Magnesia MgO

Magnesia acts as a stabilizer in glass chemistry, and inhibits devitrification by lowering the liquidus temperature (West-Oram 1979, p 234). The substitution of lime with magnesia, up to 5 % lowers the melting point of soda – lime – silica glasses by 100 °C (Rehren 2000b, p 15). Magnesia has not been a constant

component of glasses and is therefore a useful characterising tool. It is found in the earliest glasses at levels of 2 – 6 %, but during the Roman period it was typically less than 1 %. Caley (1962, p 65) states that magnesia has been an almost universal component in ancient glasses, usually 2 – 5 %, sometimes < 1%. Early glass scientists saw the high magnesia content of 2nd Millenium BC

Egyptian glasses as an indicator of its deliberate inclusion, in the form of

Dolomitic (i.e. magnesium-rich) limestone, as the source of lime (rather than purer forms of limestone) (Forbes 1966, p 128, Caley 1962, p 79, Matson 1948, p 53 and 1951, p 84). It was Turner who was able to discern the likelihood of the magnesia in Egyptian glasses being derived from plant-ashes (Turner 1956c), based on compositional analyses of ashes. Limestone was not therefore required as a source of calcium oxide, since sands often contain a high proportion of calcium carbonate (Turner 1956b, p 176).

High MgO 2.5-6 %

Re. High K2O 2.5-3.5%

High magnesium soda lime silica glass (soda-rich

plant ash) High Na20 14-17%

Low K2O 1%

Low MgO <1%

Low magnesium, soda lime silica glass (natron)

High Na2O 14-17%

High K2O 6.5-14%

Mixed-alkali glass

Low MgO 0.4-1%

Table 3.2. Key characteristics of 2nd and 1st Millenium BC glasses.

The compositional changes in glasses from the Eastern Mediterranean around the 8th C BC defined the adoption of natron as the principle source of soda, and was

accompanied by a three-fivefold decrease in concentrations of magnesium and potassium oxide (Sayre and Smith 1967 p 281, Sayre 1963, p 269).

This picture of high-magnesia glass being replaced by low-magnesia glass in the 8th Century BC has been complicated by the investigation of 15th and 14th Century BC cobalt-coloured glasses from Egypt. Whilst this group of glasses are high in magnesia, it is suggested that this is derived from the cobalt-rich alum used as a colourant rather than a plant ash source, therefore indicating an earlier than anticipated use of natron in glass manufacture (Shortland and Tite 2000, also see 3.7.20 below).

There is evidence for continuity of production of high magnesia glass beyond the 8th Century BC around the Euphrates and Tigris valleys across to India, where it was used alongside low magnesia glasses (Sayre 1963, p 273, also Caley 1962, p 84)). During the Islamic period high-magnesia glasses becomes the dominant type used in the Levant, which may reflect the uninterrupted continuity of

compositional type from the 2nd Millenium BC (Smith 1963 p 285).

Variation in the magnesia contents of glasses from Raqqa (Syria) into discreet groups suggests that there is some variation within the soda-rich plant ash

tradition of 8th-9th Century AD Islamic glass production. It is difficult to establish for certain if this reflects either the selection of a different plant for the ash, or a distinct processing of the usual ash before use (Henderson 1996b).

During the 1st Millennia BC and AD, high magnesia glasses were also produced on the Indian subcontinent, however these can be distinguished from that

produced in the Middle East, because the Indian material is associated with higher alumina and lower lime contents (Hall and Yablonsky, 1998, p 1242).

Raised levels of magnesia are also associated with potash–rich plant ashes, and the transition to potash glasses in Medieval Europe sees a corresponding increase in the magnesia contents of glasses over the preceding Roman natron soda-lime- silica glass. The analysis of Roman and medieval glasses from the Weald clearly illustrates the difference in magnesia content: 0.07 wt% in the Roman glasses from Colchester, and 4.26 wt% from the Wealden glasses (Green and Hart 1987)

Whilst Roman enamels and tesserae were basically the same composition as other Roman glasses, white enamels have been shown to have comparatively elevated magnesium levels suggesting a plant ash source for the base glass (Biron et al. 1996 p 58)

3.7.5 Alumina Al2O3

Alumina acts as a stabiliser, inhibiting devitrification and lowering the melting point of soda-lime-silica glasses in small proportions (West-Oram 1979, p 227, Brill 1987). The presence of alumina as a major constituent improves the

durability of the glass (Cole 1966, p 46). Above 7 % alumina, the glass viscosity begins to increase rapidly, making a glass more difficult to melt. It was first used as a deliberate component in glasses in the 19th Century. Study of the thermal

history of glasses suggests that repeated re-melting of alumina containing glasses results in a relative enrichment in the alumina levels compared with other

components (Jackson 1996, p 292-293). Alumina is a universal component of ancient glasses, usually in the range of 1 – 5 % (Caley 1962, p 65). Early reports of glass analyses before 1953 may tend to over-estimate the alumina content at the expense of the calcium oxide levels. Until Geilmann and Jeneman’s 1953 paper identifying phosphates in glass, the unidentified P2O5 would have been

precipitated out as calcium phosphate, which would be counted as alumina, leading to underestimation of calcium oxide content and exaggeration of the alumina (Geilmann and Jeneman 1953, Caley 1962, p 54).

Analyses of sand have demonstrated that alumina in glasses may easily be derived from this silica source. For example, samples taken from the beach at Volturno in Italy have alumina contents as high as 12.5 %, although it is has not been clearly demonstrated if this particular sand was ever used in glassmaking (Brill 1999b, p 475). Analyses of Indian glassmaking sands have given values of 7.17 %, and a silica:alumina ratio of 11:9, which matches the high alumina compositions of many Indian glasses (Brill 1987 p 7 and p 22). Indian glasses from Kopia from 3rd C BC – 3rd C. AD have consistent compositions and are characterised by their high alumina content (mean = 6.67 %), low lime and magnesia (2.93 and 1.61 % respectively) (Caley 1962, p 86, Brill 1987).

The alumina contents of natron are very low, typically less than 0.5 %, and whilst plant ashes have variable compositions, ranging from 0.45 – 5.9 %, they are typically less than 2 % (Brill 1999b p 482 – 486). Roman glass invariably contains

2.5 % +/- 0.5% alumina (Henderson 1999), which is consistent with the use of sands such as that from the River Belus (Brill 1988). Modern glassmaking sands can contain up to 2 % alumina (Gould and Hampton 1930).

A number of studies have examined the contribution of the crucibles employed in glass-melting to the final glass composition (Forbes 1966, p 118), with alumina possibly originating in alumino-silicates in the refractory clays used for making crucibles. Recent work investigating the glass-ceramic interface of excavated refractory materials have identified that there is a slight enrichment in alumina in the glass immediately adjacent to the crucible (Velde 1990, p 112, Merchant et al. 1998). However, the contaminated zone is approximately 0.5 mm thick, and does not contribute a significant amount of alumina into the main body of glass melt. In a study of Roman glasses from France, Velde cannot identify any other

component correlated to the alumina , and suggests a possible independent mineral source in the raw ingredients. However, this seems unlikely since the value of alumina to glass compositional stability was not known until the 19th Century: Brill’s experimental work with Belus sand and natron to reproduce glass compositions from Jalame suggest that the alumina content in Roman glasses is most likely to be derived from the sand (Brill 1988).

Glass from late 8th- early 9th C Islamic contexts at Raqqa (, Syria) has been noted as a variation upon the “Roman” type natron glass: it has a high alumina content (range 1 – 4 %), but low magnesia levels suggesting the soda is from a mineral source (Henderson 1999). This range is still within the reported glass analyses for the Roman period in Northern Europe, which are identified as natron soda-lime-

silica glasses (Velde 1990, p 114, table 5). The low alumina-high MgO

characteristic of 12th C Islamic glasses, are present in Raqqa in late 8th-early 9th C material ahead of rest of Middle East’s transition to this type (Henderson 1999). It has been suggested that the lower viscosity arising from the reduced alumina levels of the new recipe facilitated the manufacture of cast artefacts, leading to the widespread adoption of the low alumina glass type (McLoughlin et al. 2001). High levels of alumina noted in blue enamel on a “13th Century” Persian plate helped to inform the decision that this was faked in the 19th or early 20th Century (Carboni et al. 1998, p 90 - 91).

3.7.6 Calcium oxide