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 What makes up a piece of glass? Learn Here! Composition of Glass Glass can be generally divided into two groups: oxide glass and non-oxide glass. The ingredients of oxide glasses include oxides (chemical compounds that include oxygen). Non-oxide glasses are made from compounds that contain no oxides, and which often instead contain sulfides or metals. Oxide glasses are much more widely used commercially.

The common types of glass discussed below are all oxide glasses. Soda-lime glass is the kind of glass used for flat glass, most containers and electric light bulbs, and many other industrial and art objects. More than 90 percent of all glass is soda-lime glass. It has been made of almost the same materials for hundreds of years. The composition is about 72 percent silica (from sand), about 13 percent sodium oxide (from soda ash), about 11 percent calcium oxide (from limestone), and about 4 percent minor ingredients. Soda-lime glass is inexpensive, easy to melt and shape, and reasonably strong. All glass container manufacturers use the same basic soda-lime composition, making the containers easy to recycle. Manufacturers sort the glass by color and then later reuse it in the production of new containers. Soda-lead glass, commonly called crystal or lead glass, is made by substituting lead oxide for calcium oxide and often for part of the silica used in soda-lime glass. Soda-lead glass is easy to melt. It is much more expensive than soda-lime glass. Soda-lead glass has such beautiful optical properties that it is widely used for the finest tableware and art objects.

In addition, lead oxide improves the electrical properties of glass. Borosilicate glass is heat-shock resistant and better known by such trade names as Pyrex and Kimax. It contains about 80 percent silica, 4 percent sodium oxide, 2 percent alumina, and 13 percent boric oxide. Such glass is about three times as heat-shock resistant as soda-lime glass and is excellent for chemical and electrical uses. This glass makes possible such products as ovenware and beakers, test tubes, and other laboratory equipment. Fused silica glass glass is a highly heat-shock resistant glass that consists entirely of silica. It can be heated to extremely high temperatures and then plunged into ice-cold water without cracking. Fused silica is expensive because exceptionally high temperatures must be maintained during production. It is used in laboratory glassware and optical fibers. 96 percent silica glass resists heat almost as well as fused silica, but it is less expensive to produce. It consists of a special borosilicate composition that has been made porous by chemical treatment. The pores shrink when the glass is heated, leaving a smooth, transparent surface. The glass is sold under the trade name Vycor.

Colored glass gets its coloring from certain oxides that are added to the glass. For example, 1 part of nickel oxide in 50,000 produces a tint that may range from yellow to purple, depending on the base glass. One part of cobalt oxide in 10,000 gives an intense blue. Red glasses are made with gold, copper, or selenium oxides. Other colors can be produced in glass with other chemicals. Contributor: ï¿½ Steve W. Martin, Ph.D., Professor of Materials Science and Engineering, Iowa State University. Steve W. Martin, "Glass," Discovery Channel School, original content provided by World Book Online, glass | Britannica.com Glass Compagnie de Saint-Gobain-Pont-à-Mousson Glass, an inorganic solid material that is usually transparent or translucent as well as hard, brittle, and impervious to the natural elements. Glass has been made into practical and decorative objects since ancient times, and it is still very important in applications as disparate as building construction , housewares, and telecommunications. It is made by cooling molten ingredients such as silica sand with sufficient rapidity to prevent the formation of visible crystals.

Onion-shaped blown glass by artist Dale Chihuly, at the Fairchild Tropical Botanic Garden, Miami. Lynne Sladky/AP A brief treatment of glass follows. Glass is treated in detail in a number of articles. Stained glass and the aesthetic aspects of glass design are described in stained glass and glassware . The composition, properties, and industrial production of glass are covered in industrial glass . The physical and atomic characteristics of glass are treated in amorphous solid . The varieties of glass differ widely in chemical composition and in physical qualities. Most varieties, however, have certain qualities in common. They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass mixtures are fused with certain metallic oxides; they are, when cold, poor conductors both of electricity and of heat; most types are easily fractured by a blow or shock and show a conchoidal fracture; they are but slightly affected by ordinary solvents but are readily attacked by hydrofluoric acid. The Prince Rupert’s Drop is a droplet of glass formed by the rapid cooling of molten glass in cold … © Tyler A.

Gordon Examining a superhydrophobic glass surface that resists fog and glare and is self-cleaning. © Massachusetts Institute of Technology (A Britannica Publishing Partner) Commercial glass composition Commercial glasses may be divided into soda–lime–silica glasses and special glasses, most of the tonnage produced being of the former class. Such glasses are made from three main materials—sand (silicon dioxide, or SiO2), limestone (calcium carbonate, or CaCO3), and sodium carbonate (Na2CO3). Fused silica itself is an excellent glass, but, as the melting point of sand (crystalline silica) is above 1,700 °C (3,092 °F) and as it is very expensive to attain such high temperatures, its uses are restricted to those in which its superior properties—chemical inertness and the ability to withstand sudden changes of temperature—are so important that the cost is justified. Nevertheless the production of fused silica glass is quite a large industry; it is manufactured in various qualities, and when intended for optical purposes the raw material used is rock crystal rather than quartz sand. Learn if cold glass can flow.

Contunico © ZDF Enterprises GmbH, Mainz Similar Topics polymer To reduce the melting point of silica, it is necessary to add a flux; this is the purpose of the sodium carbonate (soda ash), which makes available the fluxing agent sodium oxide. By adding about 25 percent of the sodium oxide to silica, the melting point is reduced from 1,723 to 850 °C (3,133 to 1,562 °F). But such glasses are easily soluble in water (their solutions are called water glass ). The addition of lime ( calcium oxide , or CaO), supplied by the limestone, renders the glass insoluble again, but too much makes a glass prone to devitrification—i.e., the precipitation of crystalline phases in certain ranges of temperature. The optimum composition is about 75 percent silica, 10 percent lime, and 15 percent soda, but even this is too liable to devitrification during certain mechanical forming operations to be satisfactory.

In making sheet glass it is customary to use 6 percent of lime and 4 percent of magnesia (magnesium oxide, or MgO), and in bottle glass about 2 percent alumina (aluminum oxide, or Al2O3) is often present. Other materials are also added, some being put in to assist in refining the glass (i.e., to remove the bubbles left behind in the melting process), while others are added to improve its colour. For example, sand always contains iron as an impurity, and, although the material used for making bottles is specially selected for its low iron content, the small traces of impurity still impart an undesirable green colour to the container; by the use of selenium and cobalt oxide together with traces of arsenic trioxide and sodium nitrate, it is possible to neutralize the green colour and produce a so-called white (decolourized) glass. Optical and high-temperature glass Scientists Ponder Menopause in Killer Whales Glasses of very different, and often much more expensive, compositions are made when special physical and chemical properties are necessary.

For example, in optical glasses, a wide range of compositions is required to obtain the variety of refractive index and dispersion needed if the lens designer is to produce multicomponent lenses that are free from the various faults associated with a single lens, such as chromatic aberration . High-purity, ultratransparent oxide glasses have been developed for use in fibre-optic telecommunications systems, in which messages are transmitted as light pulses over glass fibres. Building Blocks of Everyday Objects When ordinary glass is subjected to a sudden change of temperature, stresses are produced in it that render it liable to fracture; by reducing its coefficient of thermal expansion , however, it is possible to make it much less susceptible to thermal shock. The glass with the lowest expansion coefficient is fused silica. Another well-known example is the borosilicate glass used for making domestic cookware, which has an expansion coefficient only one-third that of the typical soda–lime–silica glass. In order to effect this reduction, much of the sodium oxide added as a flux is replaced by boric oxide (B2O3) and some of the lime by alumina.

Another familiar special glass is the lead crystal glass used in the manufacture of superior tableware; by using lead monoxide (PbO) as a flux, it is possible to obtain a glass with a high refractive index and, consequently, the desired sparkle and brilliance. Adding colour and special properties The agents used to colour glass are generally metallic oxides . The same oxide may produce different colours with different glass mixtures, and different oxides of the same metal may produce different colours. The purple-blue of cobalt, the chrome green or yellow of chromium, the dichroic canary colour of uranium, and the violet of manganese are constant. Ferrous oxide produces an olive green or a pale blue according to the glass with which it is mixed. Ferric oxide gives a yellow colour but requires an oxidizing agent to prevent reduction to the ferrous state. Lead gives a pale yellow colour. Silver oxide gives a permanent yellow stain. Finely divided vegetable charcoal added to a soda-lime glass gives a yellow colour. Selenites and selenates give a pale pink or pinkish yellow. Tellurium appears to give a pale pink tint.

Nickel with a potash–lead glass gives a violet colour, and a brown colour with a soda–lime glass. Copper gives a peacock blue, which becomes green if the proportion of the copper oxide is increased. Wine goblet, blue glass decorated with white and gold enamel, Iran, mid-19th century; in the … Photograph by Trish Mayo. Brooklyn Museum, New York, gift of Mr. and Mrs. Charles K. Wilkinson in honor of Irma L. Fraad, 76.147.3 Connect with Britannica Facebook Twitter YouTube Instagram Pinterest An important class of materials is the chalcogenide glasses , which are selenides, containing thallium, arsenic, tellurium, and antimony in various proportions. They behave as amorphous semiconductors. Their photoconductive properties are also valuable. Certain metallic glasses have magnetic properties; their characteristics of ease of manufacture, magnetic softness, and high electrical resistivity make them useful in the magnetic cores of electrical power transformers. Glassmaking over the centuries Many different useful and decorative articles have been made from glass over the centuries.

The history of glass as a creative art has been determined partly by technical advances in its manufacture and decoration and partly by the history of taste and fashion. Molten glass on a glassblowing rod. © Royik Yevgen/Shutterstock.com Glass was first made in the ancient world, but its earliest origins are obscure. Egyptian glass beads are the earliest glass objects known, dating from about 2500 bce. Later in Egyptian civilization, a type of glass characterized by feathery or zigzag patterns of coloured threads on the surface of the glass vessel was made. The real origins of modern glass were in Alexandria during the Ptolemaic period and, later, in ancient Rome . Alexandrian craftsmen perfected a technique known as mosaic glass in which slices of glass canes of different colours were cut crossways to make different decorative patterns. Millefiori glass, for which the canes are cut in such a way as to produce designs reminiscent of flower shapes, is a type of mosaic glass. Britannica Lists & Quizzes Editor Picks: Exploring 10 Types of Basketball Movies Molded glass was also developed early, glass being pressed into a mold to form a particular shape.

Various types of decoration involving engraving and colour were also possible. Glassblowing was probably developed during the 1st century bce by glassmakers in Syria. With this technique the possibilities of shaping glass into desired forms were endless. Glass could be blown into a mold or shaped completely free-form. The Romans perfected cameo glass , in which the design has been produced by cutting away a layer of glass to leave the design in relief. The next major developments in the history of glass came during the 15th century in Venice . As early as the 13th century the Venetian island of Murano had become the centre for glassmaking. At first, Venetian glassmakers made use of many of the ancient and medieval decorative techniques to produce richly coloured and ornamental pieces having motifs characteristic of the Italian Renaissance. Later they developed a clear glass similar to crystal, called cristallo , which was to form the basis for a thriving export trade and spread throughout Europe. Simple blown glasses of this type were much in demand in the 16th century. Such glass lent itself to decoration by the engraving of delicate designs; used from the early 16th century, the technique remained popular well into the 18th century throughout Europe.

Diamond-point engraving was practiced in particular in The Netherlands and in Germany. Late in the 17th century Bohemia became an important glass-producing area and remained important until early in the 20th century. By the 17th century England was making glass in the Venetian tradition that was notable for its simplicity. The glassmaker George Ravenscroft discovered about 1675 that the addition of lead oxide to Venetian-type glass produced a solid, heavier glass. Lead crystal , as it was known, thereafter became a favourite type of glass for fine tableware . Trending Topics glass Glass What is glass and how is it produced? Glass is a hard material normally fragile and transparent common in our daily life. It is composed mainly of sand (silicates, SiO2) and an alkali. These materials at high temperature (i.e. molten viscous state) fuse together; then they are cooled rapidly forming a rigid structure, however not having enough time to form a crystalline regular structure. Depending on the final use and application the composition of the glass and cooling rate will vary to achieve the adequate properties for the specific application. These are the common ingredients to obtain glass: 1.

Sand (SiO2 silica) In its pure form it exists as a polymer, (SiO2)n. 2. Soda ash (sodium carbonate Na2CO3) Normally SiO2 softens up to 2000°C, where it starts to degrade (at 1713°C most of the molecules can already move freely). Adding soda will lower the melting point to 1000°C making it more manageable. 3. Limestone (calcium carbonate or CaCo3) or dolomite (MgCO3) Also known as lime, calcium carbonate is found naturally as limestone, marble, or chalk. The soda makes the glass water-soluble, soft and not very durable. Therefore lime is added increasing the hardness and chemical durability and providing insolubility of the materials. Other materials and oxides can be added to increase properties (tinting, durability, etc.), produce different effects, colors, etc. Main properties of glass These are the main characteristics of glass: - Solid and hard material - Fragile and easily breakable into sharp pieces - Transparent to visible light - Inert and biologically inactive material.

- Glass is 100% recyclable and one of the safest packaging materials due to its composition and properties Glass is used for architecture application, illumination, electrical transmission, instruments for scientific research, optical instruments, domestic tools and even textiles. Glass does not deteriorate, corrode, stain or fade and therefore is one of the safest packaging materials. These properties can be modified and changed by adding other compounds or heat treatment. Types of glass and market application The main types of glass are described below: Commercial glass or Soda-lime glass: This is the most common commercial glass and less expensive. The composition of soda-lime glass is normally 60-75% silica, 12-18% soda, and 5-12% lime. A low percentage of other materials can be added for specific properties such as coloring. - It has light transmission appropriate to be use in flat glass in windows; - It has a smooth and nonporous surface that allows glass bottles and packaging glass to be easily cleaned; - Soda-lime glass containers are virtually inert, resistant to chemical attack from aqueous solutions so they will not contaminate the contents inside or affect the taste.

Whereas pure glass SiO2 does not absorb UV light, soda-lime glass does not allow light at a wavelength of lower than 400 nm (UV light) to pass. The disadvantages of soda-lime glass is that is not resistant to high temperatures and sudden thermal changes. For example, everybody has experienced a glass breaking down when pouring liquid at high temperature, for example to make tea. Some of the use of soda-lime glass is primarily used for bottles, jars, everyday drinking glasses, and window glass. Lead glass: Lead glass is composed of 54-65% SiO2, 18-38% lead oxide (PbO), 13-15% soda (Na2O) or potash (K2), and various other oxides. When the content of PbO is less than 18% is known as crystal glass. - In moderate amounts lead increases durability; - In high amounts it lowers the melting point and decreases the hardness giving a soft surface; - In addition it has a high refractive index giving high brilliance glass. These two last properties make it appropriate for decorating purposes. Glass with high lead oxide contents (i.e.

65%) may be used as radiation shielding glass because lead absorb gamma rays and other forms of harmful radiation, for example, for nuclear industry. As with soda-lime glass, lead glass will not withstand high temperatures or sudden changes in temperature. Borosilicate glass: Borosilicate glass is mainly composed of silica (70-80%), boric oxide B2O3 (7-13%) and smaller amounts of the alkalis ( sodium and potassium oxides) such as 4-8% of Na2O and K2O, and 2-7% aluminum oxide (Al2O3). Boron gives greater resistance to thermal changes and chemical corrosion. It is suitable for industrial chemical process plants, in laboratories, in the pharmaceutical industry, in bulbs for high-powered lamps, etc. Borosilicate glass is also used in the home for cooking plates and other heat-resistant products. It is used for domestic kitchens and chemistry laboratories, this is because it has greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling experiments. There are other special types of glass by adding different substances.

For example: Element soda-lime glass | Britannica.com Soda-lime glass See Article History Soda-lime glass, most common form of glass produced. It is composed of about 70 percent silica (silicon dioxide), 15 percent soda ( sodium oxide ), and 9 percent lime ( calcium oxide ), with much smaller amounts of various other compounds . The soda serves as a flux to lower the temperature at which the silica melts, and the lime acts as a stabilizer for the silica. Soda-lime glass is inexpensive, chemically stable, reasonably hard, and extremely workable, because it is capable of being resoftened a number of times if necessary to finish an article. These qualities make it suitable for manufacturing a wide array of glass products, including light bulbs, windowpanes, bottles, and art objects. Reusable milk bottles made of soda-lime glass. Clawed Soda-lime glass was produced throughout much of Europe for hundreds of years. Silica, in the form of sand , and limestone were abundant nearly everywhere. Soda ash was readily obtained from hardwood forests, though Venetian glassmakers favoured potash produced by burning seaweed.

Learn More in these related articles: silica compound of the two most abundant elements in Earth’s crust, silicon and oxygen, SiO 2. The mass of Earth’s crust is 59 percent silica, the main constituent of more than 95 percent of the known rocks. Silica has three main crystalline varieties: quartz (by far the most abundant),... Silica-based ...However, for most glass products, such as containers, windows, and lightbulbs, the primary criteria are low cost and good durability, and the glasses that best meet these criteria are based on the soda-lime-silica system. Examples of these glasses are shown in the table Composition of representative oxide glasses. Science in glassmaking ...chemical terminology removed much of the ambiguity and confusion characteristic of previous work. It was the French chemist Jean-Baptiste-André Dumas who showed in 1830 that the durability of soda-lime-silica glass was maximized when the ratio of the three was 1:1:6; this is essentially the modern soda-lime-silica composition. 5 References found in Britannica Articles Assorted References Corrections? Updates? Help us improve this article! Contact our editors with your feedback.

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Please try again later. Close Date Published: August 28, 2008 URL: Access Date: January 19, 2017 Share BBC - GCSE Bitesize: Uses of limestone Next Uses of limestone Limestone, quicklime and slaked lime are all used to neutralise excess acidity - which may be caused by acid rain - in lakes and in soils. Limestone is used as a building material, and to purify iron in blast furnaces. It's also used in the manufacture of glass, and of cement (one of the components of concrete). The main uses of limestone and its products Limestone (CaCO3) can be used as a building material and in the manufacturing of iron. Glass - heated with sand and soda (sodium carbonate). Cement - heated with clay in a kiln. Concrete - mixed with sand, water and crushed rock Mortar - mixed with sand and water Quicklime - heated. Slaked lime (Calcium Hydroxide Ca(OH)2) - mixed with water Lime motar - mixed with water Glass Glass is made by melting sand and then cooling it. Flat sheets of glass for windows are made by floating molten glass on a layer of molten tin.

Glass manufacturers add sodium carbonate to sand during the manufacturing process, to reduce the melting temperature of the sand and so save energy. The sodium carbonate decomposes in the heat to form sodium oxide and carbon dioxide, but this makes the glass soluble in water. Calcium carbonate (limestone) is therefore also added, to stop the glass dissolving in water. The calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide. About 90 per cent of glass is soda-lime glass, or bottle glass. Page:

Question: The three main constituents of glass are sand, soda, and what else?
Answer: LIMESTONE