Lead oxide PbO

Lead compounds can act as network former, colourants and opacifiers in glass, as well as bringing sparkling brilliance to cut and polished translucent glasses. High lead opaque and translucent glasses have been identified from the mid- 2nd

Millenium BC onwards, and are well represented in the surviving glass literature (Henderson 1985, p 276 – 277). The inclusion of lead oxide in a glass artefact over 1 % marks it out as being of particular interest.

The earliest high lead glass analysed to date is from 1400 BC from Nippur in Mesopotamia containing 15.83 % PbO, and are also known from the

contemporary site at Nuzi, North Eastern Iraq (Henderson 1985, p 276, Vandiver 1983). High lead glazes are known from earlier contexts such as from 1700 BC Atchana in Turkey (Charleston, 1960, p 1, Caley 1962, p 85).

Increasing the lead content of a glass will also lower its melting point, and as a consequence enamels and glazes are often high in lead. Glasses containing 65 – 84 % lead oxide have melting points in the range of 740 – 760 °C (Wedepohl et al. 1995, p 65). The presence of substantial quantities of lead in glass facilitates the precipitation of cuprite to colour them red (see section 3.7.8). The lead content in

red glasses changes through time: pre- 9th C BC it is c 1%, 9th - 6th C BC it is 3 %, 6th - 3rd C BC it is 15 - 30% (Henderson 1985, p 282, p 276, Turner 1954b, p 455).

Prehistoric yellow opaque glasses are often high in lead, since they are coloured and opacified with lead antimonate (Pb2Sb2O7, see antimony section) (Brill 1988,

p 116). A high lead content facilitates the formation of tin oxide crystals when they are included as opacifying agents (Smith and Sayre 1967, p 303).

There is an unexplained presence of raised lead levels in glasses from Vergina, Greece (from the 1st Millenia BC), with a mean of 1.42 % in uncoloured glasses (which have otherwise been decoloured using antimony). One might otherwise expect lead oxide levels of 0.1 % or less. This phenomenon may be associated with higher levels of silver in 2 of the 5 glasses. The lead is missing from the contemporary Greek material from Kakouli and the Phidias workshop (Brill 1994, p 17).

Lead oxide contents of up to 44 % were found in glass being worked at Meare Lake Village West in the Late Iron Age (Henderson and Warren 1981, 1985 p 276).

Very high lead glasses were used in Anglo-Scandinavian England (up to 70 wt%) which has been suggested as a uniquely English phenomenon (Bayley 1982, p 494), although this is now untenable in light of the widespread use of high lead glasses elsewhere.

High lead glasses were recognised by Sayre and Smith as a distinct compositional group when they noted the exceptionally high lead content of glass amongst a group of Islamic glasses from 8th –10th Century AD contexts (33-40 wt%, average 36%) (Sayre and Smith 1961).

The use of lead in a batch changes the refractive index of the resultant glass: Medieval and Renaissance texts specify its use for enamels or gemstones (Theophilus, Eraclius, 15th C. Merrifield 1849, p 216, 528, 530). Ravenscroft famously invented a high lead potash composition (30-35 % PbO) for the production of “crystal” glass in 1675/6; i.e. high quality tableware previously made using a soda-rich composition. The high refractive index lends itself to cutting and polishing. It is important to note that Ravenscroft was not the inventor of high lead glasses, but did combine a number of techniques to create a profligate and emulated business (for the establishment of similar industries around Europe see Charleston 1960). Similar high-lead colourless potash glass was already being produced in Russia in the 11th – 13th Centuries (Sayre and Smith 1961, p 1826, Besberodov 1957, p 179).

An unusual combination of lead compounds for opacification of late 13th C. AD Syrian glass has been noted by Bimson and Werner (1969): a solid solution of PbSnO3 in Pb2Sb2O7. This combination has been observed elsewhere in 11th

Century AD glass from Novgorod and 14th Century AD Islamic glasses.

Whilst lead levels in medieval Islamic and Venetian enamels can be high, levels of lead over 30 % have been found for opaque white, yellow and green enamels,

but not red or blue enamels of this period. This observation led to the suggestion that enamelling on a medieval plate was forged (Carboni et al. 1998).

High lead window glass of Ayyubid date from Qas’r al-Banât (Raqqa, Syria) is the earliest known example of high-lead window glass- 66.1 % PbO (Henderson 1999 p 233)

It is possible to distinguish between antimony-rich glass recovered from Persepolis and contemporary Eastern Mediterranean sites on the basis of lead contents (these are not high-lead glasses). This has been attributed to different antimony sources (higher lead here is also associated with greater titanium and zirconium) (Smith 1963, p 285-6).

Many Early Chinese glasses have very high lead contents (c 43.2 %). During the pre-Han and Han periods (Han period = 206 BC – AD 220), these high lead glasses were frequently lead-barium silicate glasses in which barium oxide accompanies the lead (also see 2.7.24 below) (Charleston, 1960, p 1, Caley 1962, p 66, Biek and Bayley, 1979, p 17). A high lead content without the associated barium indicates that a Chinese glass is from the Han period or later (Hall and Yablonsky 1998). Very high lead-content glasses (70% PbO) are also known from 8th Century AD Japan (Henderson 1985, p 277).

Lead isotope analysis has been undertaken in an attempt to relate the lead content of ancient glasses to geological sources mined in antiquity. This technique has not been widely adopted, and there remains the problems of mapping geological

sources exploited in antiquity, and the confusion brought about by recycling of lead-containing materials may make it very difficult to identify specific sources (Henderson 2000, p 14). Relatively discrete groupings of archaeologically distinct material have been achieved (Brill 1969, 1970, Lilyquist and Brill 1993).

3.7.10 Chlorine Cl

Chlorine exists in ionic form in glass (as Cl- ions), and can only be dissolved into the glass matrix in very small quantities (typically less than 2 % in soda – lime – silica glasses). The presence of significant amounts of chlorine (up to about 1.2%) in ancient glasses is well known (Geilmann 1955, Velde and Gendron 1980, Bimson and Freestone 1983). In early glassmaking chlorine was inadvertently added via the plant ash or mineral salts used to supply the alkali, causing an immiscible scum to form on the surface of the molten glass (Turner 1956c). Removal of the scum left the glass virtually saturated in chlorine and the concentrations in ancient glass often approach the high temperature saturation concentration, measured at 1.42 wt % Cl for a soda-lime-silica glass (Bateson and Turner 1939, p 267). Since chlorine solubility in a glass decreases with an

increasing temperature, the chlorine content may be used as an approximate indicator of a melting temperature for a glass. This phenomenon has been employed to substantiate the model of reduced glass compositions relating to eutectic troughs in the soda – lime – silica phase diagrams (Rehren 2000a and b, also see 3.7 above). The manner in which chlorine is lost by vaporisation during the melting of medieval potash glasses has been detailed by Gerth et al. (1998),

explaining why relatively little chlorine may be available for inclusion in the final glass.

3.7.11 Chromium oxide