We are grateful to A. Lazzari for the kindly and fundamental collaboration.
1A study has been carried out on the mineralogical composition of unglazed white stone beads found in the course of archaeological excavations carried out at the site of Ras al Hadd in Southern Oman. These excavations come under the Joint Hadd Project, an international group working under the aegis of the Ministry of National Heritage and Culture, Oman.
2The remains of a costal community have been uncovered in the side located at the extreme eastern tip of the country, on a sandy spit between a lagoon and the Indian Ocean. The lagoon forms a natural port (Mosseri-Marlio, 1998). A thousand beads were found in 2002, in the early third millennium B.C. layers, in the area known as HD-6, which is rich in deposits, containing a large quantity of flint, worked stone and remains of bead manufacturing activity (Cattani & Tosi, 1997; Cattani et al., 2001-2002). These beads were fashioned from diverse materials. We selected some of the white beads for material analysis.
3The data provide information about the raw material and technology used for the production of the beads.
4A comparison has also been performed with the data concerning samples analysed in previous archaeometric studies by some authors relative to similar white beads found in caves used as burial place in Samad Al Shan in Oman and Peqi'in in Israel (Rosch et al., 1997; Bar Yosef Mayer et al., 2004).
5Several unglazed beads and micro-beads made of white paste, of different shapes and sizes have been analysed. The beads range, in size, from two to four millimetres in diameter and two to fifteen millimetres in height; the diameter of the hole is approximately one millimetre. The shape is tubular (truncated cone-shaped) for the biggest bead, cylindrical for the slender beads and ring-shaped for the micro-beads (fig. 1).
One truncated cone-shaped stone bead, site HD 6 - square R113 -US 703 - 2002; size: about 15 mm in height, from about 4 mm up to 5.5 mm in diameter, hole 1 mm. Three slender cylindrical stone micro-beads, site HD 6 - square Q113 - US 789 - 2002; size: about 6 mm in height, about 2 mm in diameter, hole 1 mm. Six ring-shaped stone micro-beads, site HD 6 - square Q114 - US 789 - 2002; size: about 2 mm in height, about 2 mm in diameter, hole about 1 mm.
Une perle tubulaire conique, trois petites perles cylindriques et six micro-perles circulaires sur le site HD6.
6The beads were analysed using X-ray diffraction (XRD). X-ray powder diffraction serves for qualitative analyses of crystalline phases and gives information about the crystal structure and the state of crystallinity of the sample; it can contribute to the unequivocal identification of materials used for the fabrication of ancient objects. Usually, the X-ray method is destructive (since powder sample is analysed) but we optimised the method to minimise the amount of sample necessary for analysis. We made a special sample holder which allows to carry out the analysis of small amounts of sample. Only 15 milligrams of pulverised bead were necessary using the special sample holder while in traditional holders up to 500 milligrams were needed; so a small amount of bead was sacrificed for analysis. The sample holder was made by carving a rectangular glass of the same shape and dimension as an ordinary sample-holder; we carved a wide but shallow well where the primary beam falls. We carefully filled the cavity with powder sample to obtain a flat surface. The good results obtained on some standard samples show that the method is valid for homogeneous samples even if the irradiated surface is rather small. The diffractrograms of the three types of the beads (tubular, cylindrical and ring-shaped) were superimposable. X-Ray diffraction pattern of the tubular bead is shown in figure 2 with the operating conditions. XRD analysis revealed that the beads of the three shapes are made of enstatite, a magnesium silicate belonging to pyroxene group (orthopyroxene Mg2Si2O6); in fact all reflections (peaks) of enstatite are present in the XRD patterns. Other crystalline phases do not appear. The relative intensity of the main peaks shows that the material is synthetic rather than natural enstatite.
7The strongest lines and the relative intensities for enstatite and synthetic enstatite from Handbook “Powder Diffraction File” - JCPDS (International Centre for Diffraction Data) are:
8enstatite - (File 22-714) - 4.43 (4)-3.31 (6)-3.18(100)-2.88(55)-2.54(25)-2.50(18)-2.48(18)
9syntetic enstatite (File 19-768) - 4.41(20)-3.30(10)-3.17(75)-2.87( 100)-2.53( 18)-2.49(8)-2.47( 10).
10Enstatite is a hard mineral that is difficult to carve (hardness five or six on the Mohs scale) but synthetic enstatite can be obtained from a soft and rather common magnesium-bearing mineral, steatite (Mg3Si4O10(OH)2 -massive talc- hardness one on the Mohs scale). Upon heating, steatite decomposes and recrystallizes to give (1) enstatite and a silica phase (SiO2):
112 Mg3Si4O10(OH)2 3 Mg2Si206 + 2 SiO2 (1)
Diffractogram of the tubular bead – Philips Diffractometer PW 1830 – Operating conditions: Generator tension 40 KV – Generator current 30 mA – Tube anode Cu – Filtro Ni – Divergence slit 1/2-Receiving slit 0.2 – Start, end angles 2 – 70 degree – Step size 0.02 degree – Time per step 1 s.
Diffractogramme de la perle tubulaire.
12We propose that steatite was used in the manufacturing process of the beads. The beads might have been made from soft steatite material and then hardened by firing at about 1000°C; the formerly soft beads thus became hard and durable. This hardening is due to the transformation of the steatite into synthetic enstatite which takes place at about 1000 °C.
13Several technological mode of production might have been used:
beads might have been directly carved out from bulk steatite ;
beads might have been produced by reshaped powder steatite (paste powdered steatite-water);
beads might have been produced by pastes made of powdered steatite and a flux.
14We carried out some preliminary heating experiments on bulk talc fired at 1000 °C for 9 hours. The bulk talc became a hard and compact material when heated. We used a Italian talc from Val Germanasca (Piemonte). The strongest lines were: 9.35(100)-3.67(19)-3.12(100)-1.87(7)-1.56(3). We compared diffractograms of the beads (fig. 2) and the fired massive talc (fig. 3). A very good overall resemblance of the patterns was evident. The figures show that the diffractrograms of the beads and the fired talc bulk are quite superimposable. Enstatite is identified as the only crystalline component. The high background below the main peaks of enstatite is due to amorphous silica, residual product of transformation from talc to enstatite.
15As beads may also have been produced by reshaped talk powder, we also tried to fire a talc paste made of finely powdered talc mixed with water; the paste was reshaped into cylinders. The experiment failed completely: shaping the cylinder was very difficult and the slightest touch reduced the fired cylinder to powder. The diffractogram of the fired paste (fig. 4) showed that enstatite and cristobalite were the mineralogical phases. Cristobalite is a crystalline silica phase, residual product of transformation from talc to enstatite.
16Since cristobalite did not appear when we fired massive talc at the same temperature, we deduce that the grinding of talc promotes the crystallization of cristobalite during firing.
17As heating experiments showed that the paste formed by powdered talc and water would not fuse under these conditions, we also tried two pastes obtained by mixing powdered talc, water and a flux: montmorillonite clay or sodium-feldspar.
18At our first attempt we added about 10 % in weight of montmorillonite clay from Wyoming to powdered talc; the crude paste showed a good workability and when heated became a hard and compact material. The diffractogram of the heated paste showed enstatite, cristobalite and quartz (a crystalline silica phase contained as additional component in the clay).
- 1 We used an italian feldspar from Catanzaro (Calabria). Mineralogical composition: 83 % sodium feld (...)
19In further attempts we fired at 1000 and 1100 °C a paste made of powdered talc and a sodium feldspar1 (13% in weight); the crude paste was workable enough to shape small cylinders but the slightest touch reduced the fired cylinders to powder. The diffractograms showed that the mineralogical phases of the fired pastes were enstatite, cristobalite and untransformed feldspar.
Diffratogram of the massive talc fired at 1000 °C.
Diffractogramme de talc cuit à 1000 °C.
Diffractogram of the paste made of powdered talc-water, fired at 1000 °C.
Diffractogramme d'une pâte faite d'un mélange talc-eau, cuit à 1000 °C.
- 2 The strongest lines and the relative intensities for protoenstatite arc: (File 03-0523) - 4.36(10) (...)
20Since this paste didn't fuse up to 1100 °C, we raised the temperature up to 1250 °C and thus obtained a hard and compact material. The diffractogram of the heated paste showed protoenstatite (high temperature-syntetic enstatite)2, cristobalite and a small amount of untransformed feldspar.
21Based on the results of heating experiments on talc it seem reasonable to think that the beads were directly carved out of bulk steatite and then hardened by firing.
22Similar white beads, some of which glazed, were discovered in several areas in the East: Indus Valley (Harappa), Upper Egypt, Umm An-Nar Island and Samad Al Shan (Oman), northern Galilee, Pakistan, Jebel al Emalah -(UAE). For many years this material was regarded as birds bone or shell or faience, only in 1984 the material was analysed by Ole Johnsen from the Mineralogical Museum of Copenhagen and identified as enstatite (Frifelt, 1991). In the last few years enstatite beads have been studied (Vidale, 1987) and experiments carried out to reconstruct the production process.
23Rosch C. et al. (1997) studied some enstatite beads from graves in the oasis of Samad Al Shan (Oman): some unglazed cylindrical beads (dimensions: five millimetres in length, 2.5-3 mm in diameter) and a short cylindrical bead shaped as a cogged wheel (6.5 mm in diameter and 4.5 mm in height). They used a special sample holder: an almost flat surface segment of the bead was brought onto the focussing circle of diffractometer by a sample mount which was adjustable in height. The X-ray diffraction pattern of the beads revealed an enstatite-related structure. Rosch fired a pure massive steatite at 1000 °C for 24 hours; he compared the powder diffractogram of the fired bulk steatite with that of the cogged wheel bead and obtained a good overall resemblance. He also fired pulverised talc reshaped into cylinder under the same conditions as the massive steatite. He said that the diffractogram of this fired material was different from diagrams of fired massive bulk steatite; he didn't specify these differences.
24Bar-Yosef Mayer et al. (2004) studied white beads, with traces of the glaze, found in a burial cave of Peqi'in, northern Galilee (Israel). The beads range in size from two to four millimetres in diameter, one to three millimetres in height, and hole diameter approximately one millimetre; XRD analyses revealed that the beads are made of enstatite and cristobalite. The authors propose this production process: a paste was prepared from powdered talc, water and perhaps a flux or a binding material; the paste was then shaped into tubes, probably along a thin rod, and fired at a high temperature.
25Barthélemy de Saizieu et al. (1994) studied a reconstruction of the production phases used at Mehrgarh, Pakistan; a beads making activity area was found in this site dated in the fifth millennium B.C. The authors proposed that beads were carved directly out of the solid steatite: bars of steatite were obtained by sawing and then were smoothed on four sides, perforated and sliced. Finally, the cylindrical rings thus obtained were fired.
26Vidale (1995) tried to reproduce the manufacturing sequence from bulk steatite to steatite beads. The raw material used was steatite stick commonly used in Pakistan for drawing on blackboards. Disks were cut with a copper blade and drilled with a hand drill (the point drill was a chert blade). Finally about 50 beads were strung on a hemp fibre and ground on a grinding stone to remove the corners. After about 25 minutes of grinding, he obtained well rounded beads. The author came to conclusion that the experimental simulations of the manufacturing sequence support the reconstruction proposed.
27Based on X-ray diffraction analysis of the beads and the heating experiments on talc, we propose that the beads from Ra's al-Hadd were produced from steatite; the beads were directly carved out of soft, solid steatite and then hardened by firing at a temperature of at least 1000 °C.
28We deduce this because:
In the diffractograms of the beads the relative intensity of the main peaks is characteristic of syntetic enstatite;
Peaks of different products deriving from the use of powder steatite, like crystobalite, or peacks relative to products from fluxes, have not been found.
It is possible that these beads were produced from locally available raw materials.
29The raw material of the examined beads, steatite, might have come from the ultramafic rocks of Oman Samail Ophiolite.
30Further analysis and heating experiments on steatite from Oman are required to confirm our propositions and to verify the source of the raw material of the beads.