While ancient cylinder seals have been studied and reported in great detail, especially as to their iconography, there have been relatively few reports on their method of manufacture. There are none dealing with the microscopic examination of seals. Here we shall describe such a study using the scanning electron microscope (SEM).
Cylinder seals (Fig. 1) originated in ancient Mesopotamia at the end of the fourth millennium. Their arrival at the wane of the Neolithic period coincided with several other important developments such as agricultural surplus, increasing trade, urbanization, the invention of bronze and cuneiform writing.
Essential to the political economy of the region spreading with trade in all directions, seals, for several millennia, were used as a method of designating signature, private property, ownership and authority. Seals were rolled on clay tablets (Fig. 2) on which contracts or receipts were written, or on balls of clay called bullae which were wrapped around rope to secure vessels. Seals were usually made from different kinds of stones and were often engraved with scenes of religious ritual and myth (Fig. 1). In most instances, the cylinder seals were bored through the center (Fig. 3) so that they could be worn as amulets. Stamp seals which had preceded cylinder seals and were also made after the cylindrical form had been discontinued, circa 330 B.C., were similarly bored through.
This report has several purposes:
- To describe the use of the SEM for the examination of ancient stamp and cylinder seals.
- To describe new techniques and materials used to overcome the difficulty in examining tool marks after the surface had been polished.
- To describe the tool marks found on both polished and unpolished surfaces.
- To describe and compare the tool marks on genuine and fake seals.
- To describe tool marks produced experimentally.
There seems to be a consensus amongst scholars of seals and seal manufacture that the actual cutting was not done by drills or gravers alone, but by the continued application of an abrasive, possibly sand or emery, which was rubbed into the stone by the tool.
This is given credence by Kenoyer (1977) who investigated bangle manufacturing from Muretrix shell in ancient Balakat, a Harrapan site, circa 2000 B.C. In recreating the stages of manufacture, Kenoyer concluded that wet sand was continuously replenished through out all the grinding steps.
Only two reports describe seal making tools found together with finished and unfinished seals, although tens of thousands of stamp and cylinder seals have been excavated—including those in the Egyptian and Aegean civilizations. The first is Frankfort’s report of a cache of bronze tools including borers, engravers and chisels in an excavation in Tell Asmar from the Akkad period circa 2300 B.C. In the second report, as yet unpublished, Boardman described bronze tools in the shape of files, burins, and polishers found at Mallia in Crete near the palace belonging to MMI—MMII, circa 1700 B.C. Excavating in Egypt, Petrie found no seal-making tools.
Because there are no written records describing ancient cylinder seal manufacture and because of the scarcity of tools excavated, the methods used to form, engrave and bore through have been, of necessity, inferred. For example, an ancient Egyptian wall painting shows beads being made using a bow drill (Fig. 4). Bow drills are also shown on ancient Greek vases and on a gravestone belonging to a gem cutter. Deductions about ancient engraving methods have been made from the earliest written records during the Greek and Roman period. Herodotus wrote that a stone was used for seal cutting that was also used for arrowheads. This might have been flint or obsidian.
Theophrastus, in the fourth century B.C. observed that differences exist in stones, and while some resist an iron graver, they may be engraved by means of other stones.
Pliny, around A.D. 70, refers to iron tools set with diamond splinters and other ostracias, and to the use of files. He mentioned the use of naxian, thought to be emery from Naxos, for polishing.
Because prior experience with the Scanning Electron Microscope (SEM) had shown that it was uniquely suited for examination of surface topography of irregular surfaces, it was felt that it could also be useful in the examination of ancient seals.
The SEM was developed in the 1930’s and commercialized in the mid 1950’s. While the SEM has had wide use in ultrastructural investigations in various aspects of science, its application to the study of artifacts has been relatively limited and never before on seals. Reidar F. Sognnaes, a researcher in SEM, in 1972 described its use to examine ivories, and the Greek archaeologist, Spyridon Marinatos. in 1972, to examine pottery glazes. The advantages of the SEM in the study of certain artifacts are that:
- It provides a greater depth of field than either light optical or the transmission electron microscope and is therefore especially suited to examine abraded surfaces. These can be examined directly or indirectly.
- It is more practical for scanning large and irregular surfaces than the transmission electron microscope which is better suited for flat surfaces with a more limited field, and only by the time consuming preparation of thin film, shadowed replicas. The SEM has a resolution of 200 Angstrom and a wide range of magnification and ease of operation.
- It permits a composite reconstruction of large, uneven surfaces making the subject easier to read and interpret.
- It is harmless to the object being studied.
Because a tool mark, once polished, becomes difficult to examine, it was decided to concentrate on the bore of the seal, which was very likely an unpolished surface. The size and geometric configuration of the central bore pose special problems for direct visual examination by any means including fibre optics. Although X-rays (Gorelick, 1975) have been used to examine the outline of the bore, they do not elucidate the fine details of the internal topography.
Some seals are made from materials which are nonconductors of electricity and this presents problems for the direct SEM examination. Coating with conductive metals would deface the artifact. However, the application of silicone impression techniques used in dentistry makes it possible to provide accurate, duplicated details of both the bore and the surface characteristics of the seals. Since it was determined that the impression could be examined directly or a more durable plastic cast made, both methods were used after coating with metal for examination in the SEM.
Three groups were established to include four genuine seals; four fake seals; and an experimental group consisting of uncut stones:
- Genuine Seals: The genuine seals (Gorelick Collection, Brooklyn Museum Cat. M. Noveck, 1970) varied in the type of stone and the period of manufacture. They were (a) a steatite stamp seal from the Jemdet Nasr period ca. 3000 B.C., (b) a hematite cylinder seal Provincial Old Babylonian ca. 2000 B.C., (c) a chalcedony cylinder seal from the Achaemenad period ca. 500 B.C., (d) an agate stamp seal from the Sassanian period ca. A.D. 500.
- Fake Seals: The fake seals (Metropolitan Museum Study Collection] were one carnelian, one hematite and two steatite.
- The Experimental Group: This group consisted of uncut stones made of steatite, serpentine and hematite. In the study these were cut with a variety of contemporary tools such as diamond stones, green corundum stones, round and spiral steel burs and coarse pumice. Motorized and hand rotary instruments were used to create bore forms.
For known reasons of accuracy and stability two commercially available silicone impression materials were used. These were citricon and xantopren blue. Supplied in kit form, a measured amount of material was dispensed from a tube onto a graduated mixing pad and a measured amount of catalyst added. Following thorough spatulation, a portion of the mix was placed directly in the seal and allowed to polymerize. Placement in the bore was achieved using a wood and/or metal support whose diameter was approximately half that of the seal bore. A slight rotational movement spread the impression material into the bore and onto its walls.
Working time is approximately one to two minutes. The bore is slightly over-filled at one end and the material allowed to set with the support in place. The set impression may be removed after ten minutes by grasping the support and excess material and applying a steady pull pressure. Care should be exercised to avoid tearing. When resistance is felt, the support may be removed separately, allowing temporary compression of the material into the space and away from the bore of the seal. No permanent distortion occurs due to the elastic recovery of the impression material. Small seal bores (up to 1.0 min. diameter) coupled with very irregular, marked undercuts may pose difficulties, It is to be preferred that the impression either be examined immediately or a casting made. However, impressions may be stored in dust proof containers for a few weeks without significant deterioration.
The entire bore and all or any part of a seal may be duplicated by making a cast from the impression. Several synthetic resin casting materials are available, many of which cure at room temperature. Previous experience led to the choice of stycast #1266 resin. Mixed according to the manufacturer’s directions the resin is poured into aluminum foil “boxes” fabricated to the size of the individual impression. It was customary to pour the mold of the bore in two stages. In the first the impression “floats” in a half filled box. After initial set, more resin is added to cover the impression and set aside to polymerize and harden. This was complete in 12 hours after which the casting was removed from the “base” and the impression material withdrawn from the casting. The casting of the bore may be laid open longitudinally by sectioning with a rotating, water cooled, thin diamond wheel. Unnecessary resin bulk was trimmed away at this time.
Since the impression material and casting resin are non-conductors they must be coated with a thin conductive film of metal to facilitate SEM examination. Mounted on standard, metal supports using conductive cement, the impression or trimmed casting was placed in a metal coating machine. Under vacuum this machine lays down a few hundred Angstroms of metal, e.g., gold/palladium, over the substrate. The coated samples were then examined in an A.M.R. 1000 scanning electron microscope and photographic recordings of salient characteristics made on Polaroid type 52 film. Since the bores and size of the seals often exceed the frame of examination (lowest magnification] in the SEM, it was necessary to construct photographic composites to provide a complete picture of the seal’s characteristics.
The information derived by this replication technique is only as accurate as the ability of the impression material and/or the casting to faithfully reproduce fine detail present in the original seal. A simple experiment was conducted to test the accuracy of our method.
Into a piece each of hematite and steatite several shallow holes, in a globe form, were cut using steel, corundum and diamond burs in an electric belt driven engine. A casting was made from an impression following which both casting and recovered impression were coated for SEM examination. The engraving marks left by the tools in the original stones were examined directly in the SEM and then compared with those reproduced in the impression and the casting. Figure 5, A, B and C, attests to the accuracy with which fine abrasion or engraving anomalies in the original stone (Fig. 5A) are reproduced first in the impression (Fig. 5B) and then in the casting (Fig. 5C).
It is to be concluded that this simple technique will provide accurate information while preserving the integrity of the original artifact. It was also determined that the impression itself could be coated and examined without the need for a stycast model.
While the model is a durable record, they are equally accurate.
Both impressions and castings of the bores of genuine seals showed evidence of tool or abrasive slurry marks left during manufacture. Three of the four bores examined were tapered toward the center. Figure 6A shows the bore of a steatite stamp seal of the Jemdet Nasr period. The bore is somewhat uniform with predominant, concentric rings or grooves. The spacing and size of the grooves is variable and more pronounced in some parts of the bore than others. In this particular bore there are relatively deep longitudinal grooves at one end which may be unrelated to fabrication.
Figure 6B shows the bore of a hematite cylinder seal, Provincial Old Babylonian, with an irregular profile toward the center of the seal. Concentric abrasion anomalies were present principally in the outer third of the bore while the central portion appeared smooth.
Figure 6C shows the bore of a chalcedony cylinder seal from the Achaemenad period.
It has an irregular geometric outline with two-thirds appearing tapered and the remainder relatively cylindrical. While evidence of concentric abrasion marks exists in the tapered portion, the shorter cylindrical portion has numerous relatively parallel, concentric rings or grooves of varying width and depth.
Figure 6D shows the bore of an agate stamp seal from the Sassanian period in which shallow concentric grooves can be seen with occasional deeper grooves as well as smooth areas. The over-all shape of the bore is relatively straight.
Fake seals showed a similarity between their bores and differences from the bores of authentic seals. Two of the four seal bores were tapered toward the center with one end barely meeting the other at the center of the hematite seal. The hematite seal (Fig. 7A) showed conspicuous abrasion anomalies. Toward the center of the seal these were concentric, though longitudinal marks predominated toward both outer halves of the bore. In contrast, the bore of the carnelian seal exhibited a slightly rough texture (Fig. 7B) without abrasion marks. Of the steatite seals, one (Fig. 7C) was very smooth, the other (Fig. 7D) rough.
The engraving marks on the experimental group left by contemporary motorized tools such as rotary steel, diamond and corundum were significantly different from those on both fake and authentic seal bores. While the size of the abrasion markings varied according to the size of the abrasive particle or the shape of the tool’s cutting edge, a multitude of concentric closely packed abrasion marks were evident (see Figure 5A). This was found to be true for both the hematite and the relatively softer steatite and serpentine stones. Attempts to drill bores with steel twist drills and a hand brace were successful only in soft stones such as steatite. Concentric cutting marks were left when a coarse pumice slurry and a rod under hand counter-oscillation were used (Fig. 8) to enlarge bores cut with a twist drill. These resembled the concentric markings on the genuine seals.
During the process of work, tools eventually wear and manifestations of the pattern of wear usually appear in the material upon which they are used.
Other striking examples of possible tool wear can be seen in the globe forms (Fig. 11) representing Pleiades (see Fig. 1). Such forms may possibly have been prepared with a small rounded tool showing random patterns in which the globe is either continuous and smooth, flat, dimpled or nipple-shaped. If the globe forms were cut sequentially it may be postulated that the change in pattern may be a consequence of wear through continued use of the same tool. Concentric abrasion anomalies are also seen which appear less marked on the seal surface than in the bore because the engraved surface is polished.
The wear profiles of a rounded tool are shown diagrammatically in Fig. 12. Similar matching profiles were seen in contemporary corundum and diamond stones and steel burs used in the experimental group.
It is to be concluded that:
- Bores and other parts of seals can be copied with extreme accuracy using silicone impressions and acrylic castings without damage to the seals.
- Scanning microscopy is an ideal method for observing and recording fine topographical detail reproduced in the impression or casting.
- Since the bores of the genuine seals showed notable similarities and since the time span of these seals ranged from 3000 B.C. to about A.D. 500, it may well be that the method of bore manufacture remained essentially the same. A more comprehensive study to include different periods and regions would be necessary for further verification.
- Although the sample was limited, notable differences were observed between the abrasion characteristics of the genuine and fake seals. (Compare Figures 6A-B, and 7A-B). However, the limited number of specimens examined suggests the need for a more comprehensive study of both genuine and fake seals. This should include both the engraved surfaces and the bores.
- Experimentation on an uncut steatite stone, using an abrasive slurry of coarse pumice, produced a finding consistent with the previously cited observation that the continued application of a slurry was the primary method of abrasion in ancient times. Further experimentation using other stones and other abrasive slurries should be additionally enlightening.
- Differences found in the globe forms of the Pleiades may be explained by the wear of the tool used in their manufacture and the degree to which they were polished. If the observations noted in paragraphs 3 and 4 are borne out by SEM studies of a large number of seals, both genuine and fake, we will have an additional method for determining the authenticity of seals for which there is no excavation history.