Egyptian faience

The discovery of faience glazing has tentatively been associated with the copper industry: bronze scale and corrosion products of leaded copper objects are found in the manufacture of faience pigments.[12] However, although the likelihood of glazed quartz pebbles developing accidentally in traces in copper smelting furnaces from the copper and wood ash is high, the regions in which these processes originate do not coincide.[13]

Although it appears that no glass was intentionally produced in Egypt before the Eighteenth Dynasty (as the establishment of glass manufacture is generally attributed to the reign of Thutmose III), it is likely that faience, frit and glass were all made in close proximity or in the same workshop complex, since developments in one industry are reflected in others.[10] Such close relationship is reflected in the prominent similarity of the formulations of faience glaze and contemporary glass compositions.[12] Despite the differences in the pyrotechnology of glass and faience, faience being worked cold, archaeological evidence suggests that New Kingdom glass and faience production was undertaken in the same workshops.[7]

Faience has been defined as the first high technology ceramic, to emphasize its status as an artificial medium, rendering it effectively a precious stone.[14] Egyptian faience is a non-clay based ceramic composed of crushed quartz or sand, with small amounts of calcite lime and a mixture of alkalis, displaying surface vitrification due to the soda lime silica glaze often containing copper pigments to create a bright blue-green luster.[7] While in most instances domestic ores seem to have provided the bulk of the mineral pigments, evidence suggests that during periods of prosperity, raw materials not available locally, such as lead and copper, were imported.[12] Plant ash, from "halophyte" (salt-tolerant) plants typical of dry and sea areas, was the major source of alkali until the Ptolemaic Period, when natron-based alkalis almost completely replaced the previous source.[14] Although the chemical composition of faience materials varies over time and according to the status of the workshop, also as a cause of change of accessibility of raw materials, the material constitution of the glaze is at all times consistent with the generally accepted version of faience glazing.[12]

Typical faience mixture is thixotropic, that is thick at first and then soft and flowing as it begins to be formed.[15] This property, together with the angularity of silica particles, accounts for the gritty slumps formed when the material is wetted, rendering faience a difficult material to hold a shape.[16] If pressed too vigorously, this material will resist flow until it yields and cracks, due to its limited plastic deformation and low yield strength.[14]

This faience vessel from the reign of Amenhotep III exhibits two different shades of blue that can be achieved by adding cobalt and copper pigments to faience paste.Walters Art Museum, Baltimore.

A number of possible binding agents, amongst Arabic gum, clay, lime, egg white and resin, have been suggested to help in the binding process.[17] Although traces of clay have been found in most Pharaonic faience, reconstruction experiments showed that clay, organic gums or lime while successfully improving the wet working performance, failed to improve the fired strength of the faience, or proved the gum was too sticky for the removal of objects from their molds.[7][14][17] The use of alkalis as binders, in the form of natron or plant ash, produced suitable results in experiments.[7] Pulverized glass or sintered material of similar composition could also enhance the fired strength of faience bodies: the compositions of such glasses is in fact comparable to the published compositions of New Kingdom glass.[12][18]

Three methods have been hypothesized to shape the body of faience objects: modeling, moulding and abrasion, the last being used in conjunction with the first two.[7] Modeling, scraping and grinding are the techniques most widely used in earlier times, as represented in the material qualities of Predynastic and Protodynastic faience objects.[10] Predynastic bead manufacture is essentially a cold technology, more akin to stone working than glass: a general form of faience is modeled, possibly free formed by hand, then holes are drilled to create beads.[10] In the Middle Kingdom, the techniques employed are molding and forming on a core, sometimes in conjunction with intermediate layers between the glaze and the body.[14] Marbleized faience, resulting from the working of different colored faience bodies together, so as to produce a uniform adherent body, also appears in this period.[12][14] Towards the end of the Middle Kingdom, incising, inlaying and resisting techniques appear: these were bound to become progressively popular towards the New Kingdom.[14] In the New Kingdom, beads, amulets and finger rings are produced by a combination of modeling and molding techniques.[14] In this period, sculptural detail is created using inlays of different colored faience or by scraping the body to reveal the white intermediate layer in relief.[14] Moulding was first applied to faience manufacture in the Middle Kingdom by forming a model of an object, or employing a finished faience piece, impressing it in wet clay, and later by firing the clay to create a durable mold.[10][19] The faience paste could then be pressed into the mold, and following drying, be re-worked through surface abrasion before firing.[10] Moulds could facilitate mass production of faience objects such as amulets rings and inlays, as evidenced by the several thousand of small open face, earth-ware clay molds excavated at Tell el Amarna.[13] The level of standardisation that use of moulds produced varied, with a compositional and morphological study of faience ushabtis suggested that mass-production is an oversimplification of a complex process that may more accurately described as batch-processing.[20]

Falcon, Ptolemaic period, 13.5 cm high, see text

Wheel throwing, possibly occurring from the New Kingdom onwards, is certainly established by the Greco-Roman period, when large amounts of clay seem to have been added to the faience body.[10] Because of the limited plasticity of faience, rendering throwing extremely difficult, a progressive increase of clay in the faience bodies culminating in the quartz, clay and glass frit bodies of Islamic times, is observed in the archaeological record.[14][21]

Ptolemaic and Roman faience tends to be typologically and technologically distinct from the earlier material: it is characterized by the widespread use of moulding and high relief on vessels.[22] A very unusual and finely made group of figures of deities and falcons in the Metropolitan Museum of Art in New York, apparently representing hieroglyphs that are elements from a royal inscription, perhaps from a wooden shrine, is decorated in a form of champlevé (typically a technique for enamel on metal). Depressions in the faience body were filled with coloured "vitreous pastes" and refired, followed by polishing.[23]

Polychrome pieces were usually made by inlaying different colours of paste.[24]

In the application method, formerly assumed to be the only one used for faience glazing; silica, lime and alkalis are ground in the raw state to a small particle size, thus mixed in water to form a slurry which is then applied to the quartz core.[10][13][17] Partial fritting of the slurry favors the first stages of vitrification, which in turn lowers the final firing temperature. The slurry can be then applied to the body, through brushing or dipping, to create a fine, powdery coating.[14] Upon firing, the water from the melting glaze partially diffuses in the sand body, sintering the quartz particles and thus creating some solid bridges in the body.[14]

In the self-glazing process of efflorescence, the glazing materials, in the form of water-soluble alkali salts, are mixed with the raw crushed quartz of the core of the object.[15][16][25] As the water in the body evaporates, the salts migrate to the surface of the object to recrystallize, creating a thin surface, which glazes upon firing.[14]

Cup with lotus decoration, 14.4 cm high

Cementation glazing, a technique discovered in the Middle Kingdom, is also a self-glazing technique.[10] The possibility of the existence of cementation glazing, also known as 'Qom technique', followed the observation of this method being used in the city of Qom in Iran in the 1960s.[26] In this method the artifact, while buried in a glazing powder with a high flux content, is heated inside a vessel, causing the fusion of the object with the cement. During firing, the flux migrates to the quartz and combines with it to form a glassy coating.[26]

A vapour glaze reaction similar to salt glazing, as an alternative glazing process, has been suggested. In this process, the vaporization or dissociation of salts leads to vapour transport through the enveloping powder to the quartz body where a glaze is formed.[27]

Although glaze compositions vary regionally and chronologically, depending on the formation of the body and the glazing process employed, objects produced with different glazing techniques do not exhibit immediate diagnostic chemical variations in their compositions.[12][28] The recognition of the various glazing techniques, through microscopic observations of the degree of sintering and the vitreous phase of quartz body, is also ambiguous. For instance, objects with applied glazes and those which may have been glazed by efflorescence have overlapping characteristic features.[12][29] The following proposed criteria are subject to variation caused by increases in flux concentration, firing temperatures and time at peak temperatures.[14]

Recognition of application glazing- Macroscopically, applied glazes vary in thickness through the body, displaying thicker glazes on bases.[29] The traces of kiln supports, as well as the characteristic tendency to run and drip leading to pooling, may indicate the orientation of the object during firing.[14] In high magnification observations, the interface boundary of body and glaze appears well defined.[14] The absence of interstitial glass in the core is characteristic of application glazing: however, the possibility of adding glazing mixture to the quartz sand body, as well as the use of pre-melted glazes in the later periods, can predictably increase the degree of sintering of the core[22][29]

Recognition of cementation- Objects glazed through cementation display a thin even glaze all over the body, with no drying or firing marks, and portray a fairly friable and soft body[25][29] Microscopically, the concentration of copper characteristically decreases from the surface: the interaction layer is thin and well defined and the interstitial glass is absent with exception to the vicinity of the boundary layer.[14][29]

Recognition of efflorescence glazing- Pieces glazed by efflorescence may show traces of stand marks: the glaze appears thick and prone to cracking, thinning toward the edge of the piece and in concave areas.[29] In high magnification the interstitial glass is extensive; the unreacted salts which have not reached the surface fuse of the body accumulate in the core, creating bridges between the quartz particles.[29]

Most of the seven variants introduced by Lucas fail to recognize the glazing technology utilized or to suggest the stylistic and technological choices embedded in the manufacture of a faience object. However, variant A describes a technologically unique product and as such is still applicable: it has a finely ground underglaze consisting of quartz particles in a glass matrix, often revealed by incisions or depressions cut into the overlying glaze.[12][22] Glassy faience, variant E, displays no distinct outer layer from the interior, thus it has been suggested that the term 'faience' is a misnomer and the alternative name 'imperfect glass' has been advised.[12] Regarding variant F specimens, Lucas suggests the use of lead glazes, however it appears that lead glazing was never practiced in Ancient Egypt.[12][29]