Derivation of Ancient Egyptian Faience Core and Glaze Materials
Stocks, Denys A., Antiquity
An essential ingredient of the lovely blues in ancient Egyptian materials - faience, glazes, frits - is copper. How did the knowledge of that copper use arise? There is a telling congruence with Egyptian techniques in drilling stone artefacts, and the characteristics of the powder drilled out as waste.
An enigma of ancient Egyptian craftsmanship is the origin of the materials used for faience cores (bodies) and glazes. Ancient Egyptian dynastic craftsmen used copper tubular drills, with sand abrasive, to excavate stone artefacts (Petrie 1917: 45-6; Lucas 1962: 74; Reisner 1931: 180); the waste powders, rich in quartz, also contained copper from the drills. Did ancient craftsmen use these for other purposes? This article proposes that the powders make faience cores and blue glazes and, perhaps, blue frits. To explore this possibility, the characteristics of ancient faience are compared with the microstructure and composition of experimentally made ceramics.
Ancient faience: a brief description
Faience was employed to make jewellery, statuettes, small vessels and tiles, which were mostly blue or green. The first ancient Egyptian glazed material, found by Brunton & Caton-Thompson (1928: 27-8, 41) in grave deposits dated to the Badarian culture of Upper Egypt (c. 5500-4000 BC), consisted of carved and drilled steatite beads covered by a transparent and glossy glaze. It appears clear in cross-section, but in looking directly at the surface the optical effect is of translucency (Vandiver & Kingery 1986: 20, [ILLUSTRATION FOR FIGURES 1, 3 OMITTED]). Glazes containing malachite (a copper ore) produced the greenish-blue colour which imitated the rarer lapis lazuli and turquoise (Vandiver & Kingery 1986: 20).
About 4000 BC, stone cores were replaced by ceramic ones, made mainly from finely divided (ground) sand, but occasionally of comparatively coarser sand, which was modelled into shapes; cores also contain minor amounts of lime and either natron - a naturally occurring alkaline mixture of sodium salts, carbonate, bicarbonate, chloride and sulphate - or plant ashes. Often very friable, they are frequently white, or practically white in colour, but can be tinted brown, grey, yellow, sometimes very slightly blue or green (Lucas 1962: 157; Kaczmarczyk & Hedges 1983: 123; Vandiver & Kingery 1986: 20). In the core, minute angular particles of quartz are bonded together by varying amounts of interstitial glass, and covered with an alkali-based glaze, typically coloured blue by copper (Tite 1986: 39; 1987: 23-4).
A summary by Vandiver (1982: 167) of a composite range of chemical analyses of the body shows 92-99% Si[O.sub.2] (silicon dioxide), 15% CaO (calcium oxide), 0.5-3% [Na.sub.2]O (sodium oxide), with small quantities of CuO (copper oxide), [Al.sub.2][O.sub.3] (aluminium oxide), Ti[O.sub.2] (titanium dioxide), MgO (magnesium oxide) and [K.sub.2]O (potassium oxide). Most authorities accept faience firing temperatures of 800-1000 [degrees] C (Vandiver 1983: A10-11, A26ff). A significant number of ancient Egyptian faience cores (Tite & Bimson 1986: 69) show that many particle sizes are less than 50 [[micro]meter] diameter; even when coarser-grained quartz (100-200 [[micro]meter] diameter) predominates, significant amounts of fine-grained quartz, less than 50 [[micro]meter] diameter, are still present (Tite & Bimson 1986: 69). Dynastic cores, moulded, or modelled, from a stiff paste (Petrie 1909: 115-16, 118-19), were glazed by efflorescence (the firing of a core containing a glazing component, which partially rises to the surface during drying and fuses to become the glaze), or by cementation (the firing of a dry core buried in a glazing powder), or by direct application of a glazing slurry to a dry core's surface before firing.
The glaze consists of a soda-lime-silica mixture (Vandiver 1982: 167), generally 60-70% silica, 16-20% soda and 3-5% lime (Tite pers. comm. …