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Pigments et colorants de l’Antiquité et du Moyen Âge

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Institut de recherche et d'histoire des textes
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Centre de recherche sur les collections
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Équipe Étude des pigments, histoire et archéologie

Session B. Mesurer, caractériser, désigner, mémoriser les couleurs

Plasma-source mass spectrometric analysis of ancient egyptian pigments

Emma Angelini, Paolo Bianco, Elvira D’amicone and Luigi Vigna

Full text

Samples: Historical notes and description

1From many years the Egyptian Museum of Torino carries on studies and restoration activities on the objects of the collections.

2Of particular interest for research purposes are the pigments found by the Archaeological Italian Mission, during the excavations in Heliopolis, in 1903-1905 (fig. 1, 2, 3).

Fig. 1. Map of Egypt with the indication of the excavation site, Heliopolis, the holy area of the capital city Memphis, at present a quarter of Cairo.

Fig. 2. Obelisk raised by the pharaoh Sesostris I in 1930 B.C. (Archives Alinari).

Fig. 3. A page of diary of the excavations of the ltalian Archeological Mission of the Egyptian Museum of Turin.

3The total number of samples is thirty-two: 14 red pigments, 13 yellow pigments, 2 green pigments and 3 typical Egyptian blue pigments.

4Unfortunately in two cases only the date can be specified, furthermore, from the excavation reports and from the inventory register, data on the discover context cannot be found. The material is unpublished.

5The fourteenth samples of red pigments present two different aggregation state.

6In one case, sample Suppl. 3604 (fig. 4), the material has been comminuted and ground. It has been found in a small beaker, filled at an height of 4 cm; the pigment adheres to the inside wall until the top of the beaker. This one is wheel-shaped and has the following dimensions: 10 cm height, rim diameter 8.2 cm, thickness 0.6 cm. The clay has been degreased with straw and sand and the cross-section is orange-coloured; the outside wall is red-coloured. The beacker base has a conical section, the body is high and little careenated. A similar pot was found in the same place by Petrie during successive excavations of the British School of Archaeology in 1911-1912 (Petrie). This kind of beaker has been dated between XIXth and XXVIth dynasty; the comparison with similarly shaped pots could delimitate the objects to 8th-7th century B.C. (Petrie, table 10, fig. 6; Kelley, tables 81.2, nn. 24 Q,S,U; 82. 1 n. 816 s). The beaker is intact with a structurally compact mixture, without salts.

Fig. 4. Powdered red pigment, Suppl. 3604.

Fig. 6. Yellow pigments, Suppl. 3601 (left), Suppl. 4193 (right). Reference ruler = 1 + 1 cm.

Fig. 7. Green pigment, Suppl. 4161.

Fig. 8. Egyptian blue, Suppl. 3603.

Fig. 5. Finegrained red pigment, Suppl. 4152 (40x).

7The other thirteen specimens of red pigment (Suppl. 4130-4130, 4130-4157 and 4152) are aggregates of different shape, dimension and weight, whose values are the following: 1) 167.5 g: 2) 149.8 g; 3) 60.3 g; 4) 34.0 g; 5) 29.3 g; 6) 72.3 g; 7) 52.3 g; 8) 35.5 g; 9) 22. 0 g; 10) 14.7 g; 11) 6.3 g; 12) 4.2 g (fig. 5). The sample are granular with low stiffness and consequently are easily shattered.

8The thirtheen specimens of yellow pigment are aggregates, classified on the basis of increasing visual saturation in three groups identified as a, b and c. Shape, dimensions and weight are different for all the samples. Only one specimen (Suppl. 3601) belongs to type a, and weights 7.5 g; one sample (Suppl. 4149) of weight of 72.9 g belongs to type b, while in group c are the other eleven samples with the following weights: 1) 64.7 g; 2) 15.8 g; 3) 5.8 g; 4) 11.9 g; 5) 21.4 g; 6) 13.3 g; 7) 5.5 g; 8) 3.7 g; 9) 13.5 g; 10) 37.3 g; 11) 8.1 g.

9The specimens of type a and c are more compact, while the type b one is crumbling (fig. 6).

10The two samples of green pigment display two different aggregation state. Shape, dimension and weight are different. The first one weights 14.7 g (Suppl. 4191) and is fairly stiff, showing a partial surface crumbling (fig. 7). The second one weights 83.7 g (Suppl. 4192) and is regularly fine grained and compact.

11One of the three specimens of « Egyptian blue » is raw material and weights 32.1 g (Suppl. 3603) (fig. 8). The other two samples are fragments of different artifacts: the first weights 13.1 g and has concentrical stripes (Suppl. 3603); the second weights 70.8 g and has the cartouche of the pharaoh Ramesses II, 1279-1213 b.C. (Suppl. 2678). The specimens are compact and are in excellent conditions.

12Several samples were taken from the pigments previously cited; the specimens already analyzed are the following:

  • red pigments: Suppl. 3604, Suppl. 4152 (n.2)

  • yellow pigments: Suppl. 3601 (type a), Suppl. 4193 (type c, n. 11).

  • green pigments: Suppl. 4191.

  • egyptian blue pigments: Suppl. 3603 (raw material).

13In a future work all the samples will be analyzed and the composition will be compared with the raw materials trace elements in order to find out information about their origin.

Chemical analysis of pigments

14The first problem which arises in the analysis of archaeological samples is the need for non destructive techniques, or very sensitive destructive ones, in order to preserve the specimen.

15With this aim, two complementary analytical techniques were used: X-ray diffraction for structural determination and Plasma Source Mass Spectrometry for elemental analysis.

16The same small amount of sample (1-5 mg) was subjected to the non destructive X-ray diffraction analysis, and then underwent to the second (destructive) analysis.

17Apparatus - sample treatment

  • Philips X-ray powder diffractometer, Cu Kα, λ = 0.15418 nm

  • VG Elemental Plasmaquad ICP-MS

  • Perkin-Elmer Specamill

  • Nickel sieve, 10 μ

  • Bransonic 220 ultrasonic bath

  • Plastic test tubes, 10 ml

  • Hewlett-Packard 6661A Water Purifier

18While the long established X-ray diffraction technique requires no description at all, a short introduction to the basis of the plasma source mass spectrometry (ICP-MS), developed by A. Gray of Surrey University, may be useful (Gray et al.; Lorber et al.; Tolg). An overall view of the apparatus is schematically shown in Fig. 9. The sample under examination is dispersed into a stream of gas, which is injected in the core of a very high temperature plasma (8000 K) sustained with RF fields. The energy transfer from the plasma to the sample leads to its dissociation, atomisation and ionisation. The plasma core containing the sample ions is extracted into a reduced pressure region through a small orifice, and through another orifice, a portion of it is introduced in a high vacuum region, where a system of electrostatic lenses extracts the charged ions and focalizes the so obtained ion beam in a quadrupole mass filter, which only transmits ions of particular selected mass to charge ratio to the ion detector. Each element has unique and simple group of isotopes, which allow easy identification and quantification of the elements in the sample. The sample can be introduced in the plasma by means of nebulization of solutions or dispersions, or by direct vaporization of solid samples (laser ablation, arc, spark, electro-thermal sampling).

Fig. 9. Plasma source mass spectrometer overall schematic (Courtesy of VG Elemental).

19The samples (1-5 mg) were finely ground, below 10 µm, then after the X-ray diffraction analysis, were wetted with methanol and dispersed in purified water.

20The elemental analysis was performed with the VG Plasmaquad, equipped with a De Galan nebulizer and an Ismatec peristaltic pump, sonicating the test tube during sample uptake, in order to achieve better dispersion homogeneity.

21The acquisition parameters were the following:

  • scan range (m/z) 5 to 240

  • number of scan sweeps 60

  • Dwell time (µs) 500

  • Lens tuning optimized on 115In

  • Plasma gases:

    • coolant 13 1/min

    • auxiliary 0.7 1/min

    • nebulizer 0.84 1/min

  • ICP RF Power: forward 1300 W, reflected <10 W.

Results and discussion

22The red pigments proved to be based on iron oxide: sample Suppl. 3604, fig. 10, was probably obtained by direct grinding of hematite, and contains trace amounts of Ca, Ti, Cr, Mn, Cu, Zn, Sr, Ba, Hg, Pb.

23The fine grained pigment, Suppl. 4152, is mainly constituted by crystalline quartz, with Fe2O3 inclusion, and Rb, Sr, Sn, Hg, Pb, trace elements (fig. 11).

24The yellow pigments were identified, from the X-ray diffraction analysis, as yellow ochre, a mixture of silica and hydrated ferric oxide in different ratios.

25In fig. 12 ICP MS analysis for sample Suppl. 4193 shows iron as main component, with sodium, calcium and magnesium in lower concentration. The same trace elements were found in both samples analyzed (Suppl. 4193, 3601): Ti, Cu, Zn, Sn, Ba, Hg, Pb.

26The green pigment, Suppl. 4161, showed an X-ray diffraction pattern (fig. 13) typical of Sampleite, NaCaCu5(PO4)4Cl 5 H2O, with a small contribution from silica. From the elemental analysis, fig. 14, in addition to the main components were found Mg,Al,Ti,Fe and traces of Mn,Zn,Ba,Hg,Pb.

Fig. 10. Mass spectrum (range 42-70 a.m.u.) of red pigment, Suppl. 3604.

Fig. 11. Mass spectrum (range 80-125 a.m.u.) of red pigment, Suppl. 4152.

Fig. 12. Mass spectrum (range 43-95 a.m.u.) of yellow pigment, Suppl. 4193.

Fig. 14. Mass spectrum (range 23-70 a.m.u.) of green pigment, Suppl. 4161.

Fig. 15. Mass spectrum (range 42-75 a.m.u.) of blue pigment, Suppl. 3603.

Fig. 13. X-ray diffraction spectrum of green pigment, Suppl. 4161. The stars show the fit with sampleite diffraction peaks.

27The blue pigment, Suppl. 3603, was identified as ’Egyptian blue’, CaCuSi4O10, with a noticeable amount of quarz. The elemental analysis (fig. 15) showed also high concentration of sodium and iron with tin and lead as minor components.

28From the comparison of these results with literature data the following remarks may be drawn:

  • the composition of the specimen Suppl. 3604, similar to the ones previously analyzed (Lucas 346-348; LA) confirms the extensive use of this pigment in Ancien Egypt, from predynastic age to greek-roman period. Its finding in the beaker dated to 8th-7th century B.C., allows to state its use in that period.

  • the fine grained red samples can be pointed out as a new kind of finding. The further investigation will also consider a noteworthy number of samples, which might attest an extensive or anyhow important use.

  • the yellow pigment belongs to the large class of iron oxides (Lucas 349-351; LA). Unfortunately the lack of exact references on the discovery place kept the Authors from a chronological attribution of samples.

  • particularly interesting is the green pigment analyzed because of its composition completely different from the others previously described in literature (Lucas 344-345; LA).

  • on the contrary the composition of the blue pigment analyzed is similar to the well-known artificial Egyptian blue pigment (Lucas 340-344; Bayer; Saleh et al.; LA) extensively used during the all antiquity.

Conclusions

29From these preliminary results, we can draw the following conclusions:

  • The ICP-MS technique has proved to be valuable for the elemental analysis of archaeological samples, due to its great sensitivity.

  • The data obtained by means of this technique, coupled with the X-ray diffraction analysis allow the identification of the structure and main chemical composition of inorganic samples.

  • Furthermore, the determination of the concentration and isotope ratio of trace elements, compared with the analysis of the possible raw materials (rocks and minerals), can lead to the identification of the geographical origin of the samples.

Bibliography

G. BAYER, H.G. WIEDEMANN, Le bleu égyptien synthétique de l’Antiquité examine sous l’angle scientifique, Bulletin Sandoz, 40 (1976), 20-40.

A. GRAY, A. DATE, Inductively coupled plasma source mass spectrometry with continuous flow ion extraction, Analyst, 108, (1983), 1033.

A.L. KELLEY, The pottery of ancient Egypt, III, Royal Ontario Museum (1976).

Lexicon der Ägyptologie (LA), Farbe, II, Wiesbaden (1977), 115-117.

A. LORBER, Z. GOLDBERT, Convenient method for the determination of trace elements in solid samples using an inductively coupled plasma, Analyst, 110, (1985), 155-157.

A. LUCAS, Ancient Materials and Industries, London 4th ed. (1962).

W.M.F.L. PETRIE, E. MACKAY, Heliopolis, Kafr Ammar and Shurafa, London (1915).

S.A. SALEH, Z. ISKANDER, A.A. EL-MASRY, F.M. HELMI, Some ancient egyptian pigments in A. BISHAY, Recent advances in science and technology of materials, 3th New York and London (1974), 141-155.

G. TOLG, Extreme trace analysis of the elements, the state of the art today and tomorrow, Analyst 112 (1987), 365-376.

© CNRS Éditions, 2002

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