Archaeotechn%gy

The Analysis of Ancient Glasses Part II: Luxury Roman and Early Medieval Glasses

Julian Henderson

Author's Note: In Part I (November 1995 JOM, pages 62--64), ancient glass as a material was described, including the raw materials used for its production and its resulting working properties. Some scientific techniques of analysis were out­lined, as were the ways in which they were used in the investigation of ancient glass. The gross compositions, the trace impurities, the colorants, and the opacifiers can all he identified and quantified using a range of techniques; this can lead to clearly defined characteristics for glass made at specific times and in specific regiOns and places. The first example of glass production discussed was prehistoric Ital­ian glass-the first true European glass produced.

LUXURY ROMAN GLASS

the opaque white glass contains a rela- neither the opaque nor translucent tively high level of lead oxide4 and is glasses used to make them contained a opacified with calcium antimonate crys- high level of lead oxide. It is, therefore, tals. possible that the stock of opaque white

A series of analyses of opaque white glass used for the manufacture of large glasses used in the manufacture of mo- luxury glass vessels such as the Portland saic pillar-molded ribbed bowls (some vase and the Kunsthistorische Museum of which probably date to the late 1st vessel, both probably made in the Medi-century B.C., and the rest date to the 1st terranean world, was manufactured in century A.D.) has revealed that the opaque the same tradition, irrespective of vessel white glass used contains significantly form. To the author's knowledge, none

Although a limited amount of analyt- lowerlevels oflead oxideS and can easily of the common mosaic-ribbed bowls ical research has been carried out on be distinguished from the glass used in have been found to contain opaque white luxury Roman glass vessels, the small the Portland vase and the vessel in glass with high-lead oxide; this distinc-amount that has been done is instructive Vienna. This simple distinction appears tion shows that there was a difference in in that it shows interesting compositional to be related to the difference in the workshop practice reflected in an in-characteristics that must be explained. technology used. The Portland vase of creased volume of production. The Perhaps one of the best known --------------------- ribbed bowls continued to be Roman glass vessels is the Port- common in the 1st century A.D.

land vase.! It has recently re- Another luxury Roman glass ceived media attention because vessel that exhibits the refined it has been restored, and its his- use of colorants to produce an tory has been thoroughly re- impressive visual result is the searched. Partly through the use Lycurgus cup. When viewed in of art-historical factors, the Port- reflected light, the Lycurgus cup land vase can probably be dated appears to be a pale green color; to the late 1st century B.C. During in transmitted light, however, it the time that the British Museum appears to be a pink color (dichro-carried out the restoration, small ism). In physical terms, the effect samples of glass were removed can be explained by the presence for chemical analysis.2 The blue of minute par tides of gold-silver matrix of the vase was found to alloy that refract light, and be-be colored by cobalt oxide, and cause thepartides are so small in the scene carved on the outside transmitted light, the matrix glass in opaque white glass was found Figure 1. A scanning electron micrograph of opacifying cuprite allows light to be transmitted,

crystals in early Roman opaque liverish-red glass. Each crystal to be colored and opacified by but it also modifies the color.6

is -111m across. calcium antimonate in a glass Placed in a broader context, the containing 12.48% lead oxide. The white the 1st century B.C. was cast and cased, coloration of many early Roman red glass would have been carved into a with the opaque white glass being glasses is due to a dispersion of opaque decorative frieze on the outside of the carved; the Viennese glass vessel, al- red crystals within the glass (Figure 1). vessel, so it is evident that the relative though probably not carved, was highly The composition of these crystals is the softness of the high-lead glass was ex- polished so the relative softness imparted reduced form of copper (cuprite or cu-ploited and that the glass artisans un- by the high-lead oxide level would have prous oxide) and sometimes metallic derstood their materials well. been used by the glass artisans. copper. Like dichroic glasses, opaque

Another vessel that possibly dates to Whereas the Portland vase is arguably red glasses were difficult to produce, the 1st century B.C., and which incorpo- the most famous Roman glass vessel particularly when it came to the control rates opaque white glass, is currently in because of its intricately carved scene of the oxidizing-reducing conditions in the Kunsthistorische Museum in Vienna, and the Kunsthistorische Museum ves- the glass furnace. Other factors that in-Austria. It is a large spherical two-hand- selcan also be considered a tour de force, fluenced the degree of success in pro-led mosaic jar that had been cast and the mosaic-ribbed bowls with which we ducing opacifying crystals were the max-polished.3 Here, the opaque white glass compared them are considerably more imum temperatures achieved, the bulk is marbled with translucent purple and common. The mosaic-ribbed bowls were composition of the glass, and the heat brown glass, colored mainly by manga- mass-produced, and, although they were treatment used to develop the crystals. nese and iron oxides, respectively; again, cast, they were not carved or polished; The crystals that were developed in

Over the last 30 years, there has been a discemible increase in the number of scholars who have focused their research on early industrial organizations, a field of study thet has come to be known as Archaeoleehnology. Archaeologists have conducted fieldwork geared to the study of ancient technOlogies in a cultural context and have drawn on the laboratory analysesdeveioped by materials scientists as one portion of their interpretive program. Papm forthis bimonthly departmenl are solicited and reviewed byRobert M. Ehrenreich of the Natloflal Matenals Advisory Board of the National Research Council.

62 JOM • February 1996

Glass is a complex malerial thai nOlTllaily exl$ls il'l a single physical phase .containing lI1ixw_Of .eIemen1 oxides, whichdetenil'l$ itsmellirlgaflddeningpoifltS, working period, weng1h, vIsoosHy, QQIor ,,durability. transparency, afld opacity, It has been used for the manufacture Of vessels, WIndows, ertamets, 1esseree, and glazes.

After some experimenlaliQn, 1he$8lecliM ot raw materials to makeijlass . fusiOl'l al'ldwmking 1emperalUres led wide ranged arlifac1 fons and the d~lopment of a wide rMge of colors. During each cultural phase, people ctmse to manufacture glass not only of a __ .sIlape,. DuI also of a characteristic.chemical composition.

In the seieAtific irwesIigaliol'lo! _~micaloom­positiOlls can ofI&rtthoW1ha1~hindsotfaWma1eri* used chMgi!d _time, piOvimng the artaly$1 with a chemical flng&lprint forglass~on ata particUlar timeal'ldplace. Many~wet&Quit$conse~ about selactlrtg raw mal&ftal$ for glassmanufactura, afld it is, therafore, possil:lle to pfa.Ce'EI glass ~QSi. tiM into a technological amlior ,dbrOOotogiCltll!lrallket accordil'l9 to the kwefs,ot ma,klr. millOr, aI'Id trace eIe­ment oxidesPre&ent, ~u~ot apta~ as~ I1therthan a mil'l$ral aIkaIisourcei~.~ millOr and trace elements Into the glass ., which provides a means of characterizll'l9 the glass. Sand. the princiPle sHica source in aooieI1t glasS,is also. associat$d with various ievels1lf mineral impurities. whichprov!d!!s another way in which the glass ' •. I!ie Characterized. Locating the original SOl/rca otthe sal'ld is almost out Of the queStiOl'l, given the .complex depo$itiOl'laibistories of many sand deposits.

opaque red glasses were, however, sig­nificantly larger than the dispersion of metal droplets contained in the Lycurgus cup. When opaque (sealing-wax) red cuprite glass is full of dendritic crystals, it is rendered totally opaque; reddish glass of a browner ("liverish") hue nor­mally used by the Romans, however, contains smaller crystals, and the over­all color is likely to be dominated more by the major components in the clear glass matrix than by the crystals in trans­mitted light?

The extent to which Roman glass mak­ers were able to control the environment of glass production deliberately is, to some extent, an open question. It is, nev­ertheless, an undeniable fact that the raw materials, both major components and the colorants, used to make a wide range of utilitarian and luxury glasses were closely controlled.

EARLY MEDIEVAL GLASS IN EUROPE

An interesting example of the use of chemical analysis to investigate ancient glass is the study of Anglo-Saxon and Viking-age material. Much of the dis­cussion that follows is based on data that is about to be published or is unpub­lished.

As described in Part I of this series, some Bronze Age glass is of a soda-lime­silica (SLS) composition, with a high magnesia content-between -2.5% and 5.0% (HMG); Roman-age glass is princi-

1996 February • JOM

SERIES SUMMARY

At atlird leval.1he u5eof glass colorants, opacHiers, al'ld d&cotorantsioften .rtvlnlJ either diredly or il'ldi­r&mlyfrom mimlrais,also inti'Odu<:e$iln.intsresl!ng de . ot trace eIemeI1~. There are. therefore, alleas11hree tevel$.~ ~lCh glaSs can be chemically characteiized. Givensuffieient ~ data for W&II-daIed arM chaeotogK:al·. IJtasses, it becomes relatively easy to identi\' a~e Ot a later copy of a glass vessel.

Thell!li1lrl1ifk1 analysis of elK9Pean Bron;r:& Age glass has< f$Vea1ed theexist&rte& otan entirely new . composition ilia wodd 00I'I1exI. Its COI'Iten1is charac­teriZedby mi~alkalis.lowmagrtesia,and tow cafciUm oxide, This composition has been found in a large concentration1lfglass in I'IOI1l1em Italy atthe late 111h-10th <rE!Rtllry B.C. site of Frattesina, where the scale of \he~~ issooh thatitcol'lSti\utes 01'1$01 the mQst importent aooiem giass-worklng Site&. The especially .Importent f&alure of the glass is thai it is en!irely diflerantfr~ gll\$$ used in Graec&, Egypt, al'ld Mesopotamts in1he 2nd millennlum B.C.

Whlle the scafe of glass prOdwfron was oIlviously larger in the .RQman~, .a_nation of IIlXUry

. Roman glass _as rMaled.ahigb level of sophistication an~$!*ial~ ill.1he.I/ldI.l$llY •. WI1en .. the klmoQs calVed OpaqIje white 0I4e' face CIt the Por'lland vase is ~~IIYWlthopaqlJ&whitegla$Sused for . . • .. of rltJbedplllar mOlded boWls, it is .. . higlllead oxideoomellt oi the Ponial'ld vasewasdeliberarely usedl!lecatJse H would have I!Ieen so1lerto ~.lhan the white glass used in \he d&oora­tiM ot.pillar·moIdedbqwls;fI:ie IaIlerhas beerI fOlJrld to conI!dn~igibleleVelspfleadoxide •. Anoiherexample ot abigllleVel.oi sophtslicalion in Roman glass techno!-

pally soda-lime, but with a low magne­sia content-between -0.5% and 1.0% (LMG). Much Anglo-Saxon vessel glass is also of this "Roman" SLS composi­tional type,8 which raises the obvious, though problematic, question: Is Anglo­Saxon glass recycled Roman glass or did Anglo-Saxon glass houses continue to use the same basic raw materials? A low potassium oxide content in Roman glass (less than 1 %) accompanies the low mag­nesia level-this means that the prin­ciple alkali is likely to be a mineral form of soda or a mineral alkali with the same chemical characteristics (see Table I in Part I). Occasionally, there are excep­tions when we find HMG in the Roman period, such as some examples from Sil­chester, England.9 Conventionally, this would infer that the glass was derived from the Middle EastlO-which may in­deed be the case, but it would still lie within the Roman Empire.

Among the Anglo-Saxon glasses ana­lyzed, there are a small number of ex­ceptional compositions possessing el­evated potassium oxide levels, particu­larly among red glasses. Analysis of Roman red enamels and glass mosaic tessera has revealed that similar anoma­lous compositions were in use between the 1st and 4th centuries for both. For some reason-possibly because of the technological difficulties of making it­the red glass used for Roman enamel has this unusual chemical composition, so it is well characterized and easily distin-

ogy is the inclusion of minute droplets of metal or metal alloys to produce rooy glasses, imparting a dichroic effect in the lyoorgus cup (1he glass seems to be a differem color in reflected and transmitted light).

At the eI'Id of the first millennium AD., European early medieval glass is often considered to be of a soda-lime­silica (so-called "Roman") compositiol'l and to provide EMdenceforthe continuation ll'lthe use of Roman glass. There are now specifIC examples, particularly in red glasslertamel, where this has been foundto be the case; there are also now examples of early medleval glasses where trace element oxides il'ldicate that the Roman .glass was. at least, modified in Us composition. K not made indepel'ldenUy.

By the end oi the first minertnlum A.D., in the 9th and mUI centuries, glass was l!ieil'l9 used that appears to re!led:a changeoverkornthe Roman composition tolhe High Medieval composition in Europe and can be con­sidered technologically transitional. Here, glasses used tOt wil'ldows, vassels, and beads from recent el(cava­tiOl'lS have been foul'ld to contain alkali and lime IElI/eis that can be described as transitional. The explanation offered is that demafld .000tstripped supply oi the awi!­aIlle glass raw materials and scrap glass ill the 9th and 10th centuries and thet new raw materials wera added tothe existing stocks. This indiCaIesthat experimentation with raw materials look place. It is also possil:lle that a political disruption of the alkali supply (perhaPS the Islamicisation of northern Africa forced glass makers to seek alterrtative alkali sourcas). The rise in population, coupled with the demal'ld for window glass in cathe­drals, evantually l!Irougllt aIlout a complete change to the use ot potassium-rich wood ash.

guished from the compositions of other colors of glass. In this instance, it is highly likely that Roman red glass was recycled, although if we look at a Continental ex­ample (e.g., thatoftheMerovingian [6th-7th century], Netherlands), the coloring technology has been adopted, but there is a local variant. l1 From the medieval period in Europe, there is some evidence that Roman opaque (tesserae) glasses were used in Limoges enamels.12

It is when Viking-age glass of the later first millennium A.D. (9th to 10th centu­ries) is examined that one detects dis­tinct variations from the "normal" (rela­tively tightly defined in compositional terms) Roman or Anglo-Saxon soda­lime-silica glass compositions. These differences, so far, have only been found to have occurred in some English and Scandinavian glasses.13 When compared with High Medieval glass compositions from York (for example),14 the Viking­age low-lead glass is found to be transi­tional between Anglo-Saxon and High Medieval compositions. The transitional Viking-age glass is of a mixed-alkali composition; it, therefore, contains both potassium oxide, which is normally the predominant alkali in High Medieval glass, and the soda-lime glass charac­teristic to most Anglo-Saxon and Roman glass. In addition, while Anglo-Saxon glass tends to contain -6.5-8.0% calcium oxide and High Medieval glass contains -14-24% calcium oxide, some Viking­age glasses contain -11-16% calcium

63

oxide. which is also a transitional char­acteristic.

How can we explain such a radical compositional change? A possible ex­planation for the occurrence of compo­sitionally transitional Viking-age glass is that soda (probably natron) was in short supply and had to be supplemented with terrestrial plant ash as an alkali source, which would be potassium-rich. Maritime plants used as a soda source would be characterized by elevated MgO and Kp levels,tS as found in HMG (see Part I). Perhaps an extra demand for glass was created by the construction of medieval cathedrals and the need to make window glass, as well as because of the rapid rise in the population of Europe. This new information certainly infers that glass artisans were experi­menting with new recipes as a response to a change in the scale of production and the demand for glass.

CONCLUSIONS

Overall, the scientific study of ancient glass clearly provides data that infer a complex organization for ancient glass industries, and it is difficult to find par­allels amongst the organization of other industries (ancient metal production,for example, involves a relatively restricted range of raw materials). The wide range of visual effects that can be produced in glass result from the skillful use and

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manipulation of glass raw materials and the interaction of light rays with the glass. The incorporation of impurities in the glass batch as part of the glass pro­duction ritual may have occurred be­cause secret recipes were passed from one generation of glass artisans to the next. The effect might be observed in the finished product; but apart from textual evidence, there is no scientific way of identifying the presence of additives that were incorporated as part of a ritual.

ACKNOWLEDGEMENT

I am very grateful to Drs. Colleen Batey and Hilary Cool for reading through an early draft of this paper and for making very help­ful comments.

References 1. W. Gugenrath, K. Painter, and D. Whitehouse, "The Port­land Vasen," J. Glass Studies, 32 (1990), pp. 24-62. 2. M. Bimson and I.e. Freestone, "An Analytical Study of the Portland Vase and Other Roman Cameo Glasses," J. Glass Studies, 25 (1983), pp. 55-64. 3. D.F. Grose, The Toledo Museum of Art. Early Ancient Glass (New York: Hudson Hills Press, 1989), pp. 259-260 and Fig. 152. 4. Unpublished electron microprobe analyses by the author. 5. Unpublished electron microprobe analyses by the author. 6. RH. Brill, "The Chemistry of the Lycurgus Cup," Proceed­ings of the VII International Congress on Gblss (Brussels, Bel­gium: National Institute of Glass, 1965), pp. 223.1-223.12; j. Barber and I.e. Freestone, "An Investigation of the Origin of the Color of the Lycurgus Cup by Analytical Transmission Electron Microscopy," Archaeometry, 21 (1) (1990), pp. 33-45. 7. j. Henderson, "Chemical Characterization of Roman Glass Vessels, Enamels and Tesserae," Materials Issues in Art and Archaeology II (Pittsburgh, PA: MRS, 1990), pp. 601-608. 8. D.e.W. Sanderson,j.R Hunter, and S.E. Warren, "Energy­Dispersive X-ray Fluorescence Analysis of 1st Millennium AD. Glass from Britain," J. Archaeological Science, 11 (1) (1984), pp. 53-69; M. Heyworth et al., "The Role of Induc-

tively Coupled Plasma Spectroscopy in Glass Provenance Studies," Archaeometry (Amsterdam, Netherlands: Elsevier, 1989), pp. 661-670; e. jackson, "A Compositional Analysis of RomanandEarlyPost-RomanGlassand GlassworkingWaste from Selected British Sites," unpublished PhD. thesis, Uni­versity of Bradford (1992); M.j. Baxter et al., "Compositional Variability in Colourless Roman Vessel Glass," Archaeometry, 37 (1) (1995), pp. 129-142. 9. The analytical results and discussion by j. Henderson will be published in the full excavation report by M. Fulford: Excavations at Silchester. 10. E.V. Sayre and R W. Smith, "Compositional Categories of Ancient Glass," Science, 133 Gune 9,1961), pp. 1824-1826; E.V. Sayre and R W. Smith, "Some Materials of Glass Manu­facturing in Antiquity," Archaeological Chemistry. A Sympo­sium, ed. M. Levey (phiiadelphia,PA: University ofPennsyl­vania Press, 1967), pp. 279-312. 11. Y. Sablerolles, j. Henderson, and W. Dijkman, "Early Medieval Glass Bead Making in Maastricht Godestraat 30), The Netherlands" (1995); An Archaeological and Scientific Investigation," Actes des Perlensymposiums in Mannheim, ed. U. von Freeden (Romisch-Gennanische Komission, 1996). 12. j. Henderson and I. Holand, "The Glass from Borg, An Early Medieval Chieftain's Farm in Northern Norway," Me­dieval Archaeology, 36 (1992), p. 38. 13. Glass of this composition has been found at Peel Castle, Isle of Man: j. Henderson, "An Archaeological and Scientific Study of the Glass Beads from Peel Castle," Excavations at Peel Castle,IsleofMan, University of Liverpool Monograph,ed. D. Freke (in press); and Lurk Lane, Beverley, Humberside, England: j. Henderson, "The Glass," Excavations at Lurk Lane, Beverley 1979-82, ed. Peter Annstrong, David Tom­linson, and D.H. Evans, Sheffield ExcavationsReport 1 (1991), pp. 124-130, and Mf.I. B6-C1. 14. K.j.S. Gillies and A Cox, "Decay of Medieval Stained Glassat York, Canterbury and Carlisle," Glastechniche Benchte, 61 (3) (1988), pp. 75--84. 15. RH. Brill, "The Chemical Interpretation of the Texts," Glass and Glassmaking in Ancient Mesopotamia, ed. AL. Oppenheim et al. (Corning, NY: Corning Museum of Glass, 1970); j. Henderson, "Electron Probe MicroanalysesofMixed­Alkali Glasses," Archaeometry, 30 (1) (1988), pp. 77-91.

Julian Henderson is a lecturer in scientific archaeology at Nottingham University.

For more information, contact J. Henderson, De­partment of Archaeology, University of Notting­ham, University Park, Nottingham NG7 2RD, United Kingdom; telephone 0115-951-4814; fax 0115-951-4812.

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