ISSN 1063-7745, Crystallography Reports, 2018, Vol. 63, No. 2, pp. 295–301. © Pleiades Publishing, Inc., 2018. Original Russian Text © A.Yu. Loboda, N.N. Kolobylina, A.A. Veligzhanin, Y.V. Zubavitchus, E.Yu. Tereshchenko, N.I. Shishlina, E.B. Yatsishina, P.K. Kashkarov, 2018, published in Kristallografiya, 2018, Vol. 63, No. 2, pp. 320–327.
CRYSTALLOGRAPHIC METHODS IN HUMANITARIAN SCIENCES First Russian Crystallographic Congress
Complex Study of the Spearhead Superficial Crust from Burial Mound no. 1 near Novosvobodnaya Village A. Yu. Lobodaa,*, N. N. Kolobylinaa, A. A. Veligzhanina, Y. V. Zubavitchusa, E. Yu. Tereshchenkoa,b, N. I. Shishlinac, E. B. Yatsishinaa,b, and P. K. Kashkarova,d,e a National
b
Research Center “Kurchatov Institute,” Moscow, 123182 Russia Shubnikov Institute of Crystallography, Federal Scientific Research Center “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia c State History Museum, Moscow, Russia d Moscow State University, Moscow, 119992 Russia e Moscow Institute of Physics and Technology, Dolgoprudnyi, Moscow region, 141701 Russia *e-mail:
[email protected] Received July 5, 2017
Abstract—A complex study of a spearhead dated back to IV mill. BC from burial mound no. 1 near Novosvobodnaya village (collection of the State Historical Museum) and, in particular, the material of spearhead superficial crust has been performed. The elemental and phase composition of the metal of spearhead and the superficial crust on its surface have been determined by scanning electron microscopy, jointly with energydispersive X-ray microanalysis and X-ray phase analysis. A comparative analysis of the results of studying the spearhead superficial crust and similar crusts on other artifacts from the mounds near Novosvobodnaya village suggest natural origin of the crust on copper‒arsenic artifacts. DOI: 10.1134/S106377451802013X
INTRODUCTION Mounds near Novosvobodnaya village (Maikop region, Republic of Adygea) were excavated in 1898 by Veselovskii [1]. The excavations revealed two burials of the Early Bronze Age. Each tomb was a dolmen covered by a mound fill. These burials were assigned to the Novosvobodnaya type of the Maikop culture. Adults were buried in the dolmens, each with rich funeral stock: ceramics and artifacts made of precious metals and bronze. The uniqueness of the found complexes made them an object of great interest for researchers. In different periods of time, the corresponding collections were studied by Iessen [2], Popova [3], Rezepkin [4], Korenevskii [5], Ryndina, Ravich [6], and Trifonov [7, 8]. Lately, the interest of archaeologists in interdisciplinary studies has greatly increased. Physical methods of analysis of archaeological artifacts provide unique information about the ancient manufacturing technologies and the conditions for everyday use and storage of artifacts. The second studies of old collections are also necessary, because the natural science
methods are improved significantly last decades. Researchers can study the elemental and phase composition of materials at a qualitatively new level: the measurement accuracy can now be increased several times due to the application of higher power excitation sources and modern high-speed detectors with higher sensitivity and energy resolution; microscopic samples can also be investigated. In this paper, we report the results of a complex study of the superficial crust on a spearhead belonging to the Maikop culture (Fig. 1). During the inspection of old collections, curators of the Department of Archaeological Monuments of the State Historical Museum (SHM) paid attention to the dense crust of unknown origin on a spearhead. They were greatly interested in this crust, because there were known findings of weapon samples with artificial arsenic coating among Maikop culture artifacts [9]. The purpose of this study was to determine the composition of the spearhead superficial crust and analyze the mechanism of its formation on the artifact.
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1 cm
Fig. 1. A spearhead dated back to IV mill. BC from burial mound no. 1 near Novosvobodnaya village.
SAMPLES Since the artifacts studied here belong to the SHM collection, they were denoted according to the SHM inventory (SHM: inv. no.). Main Object of Study The main object of study is a spearhead (Fig. 1), which consists of a short leaf-like blade, a round crosssection rod with a stop, and a four-sided sharpened haft to be fixed in a staff (SHM: inv. no. 89/54). A low edge passes along the blade midline from both sides to turn into a rod. There is a rounded swelling (stop for the staff) at the transition point from rod to haft. The preservation of the spearhead is well: only few small edge fragments are absent. It is coated by a dark dense crust, up to 1 mm thick. The crust layer is partially cleaved off. Corrosion products are observed on the surface beyond the crust area. The spearhead sizes are as follows: length 17.4 cm, leaf width 2.5 cm, swelling diameter 0.9 cm, and rod cross-section dimensions in the upper part 0.5 × 0.6 cm. Reference Samples (i) Socketed ax (SHM: inv. no. А89/83) with a rounded socket and bulky back part. Integrity: presence of corrosion products and lost edge fragments. Sizes: length 10.0 cm, blade width 5.25 cm, blade thickness at the socket 2.3 cm, and socket sizes 2.6 × 4.6 cm. (ii) Fragment of tool (SHM: inv. no. А89/7): a piece of a flat blade with a long thin handle. Integrity: corrosion products, fragmentation. Sizes: length 6 cm. (iii) A flat ax (SHM: inv. no. А89/54) of trapezoidal shape, having straight-line lateral sides and oval heel with rounded angles. Integrity: corrosion products and lost fragments of blade edge. Sizes: length 7.9 cm, width in the upper part 3.1 cm, width in the lower part 3.7 cm, and thickness 3.5 cm.
METHODS OF STUDY A complex study of the spearhead was performed on the instrumental base of the National Research Centre “Kurchatov Institute”; it included analysis of the spearhead elemental composition by scanning electron microscopy (SEM) at Resource Center of Probe and Electron Microscopy and X-ray phase analysis at Kurchatov Synchrotron Radiation Source (KSRS). The basic composition of the artifact metal was determined on bore chip samples from the spearhead haft neck (drilling was performed previously by other researchers). Samples of the spearhead crust for studying the elemental composition and structure were taken from the region of natural violation of layer integrity on the spearhead. Samples of similar layers from test samples in the final stage of the study were taken from the regions of local destruction of “coatings.” Visual analysis of the samples was performed on an Olympus BX51 optical microscope with a Leica DFC420C camera (magnifications of ×50 and ×100). The objects were studied with SEM and energydispersive X-ray microanalysis (EDXMA), which allowed to investigate the morphology and microstructure of the sample surface (in particular, to perform qualitative and quantitative analysis of its chemical composition) with high precision. The analysis was carried out with a scanning electron‒ion microscope (SEIM) Helios nanoLab 600i having a spatial resolution of 0.8 nm for detecting secondary electrons at an accelerating voltage of 15 kV. This instrument is equipped with an EDXMA spectrometer having an energy resolution of 128 eV. The microanalytical studies in SEIM were performed at a maximum accelerating voltage of 30 kV, which made it possible to obtain EDXMA spectra in wide energy range. The SEM study was performed on three objects: (i) metal bore chips to study of the composition of the basic metal of spear with oxide mass excluded; (ii) spearhead crust to study the composition of its external surface and internal (adjacent to the metal) surface and to analyze the profile of crust layer cleavage; (iii) coating of check samples to analyze the composition. The phase composition of the samples of spearhead superficial crust was investigated on an experimental setup at the KSRS [10]. The results of X-ray phase analysis supported the data on elemental composition, providing information about the crystal phases in the sample. This imposes certain limitations on possible conditions for the crust origin. We used synchrotron radiation with a wavelength λ = 0.9725 Å, focused on a sample into a 400 μm size spot. A sample, placed in a cryoloop, was rotated around the horizon-
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Table 1. EDXMA data on the elemental composition of spearhead samples (in wt %)
1 2 3 4 5 6
Cu
S
O
С
Si
Al
Cl
K
Ca
As
Fe
18.9 66.5 68.6 86.1 92.6 23.4
0.9 7.5 4.8 8.7 — —
6.9 4.2 7.7 5.2 7.4 17.7
72.3 21.8 18.9 — — 47.3
0.2 — — — — 0.6
0.1 — — — — —
0.2 — — — — —
0.1 — — — — —
0.2 — — — — —
— — — — — 11.0
0.2 — — — — —
tal axis during measurement to average the diffraction patterns over sample orientations. Diffraction patterns were recorded using a MarCCD165 detector, oriented perpendicular to the synchrotron radiation beam and located at a distance of 100 mm. The exposure time was 10 min. Two-dimensional diffraction patterns were reduced to the one-dimensional form I(2θ) within the Fit2D program [11]. The phase composition was determined using the Match! software [12] with use of the PDF-2 powder database. The mass fractions of phases were quantitatively estimated based on the method of reference intensity ratios [13]. RESULTS AND DISCUSSION Optical microscopy of the spearhead superficial crust revealed a soft lead blaze of its surface, with local rainbow and blue tints. A material cleavage exhibited nonuniformly distributed copper-color inclusions. The inner surface of the crust was linked tightly to the corrosion products of copper‒arsenic alloy. According to the SEM data, the main spearhead metal was found to be a copper‒arsenic alloy, containing from 4.8 to 5.8 wt % As. The results of studying crust samples are presented in Fig. 2 and Table 1. The spectra in Fig. 2b make it possible to estimate the elemental composition of a sample and reveal its layer-by-layer structure. An analysis of the crust outer surface showed that the main components of the layer composition are copper and sulfur (up to 16.4 wt %) and revealed the presence of corrosion products and surface contaminations on the crust (6.6 wt % in total). Spot 3, located in the bulk of the crust, has a composition similar to that of the sample from the outer spearhead crust surface (spots 1, 2). Spectra from spots 4 and 5 characterize the interface between the crust and copper corrosion products. The layers are clearly separated and can be detected well: spot 4 is the interface between the outer crust layer and copper corrosion products. Spots 5 and 6 (corresponding to spectra 5 and 6, respectively, in Fig. 2) are corrosion products of the copper‒arsenic alloy, which are situated between the main-metal surface and superficial crust layers. The results of X-ray phase diffraction analysis are presented in Fig. 3 and Table 2. An analysis of two CRYSTALLOGRAPHY REPORTS
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samples 1 and 2 of the spearhead superficial crust, both about 0.5 × 0.5 mm in size, revealed (see Table 2) that the main compounds forming the crystal layer are β-chalcocite Cu2S (~30 wt % in both samples) and α-chalcocite Cu2S (about 40 wt % in only sample 1). In addition, cuprite Cu2O and copper Cu were identified; their contents in samples 1 and 2 differed by an order of magnitude. A small amount of digenite Cu9S5 (1 wt %) was also found in sample 2. This large spread of phase compositions reflects the highly inhomogeneous character of the crust surface structure and completely corresponds to natural sulfide copper minerals. This circumstance suggested natural origin of the spearhead superficial crust. Chalcocite belongs to the group of secondary copper sulfides; it is formed mainly exogenously as a result of oxidation of copper-containing minerals to a mixture of α- and β-chalcocites [14]. Cuprite, digenite, and copper arose apparently during the formation of chalcocite layer on the spearhead. To confirm the hypothesis about the natural origin of the “coating” and study more completely the specific features of the revealed crust layers, we analyzed some other artifacts from the mounds near Novosvobodnaya village for the presence of similar sulfide inclusions. To select test samples, we performed optical study of artifacts from mound no. 1 and objects from mound no. 2. Samples for comparison were chosen in correspondence with the visually detected specific features of the crust under study. An inspection of copper‒arsenic artifacts from mounds revealed the presence of traces of the layer of interest on the surface of some objects. Samples were selected from only crust crumbling areas. Note that the crust layer on artifacts was fragmentary in most cases (with different degrees Table 2. X-ray phase data on two crust samples Phase
Sample 1
Sample 2
α-Cu2S β-Cu2S Cu Cu2O Cu9S5
40.6% 29.8% 23.2% 6.4% —
— 30.5% 2% 66.5% 1%
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1 2 500 μm
Intensity, rel. units
3 5 6 4
500 μm
(b) 2.5 2.0 C Cu 1.5 Cu S 1.0 K 0.5 OAlSi Cl Ca Fe 0 5.5 4.5 Cu 3.5 Cu S 2.5 1.5 C 0 1.5 1.2 Cu Cu 0.9 0.6 0.3 CO S 0 1.5 1.2 Cu 0.9 Cu 0.6 0.3 CO S As 0 1.5 1.2 Cu 0.9 Cu 0.6 0.3 O 0 1.3 1.1 CO Cu Cu 0.8 As 0.5 As 0.3 Si As 0 2 4 6 8 10 12 Energy, keV
1
2
3
4
5
6
14
Fig. 2. (a) SEM secondary-electron images (circles indicate regions of EDXMA analysis) and (b) energy-dispersive spectra from spearhead samples (the corresponding quantitative data are listed in Table 1). EDXMA regions: (1) sulfide “coating” surface zone, (2) sulfide coating mass, (3) cleaved sulfide coating mass, (4) sulfide coating boundary, (5) cuprite, and (6) copper alloy corrosion products.
of integrity); however, as in the case of the spearhead, these were dense dark layers with lead blaze, located above the corrosion products.
surface of many tools from the mounds near Novosvobodnaya village that were investigated by us.
The SEM data on the samples from check objects of study, which made it possible to trace the general regularity in their compositions, are listed in Table 3.
The results of our study allow to speak about the sulfide crust as a peculiar corrosion layer, characteristic of most copper‒arsenic artifacts from the tombs found near Novosvobodnaya village.
Copper and sulfur dominate in the selected samples, as well as in the spearhead superficial crust. The sulfur content ranges from 4.6 to 16.4 wt %, depending on the general degree of corrosion and contamination of the sample surface. A inspection revealed the presence of a layer on seven objects; however, it was linked tightly to the surface, as a result of which samples could not be taken off. Thus, we can suggest that sulfide is present (in larger or smaller amounts) on the
To understand the nature and mechanisms of the formation of sulfide “coating” (up to 1 mm thick) on the surface of the artifacts from mounds, it is necessary to consider the specific features of both the corrosion layer on the artifact surface and the tomb layout. First, one should note a clear stratigraphy of the corrosion products on artifacts, which is undoubtedly related to the stages of layer formation and transformation. It is also important to take into account the
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Intensity, rel. units 1
Sample 1
β-Cu2S 29.8 % α-Cu2S 40.6 % Cu 23.2 % Cu2O 6.4 %
β-Cu2S α-Cu2S Cu Cu2O
0
15
20
25
30
35
40 2θ, deg
Fig. 3. X-ray diffraction curve from sample 1 of spearhead superficial crust.
good integrity of the metal base, the absence of dependence of the crust quality and thickness on the direct contact with metal, and the absence of this contact in most cases. Based on this, one can suggest that the crust source is not the main metal but the corrosion products formed by the onset of transformation into sulfide.
sition and structure), and the soil composition and specific features. In this context, the preservation of archaeological artifacts varies in a very wide range. Sometimes the metal of an artifact is almost completely destroyed and transformed into a cluster of mineral compounds, and sometimes a dense crust is formed on an artifact to protect the metal from further destruction [15].
Copper alloys contacting with air are known to be coated by a film of corrosion products, which evolves through two main stages: (i) formation of a primary layer of copper oxides and protoxide and (ii) formation of a denser film of salts of alloy metals. Its occurrence is generally caused by the presence of moisture and corrosive materials in air, and the layer consists in fact of analogs of natural minerals. The films retain a layered structure, which reflects the stages of corrosion products formation [15].
Note that chalcocite is rarely met among corrosion products of copper alloys on the whole and, in particular, as a result of soil corrosion. Generally, copper sulfides in air atmosphere are mainly transformed into copper sulfates [16], whereas the formation of copper sulfides in soil is related to the decay of organic materials [15]. In the case of the spearhead under consideration, the character of corrosion processes on its surface was significantly affected by the tomb peculiarity. According to Veselovskii’a report, the tomb mounds excavated by him near Novosvobodnaya village were not filled with soil [1]. Human remains, clay vessels, and metal artifacts were present in the dolmen chamber. The chamber was spanned by a stone roof, and the
Products of corrosion of copper alloys in soil have generally a much more complex composition than the ones formed in air, and their layering is not so pronounced. The formation of corrosion layers is affected by the alloy composition, state of artifact (presence of cracks and pores and inhomogeneity of metal compo-
Table 3. Comparison of the elemental compositions of samples of metal artifacts from mound no. 1 near Novosvobodnaya village (in wt %) EDXM A data on check samples (wt %)
Socketed ax Tool handle Flat ax
Cu
S
Al
Si
P
Cl
K
Ca
As
Fe
82.5 80.5 89.2
13.1 16.4 4.6
— — 0.5
1.3 1.0 1.1
— 0.4 —
— — 1.7
1.1 0.2 0.4
0.5 0.6 0.6
1.0 0.5 1.9
0.5 0.4 —
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Fig. 4. Schematic stratigraphy of corrosion layers on the spearhead: (1) sulfide coating zone, (2) copper inclusions, (3) cuprite, (4, 5) copper alloy corrosion products, and (6) copper alloy.
thus closed dolmen was coated by a layer of earth; i.e., the amount of oxygen in the chamber was limited. Likely for this reason the initial stage of corrosion of metal artifacts in tomb was accompanied by interaction with oxygen and resulted in a layer-by-layer stratigraphy of corrosion layers, characteristic of atmospheric corrosion (Fig. 4). Then the decay of organic materials in the chamber and the gradually formed anaerobic atmosphere facilitated the replacement of the outer layer of corrosion products with mineral sulfides and further development of a dense chalcocite layer on the artifact surface. Since earth was absent in the funeral chamber, the decaying organic materials formed a hydrogen sulfide atmosphere throughout the entire tomb and did not affect locally metal in the contact zones, as this generally occurs in soil. The thus formed chalcocite layer protected the main metal of artifacts from further destruction. The difference in the sulfide layer integrity on artifacts from the same burial is caused by museum’s restoration works. During the long-term storage in museum, the metal artifacts studied here were repeatedly subjected to full-fledged restoration and preventive preparation before exposure in exhibitions. The degree of cleaning was individual for each artifact. Due to this, a corrosion layer of the same appearance and composition was removed to a different extent from the artifact surface. This circumstance led to incorrect suggestions about the likely origin of the spearhead superficial crust. The presence of a sulfide layer on the spearhead and the absence of sulfide in significant amounts on other artifacts from the burial were interpreted as a result of finishing the spearhead surface. CONCLUSIONS Based on the electron microscopy data, the chemical composition of the main metal of the spearhead was identified as a copper‒arsenic alloy (about 95 wt % Cu and 5 wt % As). An X-ray phase analysis of a crust sample showed that the spearhead is coated mainly by a layer of copper sulfide chalcocite with composition
and structure completely identical to those of natural mineral. An analysis of samples of similar dark crust from check samples (other artifacts from mounds near Novosvobodnaya village) showed similarity of their elemental compositions, with dominance of copper and sulfur. A study of the crust cleavage profile revealed a layer-by-layer distribution of corrosion products between the main-metal surface and the outer dark layers. As a consequence it was found that (i) the spearhead metal was not significantly destroyed as a result of corrosion processes and the artifact retained an undistorted profile and (ii) the sulfide layer was not in direct contact with the metal but coated the layer formed by corrosion products. The spearhead metal is surrounded by a cuprite layer, on which copper‒arsenic alloy corrosion products can clearly be detected. It is followed by another cuprite layer, directly linked with the crust layer. The sequence of copper corrosion layers is retained undistorted, and the crust quality and thickness are independent of the direct contact with metal. Therefore, the sulfide layer originated from one of the corrosion layers rather than from the main metal of the spearhead. Thus, the under study copper‒arsenic artifacts were found to exist under very peculiar conditions: deep burial in soil, the absence of soil in the funeral chamber, anaerobic environment, and the abundance of hydrogen sulfide from decaying organic materials formed a peculiar medium in the tomb, which determined the development of corrosion layer on the artifact surface. As a result of constant contact with hydrogen sulfide, the corrosion products on the surface of copper‒arsenic artifacts were replaced with mineral copper sulfides and thus formed in a natural way a dark dense coating with lead blaze on the artifact surface. ACKNOWLEDGMENTS This study was supported by the Russian Science Foundation, project no. 17-18-01399. Measurements of X-ray diffraction were performed at the unique sci-
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entific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007).
8. 9.
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Translated by Yu. Sin’kov
