Informe financiero

Copper oxides are employed as colouring agents in glass compositions. The earliest documentary records of glass-making include the use of “fast copper”, in the Babylonian texts (Turner 1956c, Oppenheim et al. 1988). The earliest vitreous materials, glazed steatite of the late 5th Millenium BC (Moorey 1985, p 136) were bluish-green coloured due to the copper content: malachite had already a long history as a pigment. The earliest copper coloured red glass known to date is from Nuzi, circa 1500 BC (Henderson 1985, p 281 and Vandiver 1983). Egyptian blue glasses coloured by copper alone have been found to contain copper oxide in the range of 0.59 – 1.45 % (Kaczmarczyk and Hedges 1983, p 61). Humphrey Davy,

one of the first to examine ancient glass chemistry, analysed and discussed copper- coloured “frit”, a combination of sand, carbonate of soda and copper filings for comparison with material from Pompeii, before concluding that the archaeological material was coloured using cobalt (Davy 1815, p 106-109, p 120).

The role of copper is complex, and a range of colours can be achieved in glass depending upon the oxidation state of the copper and the presence or absence of other materials. Oxidised glasses can contain high concentrations of copper dissolved into the matrix as cupric ions (Cu++) producing a blue to green colour depending upon the concentration and glass type (Cable and Smedley 1998 p 153). More reducing conditions gives rise to an increasing proportion of cuprous ions (Cu+), which will give a less intense colouration or even colourless glass. The capacity of a glass to contain dissolved cuprous ions is more limited than for cupric ions, which can lead to the precipitation of cuprous oxide or metallic copper when highly reduced copper containing glasses are cooled. The formation of dendrites of cuprous oxide gives rise to a red to brown colour range (Freestone 1987). Lambert and McLaughlin (1978) confirmed the oxidation state of copper in blue and red glasses from 18th Dynasty Egypt using X-ray photoelectron

spectroscopy (XPS): red glasses: Cu+ or Cu0, and the blue/green glasses: Cu2+.

Experimental work to reproduce opaque red glasses from 6th Century BC Nimrud (Cable and Smedley 1992) elucidated the relationship between the copper oxide and other glass components. These red opaque glasses are also very high in lead (c. 25 % PbO, Freestone 1992). Simply retaining a reducing environment in the furnace was of itself insufficient to achieve the desired effect of maximum

precipitation of cuprite. Highly reducing furnace conditions can lead to localised reduction of the copper oxide on the glass surface, even to the point of reducing it to metallic copper. However, frequently the experimental melts were highly segregated, and insufficiently reduced throughout the batch. The type of raw component proved essential for increasing homogeneity and creating reducing conditions within the melt: litharge (PbO) proved better than red lead (Pb3O4), and

copper carbonate (CuCo3- similar to the pigment/ore malachite, Cu

CO3.Cu(OH)2.H2O) was more effective than cuprous oxide. The addition of tin

and antimony to the melt facilitated the reduction of the cupric to cuprous oxide. The lead content of the glass, in proportions of the region of 25 wt%, increases the solubility of the cuprous oxide during the melt thereby increasing the potential for cuprite precipitation (Freestone 1992, p 186, also see Ahmed and Ashour 1981 for experimental work on cuprous oxide precipitation in high-lead glasses). An absence of lead leads to batch segregation and brown or less densely coloured red glasses.

The copper-containing red glasses are chronologically useful: those with low or lead-free tend to be dull liverish or dark red and are known from the mid 2nd Millenium BC in Egypt onwards (Bimson 1992, p 166). The high-lead copper containing red glasses (15-30 wt% PbO) are brilliant sealing wax-red, and all dated examples are from 850 BC and later (Brill, in Oppenheim et al. 1988, p 120, Bimson 1992, p 168). The two types are used contemporaneously thereafter up to at least the 7th Century AD (e.g. the Sutton Hoo material, Bimson 1992, p 169).

Henderson has suggested the development of regionally distinct red opaque glass production between western and eastern Britain after the 2nd Century BC, on the basis of varying copper and lead contents (Henderson 1989a, p 47-48, 2001, p 476).

Recent work on early medieval opaque red enamels has defined a type of material not related to a cuprite-coloured soda-lime-silica glass, but a copper oxide-lead oxide-silica material produced as a by product of metallurgical raffination process (Stapleton et al. 1999).

The raw materials employed to introduce copper into the glass melt have rarely been investigated to date, considering the central role it has played as a colourant.

Common copper bearing ores for metal extraction include cuprite (red oxide, Cu2O), melaconite (black oxide CuO), malachite (green basic carbonate, Cu

CO3.Cu(OH)2. H2O) azurite (blue basic carbonate, Cu3(OH)2(CO3)2), the sulphate

mineral chalcanthite (blue vitriol, CuSO4.5H2O), the chloride mineral atacamite

(Cu2(OH)3Cl) and the silicate chrysocolla (CuSiO3.2 H2O). More complex

sulphide ores include chalcocite (copper glance, Cu2S), covellite (CuS), the iron

sulphide minerals chalcopyrites (copper pyrites, Cu2Fe2S4) and bornite (peacock

ore, Cu5FeS4). More rare, but significant in the development of copper alloys, are

those ores which also contain arsenic and antimony: tetrahedrite, containing iron and antimony ((Cu, Fe)12Sb4S13), bournonite containing lead and antimony

(CuPbSbS3), tennanite containing iron and arsenic and enargite containing

The cuniform instructions for introducing the copper colourants to a glass (Oppenheim et al. 1988, p 121-123) conform to the manufacture of blue translucent and red opaque glasses respectively. Although there is some

discussion concerning the nature of the copper-bearing materials, it seems most likely that in both cases, the “slow copper” and “fast bronze” are both copper- based metallic alloys (rather than mineral ores, Egyptian blue or metal slags) (Brill 1988, p 121). Furthermore, a large number of copper-containing ancient glasses have copper: tin ratios comparable to contemporary bronzes, suggesting that it is to metallurgy that glass scientists should look for the source of copper colourants in glass (Brill 1988, p 121, 123). Sayre and Smith (1967, p 307-9) compare the copper: tin: lead ratios of a number of glasses with contemporary bronzes, suggesting that oxidised bronzes have been used to colour the glasses. This is a line of enquiry, which deserves further exploration.

Recent excavations of 13th Century BC deposits at Pi-Ramasses in Egypt have located glass colouring activities at the heart of a bronze-casting factory (Rehren et al., 1998). Some of the copper coloured blue glasses from Final Bronze Age Frattesina are accompanied by tin, in proportions indicating a bronze as the source of the colourant (Brill 1992, p 14). The appearance of traces of zinc and lead oxides in Egyptian copper coloured blue glasses, which were absent in

contemporary copper metal artefacts may indicate the use of an ore rather than the refined metal (Kaczmarczyk and Hedges 1983, p 63). The absence of a correlation with alumina or iron elsewhere might suggest that a copper-containing

Copper and cobalt are correlated in cobalt-coloured blue glasses from Manching (260-50 BC), indicating the use of a colourant containing both components in these particular glasses (Gebhard et al. 1989, p 208).

3.7.9 Lead oxide