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Colour and chemical composition in ancient glass: An examination of some roman and wealden glass by means of ultraviolet-visible-infra-red spectrometry and electron microprobe analysis
Jo~mml ~f’Archcmhgica1
Science 1987, 14, 271-282
Colour and Chemical Composition in Ancient Glass: An Examination of some Roman and Wealden Glass by means of Ultraviolet-VisibleTnfra-red Spectrometry and Electron Microprobe Analysis Lorna R. Greena,band F. Alan Hart0 (Received 28 March 1986, revised manuscript accepted 17 September 1986) sherds from the Roman town at Colchester and from the post-mediaeval glasshouse site at Knightons near Dunsfold in Surrey have been investigated by ultraviolet-visible-infra-red spectrometry and by electron microprobe analysis. The glass is coloured by visible absorption bands of iron, cobalt and manganese, whose origins are discussed.
Glass
Keyword.~:ULTRAVIOLET, VISIBLE, INFRA-RED, SPECTROSCOPY, ELECTRON MICROPROBE, COLOURED GLASS, ROMAN, POSTMEDIAEVAL, IRON, COBALT, MANGANESE.
Introduction There is an interesting relationship between the colours of ancient glasses, the technology of their manufacture and their chemical composition. The considerable variation of colour which is observed in different samples may be due to the presence of small quantities of transition metal ions such as iron, cobalt or manganese in particular oxidation states. The presence of such ions and the consequent colour of the glass may be either involuntary or intentional and reflects the state of technology current at the time of manufacture. The technique of ultraviolet-visible-infra-red (uv-vis-ir) spectrometry offers considerable advantages in investigations of ancient glasses but has been rather little used, although valuable studies of 15th- and 16th-century German forest glasses (Sellner et al., 1979) and 4th-century Galilean glass (Schreurs & Brill, 1984) included uvvvis-ir and uvvvis spectra, respectively. Advances in modern instrumentation enable a well-resolved spectrum to be obtained without any damage or alteration to the specimen whatever, even if its surface is not in good condition. Minimum sample sizes are about 1 cm2, or even much smaller if special optics are used. Uv-vis-ir spectrometry provides information on concentrations, oxidation states and chemical environments of a number of species, mainly transition metal ions present in the glass. “Department ofchemistry, Queen Mary England. ‘Present address: Research Laboratory, 3DG. England.
College, The
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The objectives of the present work were two-fold. Firstly, to explore further to what extent useful information about ancient glasses can be obtained by uv-vis-ir spectrometry, and secondly, in conjunction with elemental analysis of the glass composition by the electron microprobe, to investigate in some detail two groups of glass fragments recovered from (1) the Culver Street excavation in the Roman town at Colchester and (2) the Wealden glassworking site at Knightons near Dunsfold, Surrey. Group (1) consisted of fragments of about l-7 cm greatest diameter which appear to belong to a variety of vessels ranging from quite finely made and decorated small flasks to larger, thick, square bottles, together with thin, flat fragments, perhaps window glass. The condition of the glass was generally very good with slight to moderate surface iridescence. The colours were typical of Roman glass in their variety, mainly pale bluish-green, but also pale blue to deep blue, nearly colourless, red-purple, yellow-green and yellow-brown. The glass sherds came from contexts whose stratigraphic date periods ranged from pre-Flavian to 350 + AD, The Wealden group (2) consisted of fragments about 3-7 cm in greatest diameter and usually l-2 mm thick. These fragments consisted mainly of crown, bottle kicks, muff and cullet. This glass was less well preserved than the Roman, often showing considerable surface deterioration. Colours were predominantly pale slightly bluish green, but with darker green, nearly colourless, blue and red-purple pieces also present. The dating of the Wealden site is mid- 16th century (Wood, 1982). The Origin of the Absorption Bands The colours of glasses as perceived by the eye are caused by the absorption of some wavelengths of the visible light which is transmitted through them, the complementary colour being observed. Thus, glass which absorbs at (say) 450 nm in the blue region of the visible spectrum (taken as 40&700 nm) will appear red. The alkali and alkaline earth silicate matrix of most glasses is inherently colourless, but the presence of small quantities of absorbing species, which in the present discussion are ions of the transition metals iron, manganese or cobalt, produces regions of absorption (bands) which impart colour. There may also be bands in the infra-red and ultraviolet regions which are similarly produced and which may be informative, but these of course do not influence the colour. Absorption bands in the electronic spectra of transition metal compounds may be produced by two main processes. The energies of the excited (high energy) states of the d electrons belonging to a transition metal ion such as iron, cobalt, or manganese in a particular oxidation state vary according to the nature and disposition of the other atoms (ligands) surrounding the metal ion. The d electrons can be excited from their normal ground state to the higher energy levels by photons of light of appropriate energies, leading to absorption bands (known as d-d bands) at specific wavelengths. The energy levels are usually depicted graphically in a Tanabe-Sugano diagram in terms of parameters A and B, and electronic transitions between energy levels are described in terms of symbols derived from group theory, e.g. 6A 1+4E. Absorption bands can also be produced by an entirely different process, namely the transfer of an electron between a metal ion and a ligand. These bands (known as charge-transfer bands) are fully allowed quantum mechanically and are very intense, in contrast to the d-d bands which are much weaker. For a full account of transition metal spectroscopy, see Lever (1984). Iron-tinted Glass There are four regions of absorption attributable to the presence of iron in glass, and the nature of the absorbing species responsible for each of these will be discussed further below. The four absorptions are (1) a strong broad band centred on 1100 nm due to Fe’+;
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(2) a strong broad band centred on 405 nm due to an Fe3+ species; (3) a series of weak, sharp bands mainly in the region 375-450 nm, due to Fe3+; and (4) a very strong band at 258 nm due to Fe3+. These bands affect the perceived colour of the glass as follows. Band (l), which is centred in the infra-red, tails into the visible and imparts a fairly pale, pure blue tint. Band (2) would by itself give a red tint but normally occurs together with (l), giving green, amber or brown shades depending on the relative strengths of (1) and (2). The bands (3) and (4) give a very pale lemon yellow colour but are normally both present together with (l), when they impart a greenish-blue tint, or together with (2) and (1) when their effect is negligible. Band (4) is in the ultraviolet but tails slightly into the visible and is chiefly responsible for the combined effect of (3) and (4) on observed colour. Spectra which illustrate these bands are shown in Figures 1 and 2. All four types of absorption were observed in the Roman glass, but (2) was not observed in the Wealden samples. Types (3) and (4), although well known in coordination compounds of iron (Lever, 1984) seem not to have been discussed previously in relation to ancient glasses, while types (1) and (2) have, however, been considered (Schreurs & Brill, 1984). Representative spectra are shown in Figures 1 and 2. Of 78 Roman samples, 64 showed absorptions (l), (3) and (4) only, while (2) additionally appeared in nine others, where it tended to obscure (3). The Fe 2+ band (1) was absent from only five samples (four nearly colourless and one purple). In the Wealden group, of 38 spectra all showed bands (l), (3) and (4) except one red sample where (1) was absent and (3) was obscured. In order to consider further the relationship between the presence of particular iron absorption bands and ancient glassmaking practice, it is necessary to discuss the nature of the absorbing species. These will be considered in sequence of increasing wavelength. An intense absorption is produced by Fe3+ with A,,,=258 nm. The reflectance spectrum of a very pale lemon yellow synthetic glass, made at 1100 C in a platinum crucible from 99.99% (or better) constituents containing 80% SiO, and 20% Na,O to which was added 0.55% Fe,O,, showed a peak at 258 nm which is undoubtedly the Fe3+ charge-transfer band. This type of absorption is fully allowed by the selection rules and corresponds broadly with the transfer of an electron from an oxygen atom to Fe3+. In transmission spectra at this concentration of Fe3+, transmittance was zero below about 305 nm. Three fairly sharp, weak bands appear at 377,418 and 431 nm. These are the Laporteforbidden and spin-forbidden dcl crystal field bands of Fe3 + and have been observed in synthetic glasses (Kurkjian & Sigety, 1968). The Fe 3+ ion is believed, on the evidence of Miissbauer spectra, to be in a tetrahedral environment. The absorption bands may be assigned to transitions from the ‘jA, ground state to 4T2(D), 4E+4A, and 4T,(G), respectively. This is not the place to discuss these state assignments in detail, but they cannot be regarded as conclusive owing to the complex nature of Fe3+ spectra. It should be noted, however, that a predominant tetrahedral environment for Fe3+ is not inconsistent with the octahedral environment reported here for Mn3+ as the latter ion is crystal field stabilized by 0.6A in an octahedral environment while Fe3+ is not. The intense band which appears in some samples of ancient glass at 405 nm has been the subject of a recent report (Schreurs & Brill, 1984). This band has for long been considered to be due to an Fe3+/S2- species (see Weyl, 1951, for the older evidence) whose exact nature was unknown. We nevertheless thought it advisable to confirm this assignment by synthesizing glasses of 80% SiO, and 20% Na,O which (a) contained no additive, (b) contained 0.5% Fe3+ (as Fe,O,), (c) contained 0.5% S2- (as Na,S), and (d) contained 0.5% Fe3’ and 0.5% S2-. Only (d) showed the characteristic intense 405 nm band, although interestingly, (b) showed it also when a graphite crucible-doubtless containing minute quantities of sulphur-was used for the melting process instead of the platinum crucible. The exact nature of the chromophore is uncertain, but we consider it most likely
L. R. GREEN
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i
Wavelength
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Figure 1. Representative spectra of Roman glasses. Both Figures I and 2 are shown as percentage absorption, as this probably gives a better impression of the colour and condition of the glass, although an absorbance presentation would in some cases give a technically better spectrum. In studying these spectra, it should be noted that the visible region is usually taken as 40&700 nm. In each figure, band systems are as follows: 1, Fe’+; 2, Fe3+/sulphur; 3, Fe3+; 4, Fe3 + charge transfer; 5 and 6, Co’+; 7, Mn3+. In spectra of Figures l(d) and 2(e) a Mn3+ charge transfer band is expected to contribute to the intensity of band 4. Sample a: Pale greenishblue; flat; c. 2.5 mm thick; one side smooth but slightly undulating, other side flat but rough; bubbles immediately under smooth side, hence cooled on refractory surface. Probably window glass. Spectrum, showing only band systems 1,2 and 3, is a common type. Colchester reference 1.8 1 C 73; stratigraphic date ofcontext, 350+ AD. Sample h: Yellow, c. 2.2 mm thick. Spectrum shows iron bands as in sample a above, but is yellow rather than blue because of the iron-sulphur band 4.1.8 1 C 148, 1 If&300 AD. Sample c: Pale mid-blue, 4.5-5.7 mm thick, probably from square bottle. Band systems I, 2,3 present together with the cobalt bands 5 which alter the colour from that of sample a to pure blue. 1.81 B 964, 65-110 AD. Sample d: Purple, l.lL1.3 mm thick. The spectrum is dominated by the manganese band 7. 1.1% manganese was present, 1.81 A 360; 4960/l AD. Sample e: Deep blue, 1.551.7 mm thick. The deep blue samples are mostly thin and curved, probably from small enclosed vessels. The spectrum shows strong bands of cobalt, 5 and 6. I.81 B 952,65-l IO AD.
to be as in formula (1) when a fully allowed charge-transfer band would be expected at about the observed wavelength, but other absorbing iron-sulphur species cannot be excluded from consideration:
-O\ - O-Si-S -O/
--Fe3
+.
Formula (1)
Interestingly, in an experiment similar to (d) above, in a platinum crucible, but with 99.99% purity carbon powder finely mixed with the other materials, the 405 nm band did
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Cd)
00
2000 Wavelength
i jj
500
1000
1500
2000
(nm)
Figure 2. Representative spectra of Wealdcn glasses. Sample u: Bluish-green bottle kick, uneven thickness c. 2 mm. This spectrum is typical of this group and shows band systems I, 2 and 3 together with the cobalt bands 5 which would be expected to affect the colour of this sample. From the absorbance at 670 nm, the cobalt concentration is estimated as 25 ppm. Guildford Museum identification AS4803. Sample h: Blue-green rim sherd, c. 1.5 mm thick. Similar to a but more cobalt. Guildford Museum identilication AS4801. Sample c: Deep blue glass. Flat 3 cm sherd, 4 mm thick. Dominated by cobalt bands 5; also present are 1, 3, 4 and 6. Guildford Museum identification AS4798. Sum& d: Almost colourless yellow-green sherd, 1 mm thick. Rather weak Fe” band, but contains concentrations of iron and manganese close to the group means. Cobalt must be virtually absent. Guildford Museum reference AS4801, Sample e: Deep purplish red, flat, I.5 mm thick. Rendered largely opaque by an intense Mn3+ band 7. Contains 2.0% Mn. Guildford Museum reference AS4799.
not develop, the glass being a pure blue, presumably owing to complete reduction of the iron to Fe2+, thus destroying the chromophore. The strong broad band centred at 1100 nm is undoubtedly the 5T29+5E, spin-allowed crystal field transition of the octahedral d6 ion Fe 2+ The very broad character of this band is probably due to Jahn-Teller splitting of the excited state. The A value of 9090 cm ’ is very similar to that observed for [Fe(H,0)6]2+. As the Fezi/Fe3+ ratios were not estimated in this work, the extinction coefficient for this band can only be stated as a lower limit, namely E> 15. This is rather a high value, suggestive of a somewhat unsymmetrical coordination. There must always be some doubt as to whether the various glass colours were produced in an uncontrolled way or were intended. It may be thought reasonable to assume that they were intended, because glass manufacture, as an inherently costly process on account of the volume of wood and degree of skill needed, would be expected to have employed carefully standardized procedures which would normally have produced uniform products.
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in controlling the iron colours just discussed, the ratio of Fe’+ to Fe3+ is of course important. Both Fe’+ and Fe3+ may coexist in glass because although Fe3 + is somewhat more stable, furnace atmospheres can be sufficiently reducing to stabilize a proportion as the dipositive species, and carbon may have been initially present if wood ash was used as a flux. In the case of the nearly colourless Roman samples, three were analysed as described below and contained an average of 0.23% iron. This value is within one standard deviation of the mean iron content of all the Roman samples, and hence the lack of colour was obtained by use of oxidizing conditions rather than use of iron-free constituents. To eliminate any blue colour due to Fe2 + , it would presumably be sufficient to ensure that the reducing furnace gases did not pass over the surface of the molten glass but that air had some access to it for a prolonged period. It seems certain that MnO, was not added as oxidant because the concentration of manganese averaged only 0.04% in these samples. Indeed, a higher concentration might be undesirable if oxidation by air was used, as oxidation to reddish Mn3+ might occur. These remarks do not in any way suggest that the addition of MnO, is not a practical method of decolorization, merely that it was not used for these samples. Considering those samples of brownish Roman glass which showed a band at 405 nm, the redox conditions must again have been fairly closely defined to avoid on the one hand total reduction of the iron to Fe’+, and on the other, oxidation of sulphide, if a brown colour was to be produced intentionally but of course empirically. Manganese-tinted Glass Ancient glass normally contains manganese, usually as a component of the alkali flux. Under the fairly reducing conditions which appear to have been usually present in ancient glass furnaces, manganese is present as Mn 2+ . This ion imparts virtually no coloration at the concentrations normally observed. Under more oxidizing conditions, the strongly colored ion Mn3+ is readily produced. The complex interrelation of manganese concentration, an oxidizing atmosphere, temperature, basicity of the glass and colour may not have been fully understood by ancient glassmakers. Thus, Theophilus (probably to be identified as Roger of Helmarshauseqfl. c. 1100 AD) instructs as follows: But if you see any pot [of molten glass] happening to turn a tawny colour, like flesh, use this glass for flesh-colour, and taking out as much as you wish, heat the remainder for two hours. . . and you will get a light purple. Heat it again from the third to the sixth hour and it will be a reddish purple and exquisite (Hawthorne & Smith, 1979).
This appears to be a description of the accidental and gradual appearance, due to oxidation, of the rather characteristic Mn3+ colour. This phenomenon of the adventitious appearance of the purple Mn3 + colour is to be distinguished from the use of manganese, in the form of pyrolusite, MnO,, as an intentionally added decolorizing agent, in which circumstances its probable mode of action is removal of iron colours by oxidation of Fe2 + to Fe3+ (removing the blue colour of Fe2+) and oxidation of S2- to SO, (removing the brown-yellow of the Fe3+/S2- chromophore), itself being reduced to Mn’+. The uncontrolled first appearance of the purple colour in a manufacturing process which could be expected to be closely controlled is probably to be related to the very variable manganese content of wood, which is referred to below. The light absorption of the Mn3+ ion in glass occurs in the range h,,, = 47&520 nm. Absorption at a similar wavelength is shown by [MnF,13- (461 nm) and [Mn(H20J3+ (476 nm), and so there is little doubt that the absorption band in glass is due to a spinallowed ‘E,+ 5T2, transition in the approximately octahedrally coordinated Mn3+ ion.
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This ion, with a d4 electronic configuration, will show Jahn-Teller distortion in addition to any distortion from octahedral symmetry induced by the non-crystalline nature of the glass matrix. We examined two samples which showed the Mn3+ absorption band. Both were reddish purple, one being Roman and the other Wealden. The spectra of both samples are shown in Figures 1 and 2, where h,,, = 490 nm. The concentration of Mn3+ in tetrahedral sites must be very small compared with those in octahedral sites, because of the absence of the ‘T, -+ 5E band of tetrahedral Mn3 +, which must have a considerably greater E value because of non-centrosymmetric components of the crystal field, than the 5E,+ ‘T2, band of octahedral Mn3+. It is noteworthy that the intense broad Fe’+ band at 1100 nm is entirely absent in this spectrum, in line with the expectation that Fe’+ and Mn3+ cannot coexist in a glass matrix owing to the difference in redox potential [E@ for Mn(OH),/ Mn(OH), = - 0.2 V; E@ for Fe(OH),/Fe(OH), = - 0.56 V]. Analysis of the Roman sample showed Mn = 1.1%. If all manganese present in this sample were Mn’ +, the extinction coefficient value would be 13.5. For [Mn(H20)6]3+, a=5.0 (Hartmann & Schlafer, 195 l), but the lower symmetry to be expected of the coordination polyhedron of Mn3+ in glass would lead to a higher value and there is evidence that even in an oxidizing furnace atmosphere, a majority of Mn 2 + is to be expected (Weyl, 1955). Though well above the average, the concentration of manganese in these samples is not so high as to lead to the inference that it must have been added intentionally. The spectrum of the Mn 2 + ion, in which state much of the manganese must remain, is both spin- and Laporte-forbidden and is exceedingly weak. In a synthetic sample the most intense transition 6A i 4 4A,, 4E at 420 nm was barely observable at a concentration of 3.2% Mn2+ ion. In the ancient glasses, stronger bands of Fe3+ in any case exactly coincide with and mask this weak absorption. Cobalt-tinted Glass A deep blue colour in glass is traditionally associated with the presence of cobalt. However, because the concentration of cobalt which gives an acceptable blue colour is very small, and because there is frequently a requirement to preserve samples unharmed, the number of instances where the presence of cobalt as colouring agent has been objectively confirmed is limited. Furthermore, as is clear from the following results, the unsuspected presence of minute quantities of cobalt influences the colours of some specimens of archaeological glass considerably. Cobalt is expected to be present entirely as Co’+, because Co3+ is unstable relative to this ion except in the presence of strongly complexing ligands. The = Si-0 group, which must be the donor group in glasses, would not be expected to stabilize Co3+. Synthetic soda glasses containing 0.3% Co 2+ have a strong triple-peaked absorption band at 530, 585 and 647 nm, which absorbs virtually all visible light except blue, together with a less intense absorption in the infra-red at 1265, 1500 and 1740 nm. This spectrum is characteristic of tetrahedrally coordinated Co ‘+ . Thus, the species [CO(OH),]~- absorbs at 525, 590, 625 and 1210, 1370, 1590 nm with E = 170 and 45, respectively (Cotton et al., 1961) while the species [COC~,]~- has a similar spectrum shifted to longer wavelength because of the weaker ligands. The Tanabe-Sugano diagram for Co2+ shows that the two bands are 4A2+4T, (P) and 4T, (F), respectively (the 4A2 j4T2 band is too low in energy to be observed). The spectra were examined of seven samples of Roman glass and two samples of Wealden glass which were very or moderately intensely blue to the eye. These all proved to contain Co2+ as colouring agent, as shown by the presence of the strong characteristic absorption just discussed. Examples are shown in Figures 1 and 2. The feeble, sharp bands
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of Fe3+ were always also present, and the Fe ‘+ band sometimes present. The Fe3+/S2band was always absent (necessarily, or the glass would be black). Part of one specimen was analysed for cobalt by atomic absorption (the concentration was too small to be measurable by the electron microprobe), and was found to contain 540ppm cobalt. Assuming all cobalt ions are in tetrahedral sites, the extinction coefficients are 123 and 11 for the 4T, (P) and 4T, (F) bands, respectively. These values are rather low compared with the values previously mentioned and may indicate that only a proportion of the Co 2i ions occupy tetrahedral sites, in which case the broad, weak absorption of Co2+ when octahedrally coordinated (values for [CO(H~O)]~~+, A,,,,= 530 nm, E = 5) would not be observed, leading to an apparently low Evalue for tetrahedrally coordinated Co2+. The crystal field parameters for the spectrum shown in Figure 1(e) are A = 3950 cm- ’ and B’ = 790 cm I, which are chemically reasonable values for Co” in a tetrahedral environment of oxyanions. Examination of the spectra of the Roman and the Wealden samples shows clearly the presence of cobalt in trace quantities in many of them. A concentration of only 50 ppm of Co2+ is easily detectable in the spectrum of a sample of about 2 mm thickness, and the concentration can be estimated from the absorbance values. It is clear that several of the samples owe much of their blue colour or greenish-blue appearance to the Co2+ absorption, in addition to the contribution from the Fe2 + absorption band which is alone usually held responsible for pale blue colours, or in combination with Fe3+/S2- absorption, for greenish-blue colours. Examples of such spectra are shown in Figures 1 and 2. Of 84 Roman samples, cobalt was detected in 37, of which six were deep blue and eight showed Co2+ bands which were judged to be sufficiently strong to influence the observed tint appreciably (i.e. the absorption of visible light due to Co2+ was comparable with that due to Fe2+). The Wealden glasses showed a stronger influence of cobalt. Of 48 samples, only 11 showed no Co2+ bands, while two were deep blue and 12 had bands judged to influence the tint. An interesting effect was shown by the short wavelength component of the strong Co2 ’ triplet. This occurs at 535 nm in the Roman samples but at 520 nm in the Wealden, both values with a range of about f 5 nm. This is doubtless due to the different compositions of the two types of glass with respect to Na+, K’ and Ca2’ modifying the ligand field experienced by the Co 2f ion. Provided observable cobalt bands are present, this provides a simple distinction between glasses of these compositions. Further work using traces of cobalt as a spectrophotometric indicator in synthetic glasses of various compositions is needed to define the extent and sensitivity of this effect. Abundances of Iron, Manganese and Cobalt in Wood Ash Available data suggest that there can be a surprisingly high concentration of manganese in wood ash; values up to 13.7% Mn,O, are quoted (Newton, 1978; see also Newton, 1980) for beechwood ash, and 1.41% Mn has been reported (Sanderson & Hunter, 1981) in Wealden oak ash. We have made analyses by ICP emission and atomic absorption of 11 samples of each of beech, birch, hornbeam and oak from Epping Forest (D. J. Galloway & F. A. Hart, unpublished data). The results will not be discussed here other than to state that the ratios of the concentrations of Ca, K, Mg, Na, Mn, Fe and Co are entirely consistent with the ratios of these ions found in the Wealden glass samples (although of course the soils of the two wood sources are different). For cobalt, the concentrations of Co found in the wood correspond to those in the glass other than the deep blue glass; the latter must contain cobalt added intentionally, as would be expected. In the case of the manganese content, this was particularly variable. For the oak samples, the mean concentration of Mn expressed as a percentage of the dried wood was 0.0234, but the
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standard deviation was as much as 0.025. Results for beechwood were similar. In these circumstances, it is understandable that Theophilus regarded the appearance of the purple colour as uncertain. Bulk Composition of the Glasses A selection of typical glass sherds chosen from the samples whose spectra had been obtained was analysed by the electron microprobe. A total of 36 Roman and I.5 Wealden samples were analysed. Table 1 shows the compositions found. It is clear that the Roman and the Wealden groups have compositions which are consistent within each group and that each group is, as may be expected, very clearly distinguishable from the other. The Roman glasses are soda lime glasses with very little potash, while the Wealden glasses are potash lime with very little soda. This undoubtedly reflects the different origins of the alkaline fluxes used; in the one case, probably natron or purified Mediterranean marine plant ash and in the other, forest wood ash. The minor elements also show distinct differences, the most obvious being magnesium, which is barely detectable in the Roman samples but averages 2.3% in the Wealden. Multivariate statistical analysis could be used to separate the Roman from the Wealden glasses, but it would be superfluous in this instance of two types of glass of very different origin and clearly different composition. No attempt was made to cluster the Roman and the Wealden glasses into subsets, as this was not an objective of this work and the data are in any case insufficient. Because glassmaking sand would normally be expected to be almost pure silicon dioxide, the notable constancy of the silicon content within each group must indicate that the proportion of sand to alkaline flux was very closely controlled. The contexts from which the Colchester glass was obtained have a wide date range, and the constancy of silicon content under these circumstances is of some interest. The composition of the alkaline flux itself is much less constant within either group of glasses. The composition of the Colchester glasses is, except for their low magnesium content, similar to some early Roman period glasses from Castleford and elsewhere which have recently been analysed by X-ray fluorescence (Sanderson et al., 1984). Experimental Methods Uv-vis-ir spectra were obtained by means of a Perkin-Elmer 330 spectrophotometer operated in the diffuse transmission mode using a Hitachi integrating sphere accessory. In this mode, the sample beam passes through the sample, and though refracted and scattered by the irregularly shaped and imperfectly preserved glass, the emerging beam is collected by the integrating sphere and its intensity is measured. If the glass samples had been expendable, they could have been ground flat and polished, the spectra being obtained in the conventional transmittance mode. Alternatively, the samples could have been powdered and spectra obtained in the diffuse reflectance mode. This last procedure was used to obtain spectra of some of the standard synthetic glasses. Samples which were badly weathered on the surface were painted with a 1 : 1 mixture of glycol and ethanol just before the spectrum was obtained, in order to reduce scatter. This liquid subsequently evaporates cleanly, leaving the sample unaffected. Electron microprobe analyses were obtained by means of a Hitachi S450 Scanning Electron Microscope fitted with a Link Systems 860 energy dispersive spectrometer. A fragment of glass about 1 mm in diameter was removed from each of 51 representative samples and concordant duplicate analyses were obtained from the fresh surface of the fragment. Mean analytical values, as obtained, are shown in Table 1. In order to estimate accuracy and precision, two synthetic glasses, approximating to the Roman composition as shown in Table 1, were prepared in platinum crucibles at 1150 C from accurately weighed pure materia!s, mainly carbonates. The loss of weight (CO,) on heating was
$0.86 (1.16) 0.62
Wealden glass (15 samples)
7:06 (8.50) 0.28
0.47 (0.57) 0.24
K
compositions
2.57 (4.26) 0.34
13.22 (18.50) 0.18
0.22 (0.37) 0.38
*0.08 (0.13) 0.70
to.04 (0.07) 2.5
5.88 (8.23) 0.21
Ca
Ti
and Weal&n Mg
qf Colchester Mn
0.84 (I .08) 0.44
0.34 (0.44) 0.85
glass
0.59 (0.84) 0.26
0.37 (0.53) 0.59
Fe
I.55 (2.93) 0.29
1.40 (2.65) 0.19
Al
27.29 (58.39) 0.040
32.56 (69.67) 0.024
Si
I.30 (2.98) 0.30
na.
P
0.28 (0.70) 0.23
0.31” (0.77) 0.28
s
0.39
0.40
0.12
0.92
Cl
Elemental compositions of Colchester (Roman) and of Wealden (post-mediaeval) glass shown as means (jr) and coefficients of variation (s/Z), where X is the weight percentage of element and s is the standard deviation. Where the instrument sensitivities, as defined by 20 above background, are comparable with or greater than ic, as in the case of less sensitively determinable elements having low values of%, this is indicated by * (values of2o: Na, 0.47; Mg, 0.40; Ti, 0.13%). Wealden samples: one bottle kick, two vessel sherds, one muff, five crown, two cullet, two blue, two red. The cullet analyses were unexceptional. The values in parentheses are the corresponding weight percentages of the conventional oxides (Fe as Fe,O,, Mn as MnO, P as P,O,, S as SO,). These oxides of course are not present as such, nor is the element in some cases necessarily completely in the oxidation state indicated by the oxide. “Thirty samples only; na., not analysed.
II.98 (16.15) 0.15
Colchester glass (36 samples)
Na
Table 1. Elemental
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theoretical. The synthetic samples were analysed using the electron microprobe (21 determinations). Coefficients of variation were: Si, 0.015; Na, 0.07; Ca, 0.09; K, 0.10; Fe, 0.16; Mn, 0.13; S, 0.14. Ratios of true composition to mean microprobe-determined composition were: Si, 1.015; Na, 0.96; Ca, 0.97; K, 1.31; Fe, 1.12; Mn, 0.85; S, 0.88. Considering the low content of K, Fe, Mn and S in these glasses, and the unfavourable energy of the sodium emission, the implications of these results for the accuracy of the values in Table I are reasonably satisfactory, except possibly for potassium in the Colchester (low-potassium) glasses. A single specimen of deep blue Roman glass was analysed for cobalt by atomic absorption by means of a Pye-Unicam SP950 spectrophotometer. A fragment was removed, ground, dissolved in aqua regia (two drops) and hydrofluoric acid (2.5 cm3) on a steam bath for 20 min, cooled, treated with boric acid (1.5 g) and made up to 50 cm3. Atomic absorption was used because the concentration of cobalt present in even deep blue glass is below the measurement limits of the electron microprobe. Synthetic glasses were prepared by heating appropriate mixtures of Na,CO,, K,CO,, CaCO,, SiO,, Fe,O,, Co0 or CoS0,.4H,O, MnO, Na,S and NaCl to 1150 C in graphite or platinum crucibles for 24 h. A.R. materials were normally used but for the tests on the Fe3 ‘,S’ - chromophore, 99.99% or purer grades were used. The synthetic glass discs were either ground and polished and examined by transmission, or powdered and examined by reflectance. All spectral assignments were checked by comparison with these synthetic glasses. Acknowledgements We thank Mr S. J. Adams for help with the atomic absorption measurements and Mr C. Mole for operating the electron microprobe (both members of the Department of Geography and Earth Science of this College). We thank S.E.R.C. for financial support for L.R.G., during which the last part of this work was completed. We are very grateful to the Director of the Colchester Archaeological Trust and the Curator of Guildford Museum for kindly loaning the glass samples. References Cotton, F. A., Goodgame, D. M. L. & Goodgame, M. (1961). The electronic structures of tetrahedral cobalt complexes. Journal ofthe American Chemical So$ety 83,469@4699. Hartmann, H. & Schlafer, H. L. (1951). Komplexionen dreiwertiger Ubergangselemente mit octaedrischer Symmetrie. Zeitschrift.fiir Naturforschung 6a, 760-763. Hawthorne, J. G. & Smith, C. S. (1979). On Divers Arts (annotated translation of Theophilus). New York: Dover Publications Inc. Kurkjian, C. R. & Sigety, E. A. (1968). Co-ordination of Fe 3 + in glass. Physics and Chemistry of Glasses 9,73-83. Lever, A. B. P. (1984). Inorganic Electronic Spectroscopy, 2nd edition. Amsterdam: Elsevier. Mestdagh, M. M., Dauby, C., Van Cangh, L. & DuPont, C. (1983). Optical and electron paramagnetic resonance investigations of colour instabilities in amber glass as a function of melting temperature and batch redox conditions. Glass Technology 24, 184-191. Newton, R. G. (1978). Colouring agents used by mediaeval glassmakers. Glass Technology 19,
59-60. Newton, R. G. (1980). Recent views on ancient glasses. Glass Technology 21, 173-183. Sanderson, D. C. W. & Hunter, J. R. (1981). Composition variability in vegetable ash. Science and Archaeology 23,27-30. Sanderson, D. C. W., Hunter, J. R. & Warren, S. E. (1984). Energy dispersive X-ray fluorescdnce analysis of 1st millenium AD glass from Britain. Journal of Archaeological Science 11,53-69. Schreurs, J. W. H. & Brill, R. H. (1984). Iron and sulphur related colours in ancient glasses. Archaeometry 26, 199-209.
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Sellner, C., Oel, H. J. & Camara, B. (1979). Untersuchung alter Glher (Waldglas) auf Zusammenhang von Zusammensetzung, Farbe und Schmeltzatmosphare mit der Elektronenspektroskopie und der Elektronenspinresonanze (ESR). Glustechnische Berichte 52, 255-264. Weyl, W. A. (1951). Coloured Glasses. Sheffield: The Society of Glass Technology. Wood, E. S. (1982). A 16th century glasshouse at Knightons, Alfold, Surrey. Surrey Archaeological Collections 73, 147.
Science 1987, 14, 271-282
Colour and Chemical Composition in Ancient Glass: An Examination of some Roman and Wealden Glass by means of Ultraviolet-VisibleTnfra-red Spectrometry and Electron Microprobe Analysis Lorna R. Greena,band F. Alan Hart0 (Received 28 March 1986, revised manuscript accepted 17 September 1986) sherds from the Roman town at Colchester and from the post-mediaeval glasshouse site at Knightons near Dunsfold in Surrey have been investigated by ultraviolet-visible-infra-red spectrometry and by electron microprobe analysis. The glass is coloured by visible absorption bands of iron, cobalt and manganese, whose origins are discussed.
Glass
Keyword.~:ULTRAVIOLET, VISIBLE, INFRA-RED, SPECTROSCOPY, ELECTRON MICROPROBE, COLOURED GLASS, ROMAN, POSTMEDIAEVAL, IRON, COBALT, MANGANESE.
Introduction There is an interesting relationship between the colours of ancient glasses, the technology of their manufacture and their chemical composition. The considerable variation of colour which is observed in different samples may be due to the presence of small quantities of transition metal ions such as iron, cobalt or manganese in particular oxidation states. The presence of such ions and the consequent colour of the glass may be either involuntary or intentional and reflects the state of technology current at the time of manufacture. The technique of ultraviolet-visible-infra-red (uv-vis-ir) spectrometry offers considerable advantages in investigations of ancient glasses but has been rather little used, although valuable studies of 15th- and 16th-century German forest glasses (Sellner et al., 1979) and 4th-century Galilean glass (Schreurs & Brill, 1984) included uvvvis-ir and uvvvis spectra, respectively. Advances in modern instrumentation enable a well-resolved spectrum to be obtained without any damage or alteration to the specimen whatever, even if its surface is not in good condition. Minimum sample sizes are about 1 cm2, or even much smaller if special optics are used. Uv-vis-ir spectrometry provides information on concentrations, oxidation states and chemical environments of a number of species, mainly transition metal ions present in the glass. “Department ofchemistry, Queen Mary England. ‘Present address: Research Laboratory, 3DG. England.
College, The
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The objectives of the present work were two-fold. Firstly, to explore further to what extent useful information about ancient glasses can be obtained by uv-vis-ir spectrometry, and secondly, in conjunction with elemental analysis of the glass composition by the electron microprobe, to investigate in some detail two groups of glass fragments recovered from (1) the Culver Street excavation in the Roman town at Colchester and (2) the Wealden glassworking site at Knightons near Dunsfold, Surrey. Group (1) consisted of fragments of about l-7 cm greatest diameter which appear to belong to a variety of vessels ranging from quite finely made and decorated small flasks to larger, thick, square bottles, together with thin, flat fragments, perhaps window glass. The condition of the glass was generally very good with slight to moderate surface iridescence. The colours were typical of Roman glass in their variety, mainly pale bluish-green, but also pale blue to deep blue, nearly colourless, red-purple, yellow-green and yellow-brown. The glass sherds came from contexts whose stratigraphic date periods ranged from pre-Flavian to 350 + AD, The Wealden group (2) consisted of fragments about 3-7 cm in greatest diameter and usually l-2 mm thick. These fragments consisted mainly of crown, bottle kicks, muff and cullet. This glass was less well preserved than the Roman, often showing considerable surface deterioration. Colours were predominantly pale slightly bluish green, but with darker green, nearly colourless, blue and red-purple pieces also present. The dating of the Wealden site is mid- 16th century (Wood, 1982). The Origin of the Absorption Bands The colours of glasses as perceived by the eye are caused by the absorption of some wavelengths of the visible light which is transmitted through them, the complementary colour being observed. Thus, glass which absorbs at (say) 450 nm in the blue region of the visible spectrum (taken as 40&700 nm) will appear red. The alkali and alkaline earth silicate matrix of most glasses is inherently colourless, but the presence of small quantities of absorbing species, which in the present discussion are ions of the transition metals iron, manganese or cobalt, produces regions of absorption (bands) which impart colour. There may also be bands in the infra-red and ultraviolet regions which are similarly produced and which may be informative, but these of course do not influence the colour. Absorption bands in the electronic spectra of transition metal compounds may be produced by two main processes. The energies of the excited (high energy) states of the d electrons belonging to a transition metal ion such as iron, cobalt, or manganese in a particular oxidation state vary according to the nature and disposition of the other atoms (ligands) surrounding the metal ion. The d electrons can be excited from their normal ground state to the higher energy levels by photons of light of appropriate energies, leading to absorption bands (known as d-d bands) at specific wavelengths. The energy levels are usually depicted graphically in a Tanabe-Sugano diagram in terms of parameters A and B, and electronic transitions between energy levels are described in terms of symbols derived from group theory, e.g. 6A 1+4E. Absorption bands can also be produced by an entirely different process, namely the transfer of an electron between a metal ion and a ligand. These bands (known as charge-transfer bands) are fully allowed quantum mechanically and are very intense, in contrast to the d-d bands which are much weaker. For a full account of transition metal spectroscopy, see Lever (1984). Iron-tinted Glass There are four regions of absorption attributable to the presence of iron in glass, and the nature of the absorbing species responsible for each of these will be discussed further below. The four absorptions are (1) a strong broad band centred on 1100 nm due to Fe’+;
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(2) a strong broad band centred on 405 nm due to an Fe3+ species; (3) a series of weak, sharp bands mainly in the region 375-450 nm, due to Fe3+; and (4) a very strong band at 258 nm due to Fe3+. These bands affect the perceived colour of the glass as follows. Band (l), which is centred in the infra-red, tails into the visible and imparts a fairly pale, pure blue tint. Band (2) would by itself give a red tint but normally occurs together with (l), giving green, amber or brown shades depending on the relative strengths of (1) and (2). The bands (3) and (4) give a very pale lemon yellow colour but are normally both present together with (l), when they impart a greenish-blue tint, or together with (2) and (1) when their effect is negligible. Band (4) is in the ultraviolet but tails slightly into the visible and is chiefly responsible for the combined effect of (3) and (4) on observed colour. Spectra which illustrate these bands are shown in Figures 1 and 2. All four types of absorption were observed in the Roman glass, but (2) was not observed in the Wealden samples. Types (3) and (4), although well known in coordination compounds of iron (Lever, 1984) seem not to have been discussed previously in relation to ancient glasses, while types (1) and (2) have, however, been considered (Schreurs & Brill, 1984). Representative spectra are shown in Figures 1 and 2. Of 78 Roman samples, 64 showed absorptions (l), (3) and (4) only, while (2) additionally appeared in nine others, where it tended to obscure (3). The Fe 2+ band (1) was absent from only five samples (four nearly colourless and one purple). In the Wealden group, of 38 spectra all showed bands (l), (3) and (4) except one red sample where (1) was absent and (3) was obscured. In order to consider further the relationship between the presence of particular iron absorption bands and ancient glassmaking practice, it is necessary to discuss the nature of the absorbing species. These will be considered in sequence of increasing wavelength. An intense absorption is produced by Fe3+ with A,,,=258 nm. The reflectance spectrum of a very pale lemon yellow synthetic glass, made at 1100 C in a platinum crucible from 99.99% (or better) constituents containing 80% SiO, and 20% Na,O to which was added 0.55% Fe,O,, showed a peak at 258 nm which is undoubtedly the Fe3+ charge-transfer band. This type of absorption is fully allowed by the selection rules and corresponds broadly with the transfer of an electron from an oxygen atom to Fe3+. In transmission spectra at this concentration of Fe3+, transmittance was zero below about 305 nm. Three fairly sharp, weak bands appear at 377,418 and 431 nm. These are the Laporteforbidden and spin-forbidden dcl crystal field bands of Fe3 + and have been observed in synthetic glasses (Kurkjian & Sigety, 1968). The Fe 3+ ion is believed, on the evidence of Miissbauer spectra, to be in a tetrahedral environment. The absorption bands may be assigned to transitions from the ‘jA, ground state to 4T2(D), 4E+4A, and 4T,(G), respectively. This is not the place to discuss these state assignments in detail, but they cannot be regarded as conclusive owing to the complex nature of Fe3+ spectra. It should be noted, however, that a predominant tetrahedral environment for Fe3+ is not inconsistent with the octahedral environment reported here for Mn3+ as the latter ion is crystal field stabilized by 0.6A in an octahedral environment while Fe3+ is not. The intense band which appears in some samples of ancient glass at 405 nm has been the subject of a recent report (Schreurs & Brill, 1984). This band has for long been considered to be due to an Fe3+/S2- species (see Weyl, 1951, for the older evidence) whose exact nature was unknown. We nevertheless thought it advisable to confirm this assignment by synthesizing glasses of 80% SiO, and 20% Na,O which (a) contained no additive, (b) contained 0.5% Fe3+ (as Fe,O,), (c) contained 0.5% S2- (as Na,S), and (d) contained 0.5% Fe3’ and 0.5% S2-. Only (d) showed the characteristic intense 405 nm band, although interestingly, (b) showed it also when a graphite crucible-doubtless containing minute quantities of sulphur-was used for the melting process instead of the platinum crucible. The exact nature of the chromophore is uncertain, but we consider it most likely
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Figure 1. Representative spectra of Roman glasses. Both Figures I and 2 are shown as percentage absorption, as this probably gives a better impression of the colour and condition of the glass, although an absorbance presentation would in some cases give a technically better spectrum. In studying these spectra, it should be noted that the visible region is usually taken as 40&700 nm. In each figure, band systems are as follows: 1, Fe’+; 2, Fe3+/sulphur; 3, Fe3+; 4, Fe3 + charge transfer; 5 and 6, Co’+; 7, Mn3+. In spectra of Figures l(d) and 2(e) a Mn3+ charge transfer band is expected to contribute to the intensity of band 4. Sample a: Pale greenishblue; flat; c. 2.5 mm thick; one side smooth but slightly undulating, other side flat but rough; bubbles immediately under smooth side, hence cooled on refractory surface. Probably window glass. Spectrum, showing only band systems 1,2 and 3, is a common type. Colchester reference 1.8 1 C 73; stratigraphic date ofcontext, 350+ AD. Sample h: Yellow, c. 2.2 mm thick. Spectrum shows iron bands as in sample a above, but is yellow rather than blue because of the iron-sulphur band 4.1.8 1 C 148, 1 If&300 AD. Sample c: Pale mid-blue, 4.5-5.7 mm thick, probably from square bottle. Band systems I, 2,3 present together with the cobalt bands 5 which alter the colour from that of sample a to pure blue. 1.81 B 964, 65-110 AD. Sample d: Purple, l.lL1.3 mm thick. The spectrum is dominated by the manganese band 7. 1.1% manganese was present, 1.81 A 360; 4960/l AD. Sample e: Deep blue, 1.551.7 mm thick. The deep blue samples are mostly thin and curved, probably from small enclosed vessels. The spectrum shows strong bands of cobalt, 5 and 6. I.81 B 952,65-l IO AD.
to be as in formula (1) when a fully allowed charge-transfer band would be expected at about the observed wavelength, but other absorbing iron-sulphur species cannot be excluded from consideration:
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+.
Formula (1)
Interestingly, in an experiment similar to (d) above, in a platinum crucible, but with 99.99% purity carbon powder finely mixed with the other materials, the 405 nm band did
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Figure 2. Representative spectra of Wealdcn glasses. Sample u: Bluish-green bottle kick, uneven thickness c. 2 mm. This spectrum is typical of this group and shows band systems I, 2 and 3 together with the cobalt bands 5 which would be expected to affect the colour of this sample. From the absorbance at 670 nm, the cobalt concentration is estimated as 25 ppm. Guildford Museum identification AS4803. Sample h: Blue-green rim sherd, c. 1.5 mm thick. Similar to a but more cobalt. Guildford Museum identilication AS4801. Sample c: Deep blue glass. Flat 3 cm sherd, 4 mm thick. Dominated by cobalt bands 5; also present are 1, 3, 4 and 6. Guildford Museum identification AS4798. Sum& d: Almost colourless yellow-green sherd, 1 mm thick. Rather weak Fe” band, but contains concentrations of iron and manganese close to the group means. Cobalt must be virtually absent. Guildford Museum reference AS4801, Sample e: Deep purplish red, flat, I.5 mm thick. Rendered largely opaque by an intense Mn3+ band 7. Contains 2.0% Mn. Guildford Museum reference AS4799.
not develop, the glass being a pure blue, presumably owing to complete reduction of the iron to Fe2+, thus destroying the chromophore. The strong broad band centred at 1100 nm is undoubtedly the 5T29+5E, spin-allowed crystal field transition of the octahedral d6 ion Fe 2+ The very broad character of this band is probably due to Jahn-Teller splitting of the excited state. The A value of 9090 cm ’ is very similar to that observed for [Fe(H,0)6]2+. As the Fezi/Fe3+ ratios were not estimated in this work, the extinction coefficient for this band can only be stated as a lower limit, namely E> 15. This is rather a high value, suggestive of a somewhat unsymmetrical coordination. There must always be some doubt as to whether the various glass colours were produced in an uncontrolled way or were intended. It may be thought reasonable to assume that they were intended, because glass manufacture, as an inherently costly process on account of the volume of wood and degree of skill needed, would be expected to have employed carefully standardized procedures which would normally have produced uniform products.
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in controlling the iron colours just discussed, the ratio of Fe’+ to Fe3+ is of course important. Both Fe’+ and Fe3+ may coexist in glass because although Fe3 + is somewhat more stable, furnace atmospheres can be sufficiently reducing to stabilize a proportion as the dipositive species, and carbon may have been initially present if wood ash was used as a flux. In the case of the nearly colourless Roman samples, three were analysed as described below and contained an average of 0.23% iron. This value is within one standard deviation of the mean iron content of all the Roman samples, and hence the lack of colour was obtained by use of oxidizing conditions rather than use of iron-free constituents. To eliminate any blue colour due to Fe2 + , it would presumably be sufficient to ensure that the reducing furnace gases did not pass over the surface of the molten glass but that air had some access to it for a prolonged period. It seems certain that MnO, was not added as oxidant because the concentration of manganese averaged only 0.04% in these samples. Indeed, a higher concentration might be undesirable if oxidation by air was used, as oxidation to reddish Mn3+ might occur. These remarks do not in any way suggest that the addition of MnO, is not a practical method of decolorization, merely that it was not used for these samples. Considering those samples of brownish Roman glass which showed a band at 405 nm, the redox conditions must again have been fairly closely defined to avoid on the one hand total reduction of the iron to Fe’+, and on the other, oxidation of sulphide, if a brown colour was to be produced intentionally but of course empirically. Manganese-tinted Glass Ancient glass normally contains manganese, usually as a component of the alkali flux. Under the fairly reducing conditions which appear to have been usually present in ancient glass furnaces, manganese is present as Mn 2+ . This ion imparts virtually no coloration at the concentrations normally observed. Under more oxidizing conditions, the strongly colored ion Mn3+ is readily produced. The complex interrelation of manganese concentration, an oxidizing atmosphere, temperature, basicity of the glass and colour may not have been fully understood by ancient glassmakers. Thus, Theophilus (probably to be identified as Roger of Helmarshauseqfl. c. 1100 AD) instructs as follows: But if you see any pot [of molten glass] happening to turn a tawny colour, like flesh, use this glass for flesh-colour, and taking out as much as you wish, heat the remainder for two hours. . . and you will get a light purple. Heat it again from the third to the sixth hour and it will be a reddish purple and exquisite (Hawthorne & Smith, 1979).
This appears to be a description of the accidental and gradual appearance, due to oxidation, of the rather characteristic Mn3+ colour. This phenomenon of the adventitious appearance of the purple Mn3 + colour is to be distinguished from the use of manganese, in the form of pyrolusite, MnO,, as an intentionally added decolorizing agent, in which circumstances its probable mode of action is removal of iron colours by oxidation of Fe2 + to Fe3+ (removing the blue colour of Fe2+) and oxidation of S2- to SO, (removing the brown-yellow of the Fe3+/S2- chromophore), itself being reduced to Mn’+. The uncontrolled first appearance of the purple colour in a manufacturing process which could be expected to be closely controlled is probably to be related to the very variable manganese content of wood, which is referred to below. The light absorption of the Mn3+ ion in glass occurs in the range h,,, = 47&520 nm. Absorption at a similar wavelength is shown by [MnF,13- (461 nm) and [Mn(H20J3+ (476 nm), and so there is little doubt that the absorption band in glass is due to a spinallowed ‘E,+ 5T2, transition in the approximately octahedrally coordinated Mn3+ ion.
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This ion, with a d4 electronic configuration, will show Jahn-Teller distortion in addition to any distortion from octahedral symmetry induced by the non-crystalline nature of the glass matrix. We examined two samples which showed the Mn3+ absorption band. Both were reddish purple, one being Roman and the other Wealden. The spectra of both samples are shown in Figures 1 and 2, where h,,, = 490 nm. The concentration of Mn3+ in tetrahedral sites must be very small compared with those in octahedral sites, because of the absence of the ‘T, -+ 5E band of tetrahedral Mn3 +, which must have a considerably greater E value because of non-centrosymmetric components of the crystal field, than the 5E,+ ‘T2, band of octahedral Mn3+. It is noteworthy that the intense broad Fe’+ band at 1100 nm is entirely absent in this spectrum, in line with the expectation that Fe’+ and Mn3+ cannot coexist in a glass matrix owing to the difference in redox potential [E@ for Mn(OH),/ Mn(OH), = - 0.2 V; E@ for Fe(OH),/Fe(OH), = - 0.56 V]. Analysis of the Roman sample showed Mn = 1.1%. If all manganese present in this sample were Mn’ +, the extinction coefficient value would be 13.5. For [Mn(H20)6]3+, a=5.0 (Hartmann & Schlafer, 195 l), but the lower symmetry to be expected of the coordination polyhedron of Mn3+ in glass would lead to a higher value and there is evidence that even in an oxidizing furnace atmosphere, a majority of Mn 2 + is to be expected (Weyl, 1955). Though well above the average, the concentration of manganese in these samples is not so high as to lead to the inference that it must have been added intentionally. The spectrum of the Mn 2 + ion, in which state much of the manganese must remain, is both spin- and Laporte-forbidden and is exceedingly weak. In a synthetic sample the most intense transition 6A i 4 4A,, 4E at 420 nm was barely observable at a concentration of 3.2% Mn2+ ion. In the ancient glasses, stronger bands of Fe3+ in any case exactly coincide with and mask this weak absorption. Cobalt-tinted Glass A deep blue colour in glass is traditionally associated with the presence of cobalt. However, because the concentration of cobalt which gives an acceptable blue colour is very small, and because there is frequently a requirement to preserve samples unharmed, the number of instances where the presence of cobalt as colouring agent has been objectively confirmed is limited. Furthermore, as is clear from the following results, the unsuspected presence of minute quantities of cobalt influences the colours of some specimens of archaeological glass considerably. Cobalt is expected to be present entirely as Co’+, because Co3+ is unstable relative to this ion except in the presence of strongly complexing ligands. The = Si-0 group, which must be the donor group in glasses, would not be expected to stabilize Co3+. Synthetic soda glasses containing 0.3% Co 2+ have a strong triple-peaked absorption band at 530, 585 and 647 nm, which absorbs virtually all visible light except blue, together with a less intense absorption in the infra-red at 1265, 1500 and 1740 nm. This spectrum is characteristic of tetrahedrally coordinated Co ‘+ . Thus, the species [CO(OH),]~- absorbs at 525, 590, 625 and 1210, 1370, 1590 nm with E = 170 and 45, respectively (Cotton et al., 1961) while the species [COC~,]~- has a similar spectrum shifted to longer wavelength because of the weaker ligands. The Tanabe-Sugano diagram for Co2+ shows that the two bands are 4A2+4T, (P) and 4T, (F), respectively (the 4A2 j4T2 band is too low in energy to be observed). The spectra were examined of seven samples of Roman glass and two samples of Wealden glass which were very or moderately intensely blue to the eye. These all proved to contain Co2+ as colouring agent, as shown by the presence of the strong characteristic absorption just discussed. Examples are shown in Figures 1 and 2. The feeble, sharp bands
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of Fe3+ were always also present, and the Fe ‘+ band sometimes present. The Fe3+/S2band was always absent (necessarily, or the glass would be black). Part of one specimen was analysed for cobalt by atomic absorption (the concentration was too small to be measurable by the electron microprobe), and was found to contain 540ppm cobalt. Assuming all cobalt ions are in tetrahedral sites, the extinction coefficients are 123 and 11 for the 4T, (P) and 4T, (F) bands, respectively. These values are rather low compared with the values previously mentioned and may indicate that only a proportion of the Co 2i ions occupy tetrahedral sites, in which case the broad, weak absorption of Co2+ when octahedrally coordinated (values for [CO(H~O)]~~+, A,,,,= 530 nm, E = 5) would not be observed, leading to an apparently low Evalue for tetrahedrally coordinated Co2+. The crystal field parameters for the spectrum shown in Figure 1(e) are A = 3950 cm- ’ and B’ = 790 cm I, which are chemically reasonable values for Co” in a tetrahedral environment of oxyanions. Examination of the spectra of the Roman and the Wealden samples shows clearly the presence of cobalt in trace quantities in many of them. A concentration of only 50 ppm of Co2+ is easily detectable in the spectrum of a sample of about 2 mm thickness, and the concentration can be estimated from the absorbance values. It is clear that several of the samples owe much of their blue colour or greenish-blue appearance to the Co2+ absorption, in addition to the contribution from the Fe2 + absorption band which is alone usually held responsible for pale blue colours, or in combination with Fe3+/S2- absorption, for greenish-blue colours. Examples of such spectra are shown in Figures 1 and 2. Of 84 Roman samples, cobalt was detected in 37, of which six were deep blue and eight showed Co2+ bands which were judged to be sufficiently strong to influence the observed tint appreciably (i.e. the absorption of visible light due to Co2+ was comparable with that due to Fe2+). The Wealden glasses showed a stronger influence of cobalt. Of 48 samples, only 11 showed no Co2+ bands, while two were deep blue and 12 had bands judged to influence the tint. An interesting effect was shown by the short wavelength component of the strong Co2 ’ triplet. This occurs at 535 nm in the Roman samples but at 520 nm in the Wealden, both values with a range of about f 5 nm. This is doubtless due to the different compositions of the two types of glass with respect to Na+, K’ and Ca2’ modifying the ligand field experienced by the Co 2f ion. Provided observable cobalt bands are present, this provides a simple distinction between glasses of these compositions. Further work using traces of cobalt as a spectrophotometric indicator in synthetic glasses of various compositions is needed to define the extent and sensitivity of this effect. Abundances of Iron, Manganese and Cobalt in Wood Ash Available data suggest that there can be a surprisingly high concentration of manganese in wood ash; values up to 13.7% Mn,O, are quoted (Newton, 1978; see also Newton, 1980) for beechwood ash, and 1.41% Mn has been reported (Sanderson & Hunter, 1981) in Wealden oak ash. We have made analyses by ICP emission and atomic absorption of 11 samples of each of beech, birch, hornbeam and oak from Epping Forest (D. J. Galloway & F. A. Hart, unpublished data). The results will not be discussed here other than to state that the ratios of the concentrations of Ca, K, Mg, Na, Mn, Fe and Co are entirely consistent with the ratios of these ions found in the Wealden glass samples (although of course the soils of the two wood sources are different). For cobalt, the concentrations of Co found in the wood correspond to those in the glass other than the deep blue glass; the latter must contain cobalt added intentionally, as would be expected. In the case of the manganese content, this was particularly variable. For the oak samples, the mean concentration of Mn expressed as a percentage of the dried wood was 0.0234, but the
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219
standard deviation was as much as 0.025. Results for beechwood were similar. In these circumstances, it is understandable that Theophilus regarded the appearance of the purple colour as uncertain. Bulk Composition of the Glasses A selection of typical glass sherds chosen from the samples whose spectra had been obtained was analysed by the electron microprobe. A total of 36 Roman and I.5 Wealden samples were analysed. Table 1 shows the compositions found. It is clear that the Roman and the Wealden groups have compositions which are consistent within each group and that each group is, as may be expected, very clearly distinguishable from the other. The Roman glasses are soda lime glasses with very little potash, while the Wealden glasses are potash lime with very little soda. This undoubtedly reflects the different origins of the alkaline fluxes used; in the one case, probably natron or purified Mediterranean marine plant ash and in the other, forest wood ash. The minor elements also show distinct differences, the most obvious being magnesium, which is barely detectable in the Roman samples but averages 2.3% in the Wealden. Multivariate statistical analysis could be used to separate the Roman from the Wealden glasses, but it would be superfluous in this instance of two types of glass of very different origin and clearly different composition. No attempt was made to cluster the Roman and the Wealden glasses into subsets, as this was not an objective of this work and the data are in any case insufficient. Because glassmaking sand would normally be expected to be almost pure silicon dioxide, the notable constancy of the silicon content within each group must indicate that the proportion of sand to alkaline flux was very closely controlled. The contexts from which the Colchester glass was obtained have a wide date range, and the constancy of silicon content under these circumstances is of some interest. The composition of the alkaline flux itself is much less constant within either group of glasses. The composition of the Colchester glasses is, except for their low magnesium content, similar to some early Roman period glasses from Castleford and elsewhere which have recently been analysed by X-ray fluorescence (Sanderson et al., 1984). Experimental Methods Uv-vis-ir spectra were obtained by means of a Perkin-Elmer 330 spectrophotometer operated in the diffuse transmission mode using a Hitachi integrating sphere accessory. In this mode, the sample beam passes through the sample, and though refracted and scattered by the irregularly shaped and imperfectly preserved glass, the emerging beam is collected by the integrating sphere and its intensity is measured. If the glass samples had been expendable, they could have been ground flat and polished, the spectra being obtained in the conventional transmittance mode. Alternatively, the samples could have been powdered and spectra obtained in the diffuse reflectance mode. This last procedure was used to obtain spectra of some of the standard synthetic glasses. Samples which were badly weathered on the surface were painted with a 1 : 1 mixture of glycol and ethanol just before the spectrum was obtained, in order to reduce scatter. This liquid subsequently evaporates cleanly, leaving the sample unaffected. Electron microprobe analyses were obtained by means of a Hitachi S450 Scanning Electron Microscope fitted with a Link Systems 860 energy dispersive spectrometer. A fragment of glass about 1 mm in diameter was removed from each of 51 representative samples and concordant duplicate analyses were obtained from the fresh surface of the fragment. Mean analytical values, as obtained, are shown in Table 1. In order to estimate accuracy and precision, two synthetic glasses, approximating to the Roman composition as shown in Table 1, were prepared in platinum crucibles at 1150 C from accurately weighed pure materia!s, mainly carbonates. The loss of weight (CO,) on heating was
$0.86 (1.16) 0.62
Wealden glass (15 samples)
7:06 (8.50) 0.28
0.47 (0.57) 0.24
K
compositions
2.57 (4.26) 0.34
13.22 (18.50) 0.18
0.22 (0.37) 0.38
*0.08 (0.13) 0.70
to.04 (0.07) 2.5
5.88 (8.23) 0.21
Ca
Ti
and Weal&n Mg
qf Colchester Mn
0.84 (I .08) 0.44
0.34 (0.44) 0.85
glass
0.59 (0.84) 0.26
0.37 (0.53) 0.59
Fe
I.55 (2.93) 0.29
1.40 (2.65) 0.19
Al
27.29 (58.39) 0.040
32.56 (69.67) 0.024
Si
I.30 (2.98) 0.30
na.
P
0.28 (0.70) 0.23
0.31” (0.77) 0.28
s
0.39
0.40
0.12
0.92
Cl
Elemental compositions of Colchester (Roman) and of Wealden (post-mediaeval) glass shown as means (jr) and coefficients of variation (s/Z), where X is the weight percentage of element and s is the standard deviation. Where the instrument sensitivities, as defined by 20 above background, are comparable with or greater than ic, as in the case of less sensitively determinable elements having low values of%, this is indicated by * (values of2o: Na, 0.47; Mg, 0.40; Ti, 0.13%). Wealden samples: one bottle kick, two vessel sherds, one muff, five crown, two cullet, two blue, two red. The cullet analyses were unexceptional. The values in parentheses are the corresponding weight percentages of the conventional oxides (Fe as Fe,O,, Mn as MnO, P as P,O,, S as SO,). These oxides of course are not present as such, nor is the element in some cases necessarily completely in the oxidation state indicated by the oxide. “Thirty samples only; na., not analysed.
II.98 (16.15) 0.15
Colchester glass (36 samples)
Na
Table 1. Elemental
CHEMICAL
COMPOSITION
IN ANCIENT
GLASS
281
theoretical. The synthetic samples were analysed using the electron microprobe (21 determinations). Coefficients of variation were: Si, 0.015; Na, 0.07; Ca, 0.09; K, 0.10; Fe, 0.16; Mn, 0.13; S, 0.14. Ratios of true composition to mean microprobe-determined composition were: Si, 1.015; Na, 0.96; Ca, 0.97; K, 1.31; Fe, 1.12; Mn, 0.85; S, 0.88. Considering the low content of K, Fe, Mn and S in these glasses, and the unfavourable energy of the sodium emission, the implications of these results for the accuracy of the values in Table I are reasonably satisfactory, except possibly for potassium in the Colchester (low-potassium) glasses. A single specimen of deep blue Roman glass was analysed for cobalt by atomic absorption by means of a Pye-Unicam SP950 spectrophotometer. A fragment was removed, ground, dissolved in aqua regia (two drops) and hydrofluoric acid (2.5 cm3) on a steam bath for 20 min, cooled, treated with boric acid (1.5 g) and made up to 50 cm3. Atomic absorption was used because the concentration of cobalt present in even deep blue glass is below the measurement limits of the electron microprobe. Synthetic glasses were prepared by heating appropriate mixtures of Na,CO,, K,CO,, CaCO,, SiO,, Fe,O,, Co0 or CoS0,.4H,O, MnO, Na,S and NaCl to 1150 C in graphite or platinum crucibles for 24 h. A.R. materials were normally used but for the tests on the Fe3 ‘,S’ - chromophore, 99.99% or purer grades were used. The synthetic glass discs were either ground and polished and examined by transmission, or powdered and examined by reflectance. All spectral assignments were checked by comparison with these synthetic glasses. Acknowledgements We thank Mr S. J. Adams for help with the atomic absorption measurements and Mr C. Mole for operating the electron microprobe (both members of the Department of Geography and Earth Science of this College). We thank S.E.R.C. for financial support for L.R.G., during which the last part of this work was completed. We are very grateful to the Director of the Colchester Archaeological Trust and the Curator of Guildford Museum for kindly loaning the glass samples. References Cotton, F. A., Goodgame, D. M. L. & Goodgame, M. (1961). The electronic structures of tetrahedral cobalt complexes. Journal ofthe American Chemical So$ety 83,469@4699. Hartmann, H. & Schlafer, H. L. (1951). Komplexionen dreiwertiger Ubergangselemente mit octaedrischer Symmetrie. Zeitschrift.fiir Naturforschung 6a, 760-763. Hawthorne, J. G. & Smith, C. S. (1979). On Divers Arts (annotated translation of Theophilus). New York: Dover Publications Inc. Kurkjian, C. R. & Sigety, E. A. (1968). Co-ordination of Fe 3 + in glass. Physics and Chemistry of Glasses 9,73-83. Lever, A. B. P. (1984). Inorganic Electronic Spectroscopy, 2nd edition. Amsterdam: Elsevier. Mestdagh, M. M., Dauby, C., Van Cangh, L. & DuPont, C. (1983). Optical and electron paramagnetic resonance investigations of colour instabilities in amber glass as a function of melting temperature and batch redox conditions. Glass Technology 24, 184-191. Newton, R. G. (1978). Colouring agents used by mediaeval glassmakers. Glass Technology 19,
59-60. Newton, R. G. (1980). Recent views on ancient glasses. Glass Technology 21, 173-183. Sanderson, D. C. W. & Hunter, J. R. (1981). Composition variability in vegetable ash. Science and Archaeology 23,27-30. Sanderson, D. C. W., Hunter, J. R. & Warren, S. E. (1984). Energy dispersive X-ray fluorescdnce analysis of 1st millenium AD glass from Britain. Journal of Archaeological Science 11,53-69. Schreurs, J. W. H. & Brill, R. H. (1984). Iron and sulphur related colours in ancient glasses. Archaeometry 26, 199-209.
282
L. R. GREEN
AND
F. A. HART
Sellner, C., Oel, H. J. & Camara, B. (1979). Untersuchung alter Glher (Waldglas) auf Zusammenhang von Zusammensetzung, Farbe und Schmeltzatmosphare mit der Elektronenspektroskopie und der Elektronenspinresonanze (ESR). Glustechnische Berichte 52, 255-264. Weyl, W. A. (1951). Coloured Glasses. Sheffield: The Society of Glass Technology. Wood, E. S. (1982). A 16th century glasshouse at Knightons, Alfold, Surrey. Surrey Archaeological Collections 73, 147.