicates that the

grain size of maghemite is much lower than that of

hematite.

In some cases (AL17, E22, E25, AD1, AD11), a

magnetic signal was seen, the HF of which was not

assignable to specific oxidic phases and may represent

poorly crystallised oxides. Furthermore, the typical

double sextet of magnetite was never observed.

Paramagnetic octahedral coordinated Fe(III) and

Fe(II) was also observed. Fe(II) is generally detected

in cores and is absent in margins, except for sample

E22, where its concentration in the margin is much

lower than in the core. In the lack of high concen-

trations of Fe-bearing silicates in the potsherds, as

suggested by the XRD data (Table 4), the paramag-

netic signal may be due to the amorphous phase

deriving from decomposition of Fe-bearing clay min-

erals such as chlorite (Maniatis et al., 1983), although

small quantities of Fe may also be contained in illite.

The formation of the amorphous phase probably

caused a reduction in porosity (Tite and Maniatis,

1975) and consequently of the permeability of the

ceramic paste. In such a way, growth of oxidic phases

at the expense of the octahedral Fe of the amorphous

matter (Murad and Wagner, 1998) was locally

inhibited. This is in agreement with the higher content

of Fe(II) in the cores and the wide grain-size distri-

bution of maghemite.

The colours of the potsherds, macroscopically

observed and described in Table 2, are congruent with

XRD and Mossbauer data. In particular, the pinkish

and reddish hues of margins are related to different

proportions of hematite and maghemite, which re-

spectively contribute with their characteristic red and


brown colours. In addition, samples AD1 and AL1

also contain diopside and gehlenite, which respective-

ly confer yellowish and buff colours to the paste.

Considering the cores, the situation seems to be

different for each finding site. The dark colour ob-

served in the cores of samples AD1 and AD11 is

attributable to the hercynite, whereas those from

Altino and Este are related to both Fe(II) in the

amorphous phases and maghemite.

The presence of hercynite in the samples from

Adria indicates strong reducing firing conditions.

Maggetti et al. (1981) experimentally verified that

hercynite forms at oxygen fugacity between 10� 12

and 10� 21 bar. Recent firing experiments carried out

by Maritan (2002) showed that hercynite may also be

obtained by firing organic matter-rich clays in an

oxidising environment. In this case, the local reducing

conditions of the cores are not related to the general

redox conditions of the kiln, but caused by firing of

organic matter.

Acknowledgements

The authors would like to thank the ‘‘Soprinten-

denza Archeologica del Veneto’’ for providing

samples and the ‘‘Istituto di Geoscienze e Geori-

sorse’’, CNR, Padova, for analytical support. They

also thank the ‘‘National Institute for Materials

Physics’’, Bucharest, Romania, for collecting 21 K

Mossbauer spectra. We acknowledge G. Walton, who

revised the English text. This work was financially

supported by the MIUR-2000 project ‘‘Development

and application of mineralogical and petrographic

investigation methodologies to the study of archaeo-

logical materials’’.

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Sandwich structures in the Etruscan-Padan type pottery