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Mike Brass. 1999 essay. The chemical composition of glass in Ancient Egypt

Introduction

It was only during the time of the Romans that glass became common place in the Mediterranean world. The people of the preceding periods considered its function to be decorative rather than utilitarian. Glass in the ancient world usually appears in the form of semi-precious stones made from materials as various as turquoise (pale blue glass) and fluorite (purple glass) (Freestone 1991). The precious quality of glass is captured in references from Mesopotamian cuneiform texts to "artificial lapis lazuli"; lapis lazuli is a gemstone that originated in Afghanistan and was traded as far afield as Ancient Egypt.

Glass in the ancient world was manufactured by melting a combination of an alkali (potash or soda) and silica (raw materials such as quartz cobbles and sand). The interaction of the heated soda and the hot sand would have formed a transparent flowing liquid that the ancients then permitted to cool to form glass (Freestone 1991). It was the ancient production of metallurgy and faience that are currently believed to have resulted in the later manufacture of glass. The Bronze Age of the Mediterranean was synonymous with vast quantities of differential metallurgical processes. The slag by-product of such workings was a glassy-like material. The ancient beads that have been analysed shown to be composed of a high percentage of such by-products back up this hypothesis. Faience consists predominantly of crushed quartz and finished off with an alkaline glaze into a ceramic body (Freestone 1991).

Glass is a non-crystalline material that is, in essence, a super cooled liquid and not a solid. It is characterised as such because of its ability to liquefy "at a much lower temperature than that required to manufacture it… [Its] rigid metastable solid [is] produced by cooling the liquid form rapidly enough to prevent crystallisation, the stiffening occurring predominantly at the glass temperature. It is characterised by an arrangement of atoms or molecules which is irregular, and thus contrasts with crystalline order… The art of glassmaking combines two distinct, independently evolving, technologies, the development of pneumatically drafted furnaces and the invention of glazes. Technically, faience, glass and vitric ceramic ware are related, in the high temperatures are necessary for their manufacture, similar raw materials are involved and all are vitreous to varying degrees." (Saitowitz 1996)

Until relatively recent times, the alkali component of the glass as well as part of the sand would be preheated and fused together before they were combined with the final components. Therefore, glassmaking consisted of two distinct stages: the raw materials were first fritted and then the melting occurred. The initial fritting process expunged unnecessary gasses and helped the subsequent melting. Commonly scrap glass was incorporated into the raw material mix with the aim of accelerating the fusion (Saitowitz 1996).

The production of glass requires several pre-requisite factors: a pneumatically drafted furnace with the ability to produce concentrated heat of between 900 - 1 000 degrees centigrade; the temperature reduced inside the furnace to that required for vitrification by means of the introduction of an alkaline flux; "a first firing of the mixture of granulated silicate and raw materials resulting in the production of a frit at a temperature of about 750 [degrees centigrade;] a second firing at a higher temperature of about 1 000 [degrees centigrade]. This firing requires sustained temperatures over lengthy periods of time. Complete vitrification can take many days to achieve; in order to speed up the vitrification process, cullet is added to the batch. Cullet acts as a catalyst in the process of liquefaction into a homogeneous mass." (Saitowitz 1996)

Reports of small glass beads and pendants have been made from sites that date to the mid-third millennium from the Near East. These are amongst the earliest known works of glass making and utilised lapidary techniques in their cutting and grinding in the cold state (Freestone 1991). Glass is not convincingly attested in Ancient Egypt before the late Middle Kingdom and is comparatively rare in the archaeological record before c. 1 550 (the beginning of the New Kingdom with the Eighteenth Dynasty) (Lilyquist & Brill 1993). It has been proposed that the craft of glass making was first introduced full-scale into Egypt by glass makers captured by Thutmose II (1 479 - 1 425 BC) from the state of Mitanni (situated between the Tigris and Euphrates river, it developed into a powerful state some time before 1 500 BC and became one of Egypt's most powerful enemies), where the technology was readily available, on one of his numerous campaigns. This hypothesis is backed by the tribute lists dating to this period, the Annals of Thutmose II at Karnak, which list glass as one of the materials (Nicholson 1993). By extension, glass was then held in high esteem with great significance of some kind attached to it. This importance was still great enough a century later by the rule of Akhenaten (1 352 - 1 336 BC) to warrant its inclusion in official diplomatic correspondence. These are the famous Amarna Letters, in which the words ehlipakku and mekku are written (Shaw & Nicholson 1995 [5]). These are Hurrian and Akkadian terms respectively, and their importation into the Ancient Egyptian language is a possible indicator as to the eastern origin of the earliest glass.

It is this admittedly hypothetical scenario of an importation of craftsmen from abroad that is believed to account for the distinct absence of any surviving instances of trial stages in the making of glass in Ancient Egypt (Shaw and Nicholson 1995). Instead, glass making emerges as a fully-fledged industry. Accordingly, technologically arduous compositions, such as clear decolorised glass, are known from as early as the reign of Hatshepsut (1 473 - 1 458 BC) and the colourless glass inlays imbedded in the throne of Tutankhamun (1 336 - 1 327 BC).

Apart from its utilisation in amulets, beads and inlays, attempts were made to use glass in more ambitious projects, including that of vessels. The technique of vessel manufacture by means of blowing was only introduced into Egypt during the Roman era. The other means was core-forming: a handling rod would be used around which the craftsman would form and shape a core of mud and sand in the shape of the vessel's interior (Shaw & Nicholson 1995). The next step would be for the core to be submerged into the viscous molten glass (an alternative method would be for the glass to be trailed over it) and leveled by means of rolling the whole on a flat stone called marver. Pincers would then be used to shape the feet and rims of the vessels. It was usually from here that the process became more complicated. The base colour of the vessel was usually either blue or blue-green. To this colour threads were supplemented so that the end effect was strands of yellow, white, red, etc., decorating the piece. These threads were sometimes pulled with a needle in order to form swag or feather patterns and then they were rolled on the marver to imprint them into the body of the glass that was still soft and therefore impressionable (Nicholson 1993).

The vessel was then placed into an oven where it was allowed to slowly cool. This is a processing termed annealing (Shaw & Nicholson 1995). Annealing permits the gradual release of stresses that had developed within the glass. When the glass had cooled, the core would then be broken up and extracted through the opening of the vessel. There was frequent difficulty in removing the core in its entirety, specifically in the shoulders of the narrow-necked vessels (Lilyquist & Brill 1993). This led to the remnants of the core often adding to the opacity of these pieces, while those that had broader necks often appear more translucent.

Glass could also be molded. In its simplest form, this involved the fashioning of clear glass forms. However, it could also be a more complicated process involving sections of different coloured glass cane being fused together in a mould to make multi-coloured vessels (Shaw & Nicholson 1995). Examples of these would include yellow eyes on a green background, or the conglomerate glass pieces with angular fragments of many colours fused into bowls.

Glass was also worked by means of cold cutting in Ancient Egypt. This process involved the working of lumps of glass, sometimes roughly molded into the desired shape, as though they were pieces of stone and thereafter shaped by means of carving (Shaw & Nicholson 1995). This extremely difficult process, that required enormous amounts of skill and practice, were the means by which some fine pieces (including two headrests of Tutankhamun) were crafted.

The available evidence suggests that in Ancient Egypt glass was hailed, or at least regarded, as an artificial semi-precious stone and, like such stones, sometimes is imitated in painted wood. It has been proposed that it is because of this connection that glass never developed forms of its own in Ancient Egypt but rather copied those traditionally made in stone, faience or other materials (Nicholson 1993). For the Ancient Egyptians of the New Kingdom glass was a costly novelty material, most likely under the control of the royalty and given as presents to the favoured officials. Until comparatively recent times the line of thought was that the production of glass declined after the 21st Dynasty (1 069 - 945 BC) and was not revived on any scale until the 26th dynasty (664 - 525 BC). However, glass making continued on a much reduced scale. Down in the Greek (Ptolemaic) era the center of glass craftsmanship was the famous city of Alexandria with the manufacture of core-formed vessels and, in Roman times, of items of cameo glass that likely included the famous Portland Vase stored in the British Museum.

Glass, because of the extensive palette of colours available for utilising in its manufacture, cannot therefore be interpreted by either its colours or its degree of transparency. Modern scientists classify it based on its atomic composition. The similarity of its atom arrangement to that of molten liquid is the same. This occurs through the strong chemical bond that binds the atoms and the stiffness prevents atomic re-adjustment during the cooling process (Freestone 1991). Therefore, it is the liquid structure of glass that is liable for many of its distinguishing characteristics.

The chemical composition of glass

The primary constituent making up the chemical composition of glass is silica (silicon dioxide), which is the most common component of the earth's crust and accounts for 50-70% of the weight of ancient glass (Freestone 1991). The silica is extracted from raw materials as freely available as quartz sand, white quartz pebbles and flint. The metallurgical furnaces and pottery kilns utilised in ancient times were not, however, capable of heating crushed quartz pebbles to the temperature required for them to melt. Therefore a flux was introduced and combined with the silica (Freestone 1991). This enabled the lowering of the melting temperature and the resultant manufacturing of glass. It has been estimated that the introduction of 20% flux would lead to the reduction of the melting temperature of quartz by as much as over 700 degrees, from about 1700 degrees centigrade to below 1000 degrees centigrade (Freestone 1991).

Alkalis were utilised in ancient times as flux. These were usually soda (sodium oxide) and potash (potassium oxide). The alkalis were gleaned either from naturally occurring minerals or from the ashes left behind by burnt plant material or wood. Nevertheless, a combination of silica and pure soda/potash would not have resulted in high quality glass (Freestone 1991). A stabiliser was needed which would lessen the solubility of the glass and prevent it corroding within a relatively short timeframe. The ancient stabiliser was more often than not lime (calcium oxide). It is questionable whether the calcium oxide was a deliberate inclusive on the part of the ancient glassmakers or whether its occurrence was accidental, either in conjunction with the silica or with the alkali, and the glass manufacturers were unaware of it as a separate component (Freestone 1991). Lime is chemical closely related to magnesia.

It is noticeable that the chemical compositions of ancient glass are relatively similar to those of the modern era, with the medieval period providing an exception. The compositions are soda-lime-silica glasses, with sodium oxide providing the flux and calcium oxide the stabiliser. Therefore, by the time of the second millennium BC, it appears that a glass formulation with a formula relatively similar to that of modern glass had been obtained and utilised with widespread familiarity.

The major chemical constituents of glass are: silica or sand (SiO2), sodium oxide (Na2O) or potassium oxide (K2O) as a fluxing or alkali agent that reduces the melting temperature of silica, and calcium oxide (CaO) from lime. Impurities within the silica include: alumina (A12O3), copper (Cu), iron (Fe2O3) and magnesium (MgO). It is likely that the fluxing agents cited above also contained traces of chlorite, phosphoric oxide and sulphate (Saitowitz 1996).

The predominant types of early glasses were soda-lime-silica and potash-lime-silica glasses. The raw materials utilised by the ancient glassmakers would first have been put through cleansing processes before they were put to use. These processes would have most probably included forms of screening, washing and burning for extraneous coarse particles, organic matter and other impurities to be expunged (Saitowitz 1996). This would have been particularly true of sand. As the ancients lacked the modern technological methods and available synthetic materials, the end result was a product which only at times was not quite as refined as those that have been obtained in the modern era. In particular, unsightly spots and other visual faults were caused by the zircon, ilmenite and rutile heavy minerals not having melted in the glassy matrix (Saitowitz 1996).

Of the differing raw materials required for glass making, sand is the most available and the least expensive. There is no fixed amount of sand that is needed during preparation, as the relatively wide limits do not lend themselves to lessening the quality of the glass produced (Saitowitz 1996). The exact origin locations of the sands that were used in the various parts of the ancient world are currently unknown, due to the lack of the necessary analytical studies and to the lack of mention in the ancient records. If it were not for the mentions of Pliny and Strabo, almost nothing would exist from the ancient accounts. Pliny singles out the sands of the Belus River on the Palestinian coast (stretching from Acre to Tyre) and Strabo the sands to north-west of the ancient Roman harbours of Pozzuali and Naples as raw material source places (Tatton-Brown & Andrews 1991).

The alkali flux soda (Na2O, NaHCO3, and Na2CO3) is one of the prominent ingredients of glass. Some soda glasses have been found to contain as much as 23% Na2O, but this content makes them vulnerable to deterioration through weathering. This impure sodium carbonate and bicarbonate form of alkali can be found in Egypt at Wadi Natrun and El Kab (Saitowitz 1996). The lakes at Wadi Natrun overlie a complex series of geological formations. Somewhat sweet water can be found in a couple of the lakes, with others containing predominantly sodium sulphate, carbonate or chloride. Lakes containing sodium carbonate would have been a natural and readily available source for glass making. Other sources of sodium carbonate were available, however: evaporates from dried seas, soils which leached salt deposits, and the salts given off by specific plants when subjected to burning (Saitowitz 1996). The Chinane (known locally in Syria as the Keli) was incredibly rich in sodium carbonate in its ash remains.

Potassium oxide (K2O) can replace sodium oxide as the flux in glass, resulting in a greater level of brilliance as well as a superior colour. The resultant glasses posses a higher melting point, and is solid and more enduring. The necessary potassium compounds are extracted from plant and wood ashes. The New Kingdoms sites of Thebes and Tel el-Amarna show significant traces of potassium oxide, in contrast to the virtual lack in glasses from Alexandria. This supports the above suggestion from the soda flux that more than one source of alkali was available to the Ancient Egyptians and utilised by them.

Calcium (lime - CaO) acts as a stabiliser to glass, allowing it to harden more rapidly during the cooling process. By making the glass more durable it has a side effect: it makes the glass more resistant to water penetration. Much limestone is derived from a dolomitic variety and contains a variable amount of MgO with the CaO. These are often present in equal ratios. As MgO is a common constituent of Egyptian glasses, it has been hypothesised that the composition of the sand may be responsible for its presence in the glass. Most of the sand of Egypt's northern coast contains calcium carbonate as an impurity, a factor that could explain a variance occurring naturally, rather than by the intentional addition of lime to the batch. Calcium derivatives commonly occur in nature as calcium oxide or lime. Calcium carbonate (CaCO3), for example, is present in sea shells, limestone and chalk. Low lead-soda-lime-silica compositions contain up to 8% CaO while lead glasses usually consist of 2.5% (Saitowitz 1996).

From the beginning of the Roman period, starting around the seventh century BC in Italy, a new second kind of glass began spreading throughout the Mediterranean world. This glass has been termed "low-magnesia glass", after its percentage of both the magnesia and potassium oxide components is less than 1% (Freestone 1991). The higher counts of magnesia and potassium oxide in the glasses from the previous millennia were the result of the utilisation in the manufacturing process of plant ash. Plant ash is not composed of pure soda. Roman glass-makers, however, achieved a result of low impurities through the application of high-quality soda or natron. The site of Wadi Natrun, an oasis that featured natural salt lakes, is located in Egypt's Western Desert. From here natron was distributed through the Mediterranean world. Indeed, it become the main depository for the Mediterranean, and thereby comprehensive and extensive trade networks are implied (Freestone 1991).

Ancient Egyptian New Kingdom glass

The dark amethyst-coloured glass of the Eighteenth (1 550 - 1069 BC) and Twentieth dynasties (1 186 - 1069 BC) owe their colour to a manganese compound, of which 0.5 - 0.7% has been calculated as oxide (Lucas & Harris 1962 [7]). It is interesting to note that white glass of ordinary quality containing manganese compounds becomes coloured if it is exposed to strong sunlight for a certain period of time. The resultant colour varies from a light amethyst tint to a deep purple colour, and it is a matter of common observation in Egypt even today to find in the desert, in the vacinity of towns, pieces of were white glass coloured in this way. This colouration is the result of the manganese compounds within the glass having undergone some chemical change, which is apparently brought about by sunlight and is not caused either by way of heat or radio-activity, although the latter produces a similar colouration as well (Lucas & Harris 1962). However, the conclusion cannot be reached that the colour of ancient amethyst glass has been caused by exposure or that it is other than original.

The colouring of the Ancient Egyptian black glass was caused by three compounds varying in proportion to each other and thereby in visual effect: copper and manganese, and iron (Lucas & Harris 1962). Although black glass was certainly manufactured in Egypt at a late date, the early black glass (which, amongst others, was used to manufacture beads) was due to the use of impure materials that contained, for instance, a large proportion of iron compounds (Lucas & Harris 1962).

The blue glasses of the Ancient Egyptians were primarily three shades: dark blues, which imitated lapis lazuli with often a great deal of success, light blues that imitated turquoise, and greenish blue that imitated both felspar and turquoise (Lucas & Harris 1962). In modern Egypt, a cobalt compound is used for colouring blue glass. Yet, cobalt only produces a dark blue colour. Consequently, the turquoise blue and the greenish-blue of some of the Ancient Egyptian glass cannot be the result of its use.

Many of the blue glass specimens from the Eighteenth and Twentieth dynasties owe their colouring to a copper compound. There is one specimen, though, from the tomb of Tutankhamun that was coloured by a cobalt compound (Lucas & Harris 1962). On rare occasions it has been found that the colouring was caused by an iron compound. Pieces have also been found from the eighth to the sixth century BC which contain both copper and cobalt. In the majority of the blue glasses that have been excavated and examined, the colour, whether the result of copper or cobalt, was modified by the presence of manganese (Lucas & Harris 1962).

The presence of cobalt in Ancient Egyptian glass at an early a date as the eighteenth Dynasty is of considerable importance. Cobalt compounds do not appear in Egypt except as traces in other minerals (Lucas & Harris 1962). Their presence in the glass points towards the Ancient Egyptian glassmakers from that era being in contact with glassmakers elsewhere who were utilising this material.

The green glass derives its colour either from compounds of copper or of iron. The colouration of the modern green bottle-glass is, for example, produced by the latter method. By contrast, the Ancient Egyptians of the eighteenth and Twentieth dynasties utilised a copper compound (Lucas & Harris 1962).

The red oxide of copper causes the colour of the Ancient Egyptian Eighteenth Dynasty red glass, as is evident from the green coating that forms on the surface when the glass decays (Lucas & Harris 1962).

The Nile limestone bluffs stretch from Luxor to just beneath Cairo. The sands bordering these bluffs therefore contain a high calcium (lime) content, which would have affected different glass-making centers situated in this region of the Nile, e.g. Fustat and Tel el-Amarna (Saitowitz 1996). The techniques for glazing of such stones as quartz and steatite, and the production of faience, had been known to the Ancient Egyptians since Predynastic times (c. 5 500 - 3 100 BC). Flinders Petrie discovered glass waste during his excavations at Tel el-Amarna, which was the capital of Ancient Egypt during the reign of the Pharaoh Akhenaten (Shaw & Nicholson 1995). This, one of the earliest examples of a substantial glassmaking industry, consisted of plain open-hearth furnaces together with small crucibles. Newer excavations during the course of the early 1990s have produced new evidence based primarily on the detailed study of kilns. These studies increasingly consider it likely that glass making was carried on alongside faience production and possibly other pyrotechnical crafts (Shaw & Nicholson 1995). Apart from El-Amarna, there are other glass working sites at El-Lisht and Malkata.

The glassy matrix

The site of Mendes in the Nile Delta was occupied in ancient times right from Predynastic times (from the fourth millennium BC onwards) down into the occupation first by the Greeks and then by the Romans (332 BC - 395 AD). The span of Mendes currently measures 1.5 square km, and its size has been reduced considerably through the devastating encroachment of agricultural fields. Mendes is also known in archaeological circles as Tell er-Ruba'a. Outside the remains of Mendes' eastern enclosure wall is the related site of Kom el-Adhem at a distance of about 100m. The literal translation of Kom el-Adhem is the "hill of bones", and it lives up to its name for it is here that animal teeth and bone are to be seen trapped within a glassy matrix in large quantities. Also relatively nearby Kom el-Adhem is the site of Thmuis. The occupational, religious and social connections and dynamics between these three sites are not completely certain. It is hypothesised, though, that Kom el-Adhem served as a harbour in its early history and later a religious function in the guise of a mortuary complex during the Graeco-Roman period. The latter assumption is based on burials excavated at the site dating to this period.

The mass aggregate of teeth and bones are found approximately 50 to 100m downslope of the burials, with sections exposed through the effects of weather erosion. One exposed area covers about 40m in width, with some blocks averaging between 0.5m in width and 1m in length, and it was from this aggregate that Magee, Wayman and Lovell (1996) extracted samples for testing its chemical and microstructural composition to determine both the processes of its formation and its resulting origins.

Volcanic activity is one common natural phenomenon that produces glassy residues as a by-product. However, on archaeological sites it has been more generally from metallurgical procedures as is known as slag (Magee et al. 1996). Slag is essentially the extraneous material that has been extracted from an ore product during the smelting process. Yet, this is not the only role played by industrial slag. The potential exists during the process of smelting of an interaction between the surrounding environment and the molten metal being attended to; slag operates as a protective measure. Slag in the archaeological literature is also used in reference to residues from glass-making, brick and tile production, lime burning, pottery manufacture, cremation, furnace and hearth vitrified fuel ash, vegetable ash, cinder ash from cow dung (e.g. Southern India) and destruction slags from burnt vitrified forts. In archaeological terms, therefore, "slag" is used to designate a glassy mass either partly or fully liquefied that was caused by silica or silicates interacting with fluxing compounds at high temperatures (Magee et al. 1996).

In applying this terminology to the glassy matrix at Kom el-Adhem Magee, Wayman and Lovell (1996) note that the slag is on a flat surface of sand and positioned such that the likelihood of it having slid down from further up the mound slope is remote. A glassy crust covers the sediments that surround the slag. The sand at the point of intersection with the glassy crust was reddened. This indicates that the formation of the slag in situ most probably coincided with the burning of the sand in the immediate vacinity (Magee et al. 1996).

The first 1m of sand beneath the surrounding surface is barren. Then, however, there is a strata containing animal bones teeth and horns from overridingly sheep and goats, with some avian also; no remains identifiable as large animals, e.g. bovids and equids, are present. These faunal remains are fragmentary and therefore observation of the processes of articulation is minimal. Directly underneath the faunal remains are the remnants of a mudbrick structure in the form of a layer of deteriorated mudbrick, making the identification of architectural features hazardous (Magee et al. 1996). It is likely that this structure was originally located uphill to the northwest and collapsed downslope. These mudbrick remains primarily contained also small quantities of goat and sheep remains. The topographic stratas of the slag and the surrounding sand follow that of the mound: northwest to southeast.

Magee, Wayman and Lovell (1996) used stereobinocular optical microscopy, combined with unaided sight examination, to determine the physical characteristics of the slag that dominated. The samples were embedded in epoxy resin, ground with silicon carbide abrasive papers to 600 grit, and polished with 6 um diamond abrasive slurry and 0.05 um aluminum oxide slurry. These polished samples were examined with an incident light optical microscope and a Hitachi S-2700 scanning electron microscope (SEM) equipped with a Link eXL energy dispersive X-ray microanalysis (EDA) system. The aim was to identify and individually analyse its microstructural constituents. Utilising standard petrographic techniques, a thin section of the slag was examined after preparation. Slag samples were ground into powder for x-ray diffraction analysis and the matrix and bone particles were separated manually so that both constituents could be analysed separately (Magee et al. 1996).

The macroscopic examinations confirmed that the aggregate was a glassy matrix containing bone and teeth. The matrix is mostly black and visicular, ranging from 1 - 6mm in diameter with their interior comprised of a reddish tint (Magee et al. 1996). The bone colour fluctuates between yellow and yellow-red, which is within the normal range of colour. Bones that have been exposed to high temperatures show signs of cracking and checking, but these are not evident on the bones from the glassy matrix. However, the point of intersection between bone and the matrix reveals the bone displays colours ranging from blue to purple. A clear, polished substance coats the tips of bones or the bone fragments protruding from the matrix. Quartz grains are, together with other mineral grains, are implanted within the matrix (Magee et al. 1996).

The results of the SEM examination revealed the samples' microstructure to consist of a complex phase mixture (Magee et al. 1996).

The identification of the differing bright, grey and dark matrix phases was done by SEM-EDA analysis. The study involved utilizing semi-quantitative elemental analyses, converted to oxide percent and normalised. Analyses of this type are commonly used in the material sciences for phase checking. When they are checked regularly against analysed standards, as happened in this case, they are suitable for general comparison with the quantitative analyses of other materials reported in the scientific literature (Magee et al. 1996).

Energy dispersive microanalysis was conducted on three areas of the sample matrix. The results demonstrate that the matrix is a heterogeneous silicate that possesses aluminum, iron and potassium. Smaller traces of calcium, magnesium, sodium and titanium are also present (Magee et al. 1996). The microstructural elements, in their form as diverse particles composing the silicate matrix, were bone fragments, iron oxide, and silicates. The bone fragments were identified through their calcium and phosphorus components. To the left of the trapezoidal bone fragment is a tear drop-shaped constituent identified as a silica quartz particle. The lower left displays a circular dark feature that is a vesicle packed with the epoxy mounting material (Magee et al. 1996). The small, bright and angular grains are probably an iron-rich phase, tentatively identified as iron oxide, and which contains small traces of magnesium and titanium. The small and wispy silicate particles have the same chemical components as the matrix. Their appearance in the matrix points towards the possibility of small crystals having formed in the glassy matrix. The cooling of the matrix during its formation process could have caused this, which caused devitrification. The tiny spherical particles are rich in calcium and phosphorus. Accurate determination of their composition is precluded through the sub-micron size of these particles being smaller than the volume analysed by the SEM-EDA technique. Their measured compositions are, therefore, those of particles together with a notable but indeterminate contribution from the surrounding matrix. Their compositions strongly point toward derivation from bone material, possibly as immiscible droplets of a phosphate-rich liquid (Magee et al. 1996).

The x-ray diffraction results obtained from the matrix material were consistent with a large portion of the sample being amorphous. Also present were diffraction peaks from many crystalline phases. These were identified as quartz (SiO2), the iron oxides magnetite (Fe3O4) and hematite (Fe2O3), and a crystalline silicate of the feldspar type that most closely resemble anorthite. This concurs with the phase identifications determined by SEM-EDA and petrographically as described above, with the crystalline silicate possibly being a devritification product of the matrix (Magee et al. 1996).

The significant volume fraction of calcium phosphate particles and their varied morphologies, together with the colour changes observed macroscopically at the bone-matrix interfaces, suggest that some form of disintegration of the bone has occurred as a result of its reaction with the hot silicate matrix. Elemental microanalysis (SEM-EDA) was employed to determine whether the presence of the hot silicate matrix had caused compositional changes in the bone fragments. No such changes were detected. However, elevated chlorine levels were discovered in several fragments including a bone fragment which was projecting from, and not in contact with, the matrix. X-ray diffraction analysis of the powdered bone sample exposed the presence of a calcium chloride hydroxide and a calcium (iron, magnesium) carbonate phase (ankerite) in addition to the expected hydroxyapatite. The carbonate is most likely an environmental contaminant. The presence of the chloride, which is consistent with the high chlorine levels found by SEM-EDA, will be discussed further below (Magee et al. 1996).

Conclusion

The chemical composition of glassy residues resulting from craft production and mortuary practices are characteristic and can be compared to the glassy matrix in order to determine both its source and the manner of its formation. The slag resulting from metallurgy frequently has a glassy matrix composition that resembles anorthite but contains massive amounts of characteristic mineral inclusions, e.g. in both the smelting slags from copper and iron the iron oxide wustite (FeO) and/or magnetite (Fe3O4) are usually found in dendritic form (Magee et al. 1996). So too are the crystals of the iron silicate fayalite (Fe2SiO4) and other related minerals. It must also be noted that particles of metal like metallic copper or iron are also relatively prevalent.

Copper workings are found in Ancient Egypt dating to as early as the Terminal Predynastic (3 300 - 3 100 BC). Bronze makes an appearance during the late Middle Kingdom (2 040 - 1 782 BC). Iron smelting only became common place after the New Kingdom and its production never rivaled that of its Near Eastern neighbours (Shaw & Nicholson 1995). Metallurgical slags have been excavated from numerous ancient Egyptian sites. However, these occurrences are typically near ore sources where industrial operations occurred, like at the mines west of Thebes, rather than within settlement areas (Magee et al. 1996). The conclusion that can be drawn from the results of the chemical analysis, summarised also in comparative form in the tables, is that the composition and the microstructural constituents of the Mendes slag are incompatible with a metallurgical origin.

The high silica and low iron constituents of the glassy matrix bar the possibility of its origins being both melting and smelting slags; as do the distinct absence of iron oxides, fayalite and metallic particles (Magee et al. 1996). Mendes has not yielded so far any evidence for smelting or other metalworking processes like either furnaces or other metallurgical residues (Shaw & Nicholson 1995).

The ancient craft of glass making can also be identified from waste material and failed production in the archaeological record. Although small quantities of glass are present in some of the Predynastic sites, glass making is generally agreed to have begun large-scale and regular production at the onset of the New Kingdom (Shaw & Nicholson 1995). Sand (silica) and flux are the main raw materials utilised in glass making. The most important flux in the ancient Mediterranean world was natron (sodium carbonate/bicarbonate containing sodium chloride/sulphate impurities), which in Egypt was extracted from the Wadi el-Natrun. The comparison between the results of elemental analysis of glass from the Eighteenth to Twenty-Second dynasties (the New Kingdom) and from the Graeco-Roman era provides a strong case against the glassy matrix possessing a glass making origin. This is because the glass possess, on average, significantly higher quantities of sodium and calcium than potassium (i.e. they are soda-lime glasses); by contrast the glassy matrix has a high potassium content that was likely incorporated from vegetable material during slag formation, as will be discussed in more detail below. Logically the bone is highly unlikely to be representative of an intentional addition to a glassmaking process. This is particularly the case when it is considered that glassmaking additions are customarily ground to the size of a fine particle in order to speed dissolution (Magee et al. 1996). Moreover, the calcium phosphate (this mineral normally appears in bone as apatite crystals that are insoluble in molten glass) are an unwanted addition since it causes opalization in glass. The results obtained from the chemical experiments on the glassy matrix strongly indicate that the Mendes slag is unlikely to be the product of a glassmaking industry (Magee et al. 1996). This stands in contrast to the slag found at other sites such as Alexandria, Tel el-Amarna, el-Lisht, Tanis and Thebes.

Both pottery production and cremation also produce archaeologically recoverable slags, from kiln wastes and molten residues respectively. It is unlikely that the glassy matrix was the result of pottery production because of the lack of the characteristic terracotta moulds used for the manufacture of objects. The site was also not a crematorium because of the absence of human bones (Magee et al. 1996) and because although the surface of the bone is charred there is no evidence of the milky-grey colour or the "checkerboard" cracking that would be expected to result from the high temperatures of up to 1 000 degrees centigrade which are frequently reached during intentional cremation (Shipman et al. 1984). Additional arguments against these possibilities include the composition differences between the glassy matrix and contemporary glassware, outlined above, argue convincingly against heat-damaged vessels being the source of the residue; and the composition of the matrix is significantly greater in sodium, aluminum and potassium, and lower in calcium, than are cremation slags (Magee et al. 1996).

In addition to the slag-producing activities discussed above, glassy residues are also produced by accidental fires and purposefully destructive actions. Amongst the best known examples of these activities are the Celtic vitrified forts (Youngblood et al. 1978). Here the rocks fragments and boulders of Iron Age fortifications have been fused within a glassy vesicular matrix (Nisbet 1982). Volumes up to several cubic metres have been melted together from the firing of timber-laced stone walls. These happenings required temperatures between 900 - 1 000 degrees centigrade and were therefore most likely the result of burning by enemies. In essence, while the timbers burned and the walls collapsed, a furnace was thereby created capable of conceiving sufficient heat to vitrify the walls (Nisbet 1982).

There are enough similarities between the characteristics of the glassy matrix and the Celtic vitrified forts to suggest the possibility that the Mendes material results from the conflagration of a structure (Magee et al. 1996). Mudbrick was manufactured from the local fluvial clay-rich sediments, with a temper additive. In addition, it was the common construction material, both fired and unfired, in Ancient Egypt.

A sample of fired brick from the debris of a Late Period (747 - 332 BC) structure at Kom el-Adhem was examined for comparative purposes. This was done with SEM-EDA analysis and x-ray diffraction (Magee et al. 1996). The outside of the brick was buff coloured but the cross-section displayed a black band with parallel red bands, suggestive of firing. The results show that the brick is reasonably homogeneous in composition and therefore the banding can be attributed to different oxidation states of constituent elements (Magee et al. 1996). A piece of the brick was crushed and the x-ray diffraction analysis shows that quartz together with hematite and magnetite, representing the two oxidation states of iron, are among its constituent phases. Moreover, diffraction peaks were determined are most consistent with the presence of feldspar that most closely resembles anorthite (Magee et al. 1996). It is hypothesised that this anorthite may have been formed during the firing of the brick. This could have occurred through the clay interacting with the limestone temper (Magee et al. 1996). It is notable that suitable limestone chips for use as temper is a frequent archaeological occurrence in the Mendes region. Thus the observable similarities between the glassy matrix and the fired brick suggest association, and by extension lend considerable support to the possibility that the matrix has its origins from construction mudbrick.

The exposed surfaces of the bones from the glassy matrix have an appearance that corresponds to the fusing and sintering characteristics of Stage IV in the scheme developed by Shipman et al. (1984) for catagorising the destruction of bone exposed to heat. The variables of Stage IV indicate that the bones were exposed to temperatures ranging between 440 - 800 degrees centigrade. This conclusion is consistent with the level of heating expected if a hot fluid glass cooled to normal circulating temperatures (Magee et al. 1996). The edges of the bone that are in contact with the matrix are darker in colour than the remainder of the bones. This is because of the edges having been exposed to high temperatures that the x-ray diffraction patterns show are associated with temperatures above 645 degrees centigrade (Magee et. al 1996, Shipman et al. 1984).

The analytical results that have been obtained and outlined here point towards the slag being the outcome of the fusion of a mudbrick structure that encapsulated animal bones and teeth from various species of fauna in a glassy matrix. The nearby cemetery at Kom el-Adhem has revealed remains of burnt animal bones buried during the Late Period with amulets as well as horns of rams. These animals were likely to be have been regarded as sacred. Animal cults were strong throughout all periods of pharaonic history and the practice survived into Graeco-Roman times (Spencer 1982). Sites containing the mummified remains of sacred animals such as cats, cows, dogs and crocodiles, amongst others, are a common occurrence. It is important, however, to appreciate that these sacred animal cemeteries reached their apex during the Late Period, when the preparation of the dead animals was a s detailed and subject to ritual as was that of their human burriers. Underground galleries were constructed for some of the cemeteries. At Mendes, this was accomplished by the building of trench passages cut into the desert surface, lined with brickwork and then covered with sand (Magee et al. 1996). During the course of a massive blaze these trenches would themselves have acted as a natural furnace. The wooden support structures, the mummies and the wooden funerary accompaniments would have been combustible materials, in addition to the fuel used by the perpetrators.

The presence of both calcium chloride hydroxide and, correspondingly, chlorine in some of the bones could have been derived from the use of sodium chloride and natron during the mummification process (Magee et al. 1996). During the conflagration the mudbrick would have melted and incorporated both any constituent sand utilised as temper in the bricks together with the sand surrounding the tunnel passages. This would account for the slightly higher silicon levels in the glassy matrix in comparison to the fired mudbrick sample (Magee et al. 1996). The matrix's potassium content may have originated from the mummy wrappings and possibly also the any vegetable matter utilised as mudbrick temper. The black colour of the slag and the red colour (probable visible because of weathering) of the vessels' interior surfaces came from the magnetite and hematite used as oxidation products in the melted brick (Magee et al. 1996).

The volume and numerous varieties of animals that are present in the slag blocks at Kom el-Adhem strongly point towards them having been associated with cults in which all members of a species were regarded as manifestations of gods and were revered, mummified after death and laid to rest in a close-by cemetery (Magee et al. 1996). Mendes is famous for its sacred living rams, which were worshipped throughout their whole lives as the god incarnate (Spencer 1982).

The belief system of the ancient Egyptians placed great emphasis upon the preservation of the body (Spencer 1982). The followers of the animal cults would, therefore, not have exhumed and deliberately burned the carefully prepared remains. It appears a more realistic probability that the glassy matrix represents either the purposeful destruction of cult burials during the Persian or the Christian eras. However, fire was regarded as holy by the Persians and the burning of ancient Egyptian mummified cult animals would have been held as an act of contamination (Magee et al. 1996). Despite textual evidence revealing that the Persians leveled and defiled some monuments and temples in Mendes, repairs were undertaken during the Greek occupation. Christianity arose to prominence in ancient Egypt during the late 4th-5th centuries AD. The Christian Church banned "pagan" rituals and permitted that great vandalism of traditional Ancient Egyptian religious centres that occurred (Magee et al. 1996).

The knowledge of the chemical composition of Ancient Egyptian glass was utilised in the investigation into the composition and origin of the glassy matrix. The results show that tunnel passages were constructed during the Late Period that contained mummified animal remains. A conflagration occurred, reaching temperatures great enough to cause the mudbrick to melt and flow. This in turn encased the sacred animal bones and prevented them from being totally incinerated. This event most likely occurred during the Christian era, after which the sacred animal cults never regained their former position, prestige, glory and power. The glassy matrix was the end product.


References

Freestone, I. 1991. Looking into Glass. In, Bowman, S. (ed.) Science and the Past. London: British Museum Press

Lilyquist, C. & Brill, R.H. 1993. Studies in early Egyptian glass. New York: Metropolitan Museum of Art

Lucas, A. & Harris, J. 1962. Ancient Egyptian materials and industries, 4th edition. London: Edward Arnold

Magee, M.J., Wayman, M.L. & Lovell, N.C. 1996. Chemical and Archaeological Evidence for the Destruction of a Sacred Animal Necropolis at Ancient Mendes, Egypt. Journal of Archaeological Science 23: 485-492

Nicholson, P.T. 1993. Egyptian faience and glass. Dover: Shire Publications

Nisbet, H.C. 1982. Vitrification phenomena in Hill-Forts. In, Engle, A. (ed.) Readings in Glass History, No. 15/16. Jerusalem: Phoenix

Saitowitz, S.J. 1996. Glass Beads as Indicators of Contact and Trade in Southern Africa ca. AD 900 - AD 1250. PhD thesis: University of Cape Town

Shaw, I. & Nicholson, P.T. 1995. British Museum Dictionary of Ancient Egypt. London: British Museum Press

Shipman, P., Foster, G. & Shoeninger, M. 1984. Burnt bones and teeth: an experimental study of color, morphology, crystal structure and shrinkage. Journal of Archaeological Science 11: 307-325

Spencer, A.J. 1982. Death in Ancient Egypt. London: Penguin

Tatton-Brown, V. & Andrews, C. 1991. Before the invention of glassblowing. In, Tait, H. (ed.) Five Thousand Years of Glass. London: British Museum Press

Youngblood, E., Fredriksson, B.J., Kraut, F. & Fredriksson, K. 1978. Celtic vitrified forts: implications of a chemical petrological study of glasses and source rocks. Journal of Archaeological Science 5: 99-121


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