Archueomerry 37, 1 (1995), 129-141. Printed in Great Britain

COMPOSITIONAL VARIABILITY IN COLOURLESS ROMAN VESSEL GLASS*

M. J . BAXTER,

Department of Mathematics, Statistics and Operational Research, The Nottingham Trent University, Clifton Campus, Nottingham, N G l l a N S , U.K.

H. E. M. COOL,

York Archaeological Trust, Piccadilly House, 55 Piccadilly, York, YO1 IPL, U.K.

M. P. HEYWORTH

Council for British Archaeology, Bowes MorreN House, 111 Walmgate, York, YO1 2UA, U.K.

and C . JACKSON

Department of Archaeology and Prehistory, University of Shefield. Shefield, SlO 2TN, U.K.

A statistical re-analysis is undertaken of 118 inductively coupled plasma spectrometry analyses of Romano-British glass specimens found in excavations at Colchester. There are four vessel types present, some of which are associated with chronologically distinct periods. Previous research has suggested little diference in the mean composition of direrent types. The present paper shows that there are interesting differences in the variation of compositions within types, with some showing much greater compositional stability than

others. Some possible models to explain this phenomenon are discussed.

KEYWORDS: BRITAIN, COLCHESTER, ROMAN, INDUCTIVELY COUPLED PLASMA

DISCRIMINANT ANALYSIS, COMPOSITION, VARIABILITY SPECTROMETRY, GLASS, BOX-AND-WHISKER PLOT, PRINCIPAL COMPONENTS,

INTRODUCTION

The discovery of glass blowing during the first century BC (Israeli 1991) enabled glass vessels to be produced cheaply and rapidly for the first time. As a consequence glass vessels ceased to be purely luxury items and rapidly became items of everyday use both as tablewares and containers. Although the products of the Roman glass industry are very common finds, the organization of the industry, the location of the glass-houses and the places of origin of particular types of glass vessels are more often a matter of supposition rather than of concrete knowledge. This is because there are no equivalents of, for example, the kiln sites and associated waster dumps that there are for the pottery industries (Price and Cool 1991,27). Glass furnaces are frequently small and tend only to be discovered by accident, and the normal practice with wasters will be to recycle rather than to dump them. Models of how the Roman glass industry spread into the provinces, therefore, tend to rely

* Received 8 March 1994, accepted 9 June 1994.

129

130 M . J. Baxter, H. E. M . Cool, M . P . Heyworth and C . Jackson

in the main on the development of recognizable regional types and study of their distribution patterns (Cool and Price forthcoming). Given the known problem that an increase in research activity is invariably associated with a changed distribution pattern with consequent re-interpretations of previous conclusions (see, e.g., Grose 1991, 16), there is an obvious need to look for other avenues of investigation.

Investigating the chemical composition of the glass is one such avenue. Recent research, however, has shown a compositional similarity both between and within groups of glasses from a variety of different sites throughout the period within Britain (Jackson 1992, 232). This may be due in part to a consistency of recipe, the recycling of cullet (broken glass), and the continual and repeated remelting of the glass. Consequently, it is unlikely that chemical analysis will help to identify the place of origin of most glass vessels.

Equally, it seems unlikely that it will be possible to distinguish between typologically distinct forms of vessels, made in the same colour of glass, from their mean compositions. Nevertheless, there do appear to be some differences between the compositions of colour- less and blue-green glass throughout the Romano-British period which may be related to the selection of specific raw materials and differences in manufacturing practice (Jackson 1992, 233).

Rather than examining mean compositions, another approach that has rarely been investigated is to examine the variability within the cornpositions of typologically distinct groups. The potential of this for illuminating how the Roman glass industry developed is explored further here.

The typologically distinct groups examined are all made in colourless glass. This type of glass has been selected for two reasons. The first is that the introduction of colourless glass in c . AD 65-70 marks an abrupt and chronologically well-documented change in glass tablewares (Harden and Price 1971, 321-2). Tableware forms tend to have short lifespans because they are subject to changes in fashion, and it is thus possible to trace the development of colourless glass from being used for luxury tablewares in the Flavian period to being used for very common ones by the late Antonine period. The second reason for choosing colourless glass is that care has to be exercised in the manufacture of deliberately decolorized glass (vose 1980, 34). The collection of cullet for remelting is known to have taken place in the Roman period (Leon 1941). That selectivity was exercised over the selection of raw materials (and possibly cullet) has been implied by recent research (Jackson 1992, 198). Therefore, it is hoped that, in order to ensure a consistently good quality glass, any additions to the batch were closely controlled, which would subsequently result in the reduction of the complexity of the glass produced.

PREVIOUS RESEARCH

The present study represents a more developed and sophisticated interpretation of a group of data originally presented in Heyworth et al. (1990) and which will also be available in Cool and Price (forthcoming).

A group of 120 specimens of colourless glass from Colchester were analysed by inductively coupled plasma spectrometry (ICPS). Specimens were selected from four typological groups which span the period from the first appearance of good quality colourless glass as a regular item of luxury tableware in c. AD 65-70 to the point in the late second century when colourless glass was in common use. These groups were colourless

Compositional variability in colourless Roman vessel glass 131

cast vessels (Grose 199 1, 12- 16), facet-cut beakers (Oliver 1984) and related externally ground vessels, wheel-cut beakers (Price 1987, 189-91) and cylindrical cups with double base rings (Isings 1957, form 85b).

The original research aims were: (1) to examine the composition of colourless Roman glass which had been little studied previously; (2) to investigate the compositional similarity within and between the typologically established groups; and (3) to see whether the change in status of colourless glass from a prestige material to a commonplace one was reflected compositionally. Apart from descriptive approaches, the main statistical methods used were multivariate pattern seeking methods, such as principal component analysis (PCA). These, unlike methods such as discriminant analysis, do not presume that the typologically distinct groups are also chemically distinct.

It proved difficult to reach definitive conclusions in terms of research aims (2) and (3). The group mean compositions did not appear sufficiently distinct, relative to compositional variability, to assert their difference. A particular problem arose from the ‘messy’ nature of much of the data, with a large number of unusual data measurements, or outliers, and some highly skewed variables.

Since the original work was carried out the typology of the glass has been refined. Subsequent reflection has also suggested that it is variability in the data, rather than mean composition, that is potentially important in discussing research aims (2) and (3). The focus in this paper is thus on the nature of the variability in the compositions of the typological groups, rather than on average compositions.

The next two sections recapitulate material on the glass groups analysed and the analytical procedure from Heyworth et al. (1990). The remainder of the paper presents statistical analyses designed to tease out different aspects of the compositional variability in the data.

THE GLASS

The glass was found at Colchester during excavations conducted by the Colchester Archaeological Trust between 1971 and 1985. Excluding two typological outliers, which re-examination has suggested were included in the original sample in error (414 and 417 using the numbering in Heyworth et al. 1990), the typological groups, numbers of specimens analysed and approximate dates are given in Table 1.

Re-examination of the original sample suggests that three of the cast vessel specimens have stylistic differences from others in that group (209, 210, 211). Since they were also found to be highly distinctive in compositional terms they will be excluded from further

Table 1 Summary of specimens analysed

Id Type n Approx. date AD

A Cast vessels 21 60-150 B Facet-cut beakers and other ground vessels 15 60- 150 C Wheel-cut beakers 29 100- 170 D Cylindrical cups 53 170-250

132 M. J . Baxter, H. E. M . Cool, M . P . Heyworth and C . Jackson

analysis in this paper. Further summary information concerning the glass is given in Tables 2 and 3.

ANALYTICAL METHOD

ICPS was used and 31 oxides and elements were determined, with silica subsequently obtained by difference. Full analytical details are given in Heyworth et al. (1990).

Elements with a very low presence that show little or no variability were not used in the statistical analysis. Based on these considerations and recent work on the use of ICPS for glass analysis described in Jackson (1992), the following 15 oxides and elements were selected for use in the present paper: SOz , A1,03, Fe203, MgO, CaO, Na20, K20 , Ti02, P2OS. MnO, Pb, Sb, Ba, Cu, Sr.

STATISTICAL ANALYSIS

Global analysis

In the first instance dot plots and box-and-whisker plots were used, in the MINITAB statistics package (Ryan et al. 1985), to examine the distribution of each variable ignoring typological information. Dot plots are similar in intent to histograms, but preserve more information and allow, for example, unusual points to be identified. That for Sb is shown, by way of illustration, in the upper diagram of Figure 1.

The lower diagram in Figure 1 is an example of a box-and-whisker plot, which is a useful summary diagram in the absence of multi-modality of the data (Baxter 1994). The limits of the box show the first and th rd quartiles so that the length of the box gives the inter- quartile range, or ‘hinge-spread’ as it is termed in MINITAB-call this H. The ‘+’ identifies the median; a ‘*’ identifies an ‘unusual’ point more than 1.5 H from the box; and ‘0’ a point more than 3 H distant. In the absence of ‘unusual’ data the ‘whiskers’ extend to the extremes of the data, and define the range; with unusual data, as in Figure 1, the whiskers are broken in order to highlight the unusual data.

It is usually best to examine both types of display together. The criteria for identifying unusual data are well defined but arbitrary, and inspection of the dot plot will show whether an unusual point is genuinely distinct from the bulk of the data. In Figure 1, for example, the first * to the right of the box does not correspond to a point separate from the bulk of the data.

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Figure 1 Dot plot and box-and-whisker plot fo r Sb.

Compositional variability in colourless Roman vessel glass 133

Table 2 Unusual specimens by oxide and element

Oxide or element Unusual or very unusual specimens

A1203

MgO CaO NazO K2O Ti02

MnO Pb Sb Ba cu Sr

p 2 0 5

222 396 415 428 446 396 446 222 224 456 446 448 427 435 217 225 222 441 222 224 427 534 218 220

402 411 446 463

463 446 456

452 456 463 438 449 452 455 456 463 504 406 404 405 409 426 439 448 452 453 456 463 530 534 225 427 435 436 438 446 449 452 455 456 463 504 534

428 514

Specimens are categorized as 'unusual' or 'very unusual' according to their appearance on the box-and-whisker plot with the latter shown in bold. Italicized specimens had low values, the remainder high values. There were no unusual specimens for Si02 and Fe203.

Numbers are as in Heyworth et al. (1990).

For each variable, 'unusual' observations were identified from the graphs. In Table 2 all specimens identified in this way are listed, with the following exceptions. In a small number of cases specimens identified with a '*' on the box plot were not visually distinct from the main body of data on the dot plot and have not been listed as unusual. For barium the plots were distorted by one exceptional value (specimen 436 with a value of 1259), and the graphs were thus re-examined omitting this. The large number of unusual points for MnO and Pb arise because the distributions are highly skewed with a small interquartile range and long tail of high values.

Table 3 presents summary information based on Table 2. In terms of variability in composition as evidenced by unusual values the following may be noted. (1) The cylindrical cups (type D) are the most stable in terms of the relative absence of

Table 3 Summary information from Table 2

Type A B C D

No. unusual (1 or more) 6 I 11 5 No. unusual (2 or more) 3 1 1 1 2 No. unusual (excl. Mn and Pb) 5 3 13 5 Sample size 18 15 29 53

The first two rows summarize the number of specimens that are unusual with respect to at least one, and at least two elements respectively. The third row is as the first hut excludes MnO and Ph from consideration.

A, B, C and D refer to cast, facet-cut, wheel-cut and cylindrical cups, as in Table 1.

134 M. J . Baxter, H. E. M . Cool, M . P . Heyworth and C . Jackson

unusual values, with 9.4% specimens unusual in some sense compared to 33%, 47% and 59% for the other three groups. (2) By the same token the wheel-cut specimens (type C) exhibit much greater compositional variability than the other groups and this is true regardless of whether or not only securely identified specimens are used. (3) If specimens with only one unusual element are disregarded, or if Mn and Pb are not used, then the facet-cut specimens (type B) also appear stable. Conclusions concerning the variability of the wheel-cut specimens remain unaffected.

Descriptibre analvses bj! typological group

Figures 2 and 3 show box-and-whisker plots of the measurements for each type for each of the 15 oxides and elements used. Figure 2 includes those oxides and elements showing the greatest differences in variation between types. The graphs were obtained from the STATGRAPHICS statistical package (Davies and Tremayne 1991). It should be noted that outliers shown in this figure are not necessarily coincident with those listed in Table 2, since they are determined with reference to each typological group rather than the full set of data.

(4) In general the interquartile ranges overlap to a high degree suggesting that the typical composition does not differ greatly between types. All types have a typical soda-lime-silica composition. (5) Looking at within-type variability in terms of unusual values the wheel-cut beakers (group C) again stand out with respect to oxides and elements such as MgO, P 2 0 5 and Pb. The distribution with respect to MnO is particularly unusual in comparison to other types, with a much greater interquartile range and range than other types, even ignoring the outliers on the graph. (6) Types A and B are relatively free of outliers, apart from Pb. Types B and D, in particular, differ with respect to the levels of Pb. Apart from two observations, there is no variation in type D, whereas there is a relatively large spread, however measured, in type B. The outliers evident for type D mostly arise, as with MnO and Sr, because the interquartile range is relatively small. (7) Two possible ways of quantifying variation in the data are by using the standard deviation or coefficient of variation of the data. This does not work well with these data because of the number of unusual data points and skewness of some of the variables. A more robust measure of spread is the interquartile range and this was calculated for all types and elements.

Relative to other types, B showed a high spread with respect to MgO, CaO and Pb; type C was high with respect to MnO and Ba; and type D was low with respect to CaO, Pb and Sr. These features can be inspected in Figures 2 and 3.

For each oxide and element the types were ranked from one to four according to the magnitude of the interquartile range, with 1 corresponding to the smallest range. Summing ranks across elements provides a crude measure of variability in the data and gives 27.5 for type D. and 30, 39 and 43.5 for types A, B and C. This again points to the relative compositional stability of type D and instability of type C. All the largest spreads occurred in type B (six elements) and type C (seven elements), apart from Fe which had its largest spread for type A.

On the basis of these graphs the following additional observations can be made.

Compositional variability in colourless Roman vessel glass 135

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136 .Vf. J. Baxter, H. E. M . Cool, M . P. Heyworth and C. Jackson

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Compositional variability in colourless Roman vessel glass 137

Multivariate analysis In addition to the univariate analyses described, multivariate analyses were undertaken. The motivation for this was to see whether any further light could be shed on the nature of compositional differences and whether, for example, additional unusual specimens could be identified.

Figure 4 (left) presents the outcome of a principal component analysis (PCA) of the standardized data, and shows a plot of the first against the second component. In this, and the discriminant analyses to be discussed, silica was omitted from the analysis because of potential statistical problems associated with the use of the full composition (Baxter 1994). The more unusual points on the plot have been labelled. Because the first two components account for only 48.7% of the variation in the data, which is poor, similar plots were also examined for the third to fifth components.

The results essentially confirmed the conclusions drawn from the univariate analyses. From Figure 4 (left), and the other plots examined, four of type A appeared as multivariate outliers (218,220,222,225) on at least one component, and one of type D (530). There were six unusual values from type B (396,397,402,404,405,409), with the rest, a majority, from type C.

Another way to display the results is to assume that the types are compositionally distinct and use linear discriminant analysis. The plot that results from this approach is shown in Figure 4 (right). The success of the analysis in terms of classifying specimens is 71% using the resubstitution method and, more realistically, 60% using cross-validation (Baxter 1994).

The figure shows even more clearly than Figures 2 and 3 that types A and D are compositionally undifferentiated. Also of interest from the perspective of the present paper is the fact that 11 out of 15 specimens of type B separate out to the left of the graph, and 14 or 15 out of 29 type C specimens separate out to the upper right. Types B and C are far more widely dispersed than type D, which is a much bigger group, or type A.

To check that these results were not produced by just one or two of the oxides used, analyses were re-run omitting Pb and MnO, both singly and jointly. The general picture was unaffected.

Other analyses

About a third of the specimens have been identified as unusual with respect to at least one element. Taking the rather drastic, and questionable, step of omitting all these and repeating the earlier univariate analyses led to the conclusion that there was little difference between types in terms of the interquartile ranges. In other words conclusions about compositional variability between the types rest on the number and distribution of unusual values in the data.

Using all 11 5 specimens, discriminant analyses were undertaken for all pairwise comparisons between types. The aim here was to see if any pair of types was composition- ally distinct, as such a feature could be observed in the previous analysis using all four types. Results using both the resubstitution and cross-validation methods are summarized in Table 4.

The cross-validation results provide a more realistic summary of the success of discrimination. Types B and D appear the most distinct, reflecting what can be seen in

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Compositional variability in colourless Roman vessel glass 139

Table 4 The suecess of discriminant analyses for pairwise discrimination by types (%)

Type A B C D

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The upper triangle of the table indicates the success in classification as measured by the resub- stitution method; the lower triangle measures the success as judged by cross-validation.

Figure 4 (right). Discrimination between type A and the other types is relatively poor (50% is the expected success rate with random allocation); that between types C and D is also not good. It can be seen from Figure 4 (right) that this appears to be because ‘typical’ compositions between types do not vary much; it is the unusual specimens within types, particularly B and C , that account for any distinctions. Although it is not a central theme of this paper it is worth noting that the ‘success’ of a classification differs by up to 21% according to which approach is used. This should caution against the use of the resubstitution method as a means of measuring the performance of a discriminant analysis with these type of data.

DISCUSSION

The gist of the statistical analyses presented is that there are evident differences between the types analysed that relate to the ‘volatility’ of the composition within types rather than ‘typical’ compositions. The late group of cylindrical cups (type D) appears to be particularly stable in compositional terms and the same appears true, to a lesser extent, for the earliest group of cast vessels (type A). The other two chronologically intermediate groups are compositionally much more volatile, particularly the wheel-cut beakers (type C). There is thus no simple relationship between compositional stability and chronology.

The variation in compositional stability may be explicable if viewed against surmized developments in the glass industry. The cast vessels and the facet-cut beakers both have distributions that are Empire-wide. It is not known in how many centres such vessels might have been made, but sources in the Mediterranean area including Italy are likely. It should be remembered that these two types of vessels were made by very different forming processes. The colourless cast vessels are the last products of the old technology that had been used to make glass vessels before the advent of the much more efficient technique of blowing. Facet-cut beakers are blown and therefore represent new technology. Though the two forms were in contemporaneous use, therefore, it is highly likely that they were being made in different centres. The glass vessel industry appears to have spread north of the Alps during the mid-first century (see, e.g., Shepherd and Heyworth 1991, 14; Follmann-Schultz 1991, 36; Foy 1991, 58-9) and during the later part of the century regional styles of vessels developed (Price 1978, 74). Wheel-cut beakers and cylindrical cups, in contrast to the cast

140 M . J. Baxter, H. E. M . Cool, M . P. Heyworth and C. Jackson

bowls and facet-cut beakers, are clearly the products of glass-houses in the north-western provinces as they have a much more limited distribution.

The compositional stability of the cast vessels may be the result of manufacture in a relatively small number of centres. As already noted they were manufactured by an old- fashioned and less efficient technology that was in decline. The greater variability of the facet-cut beakers could be reflecting a greater number of manufacturing centres. It is obvious that any new glass-houses set up at this time would have concentrated on producing blown vessels, and there was clearly a large demand for this new type of drinking vessel that so attractively reflected and refracted light through the facets.

The most volatile type compositionally are the wheel-cut beakers which were also the first colourless drinking vessels to have been made in the glass-houses of the north-western provinces. It is tempting to see these two observations as being related. The nature of the relationship, however, would depend on the model we adopt to explain how the glass vessel manufacturers of the north-western provinces worked. If we believe that they were making vessels from glass manufactured locally from raw ingredients, then the compositional variability might result from both the glassmakers wrestling with the manufacture of an unfamiliar glass and perhaps having to make use of unusual raw materials. As an alternative we might believe that the glass blowers were using raw glass imported from the Mediterranean area. Indeed it has been suggested to us (I. Freestone pers. comm.) that the transportation of raw glass would be both profitable and easier than the transportation of raw materials such as the alkali. If this mollel is followed, then the variability might be the result of the north-western glass blowers making use of raw glass from a number of different sources.

In the light of these models, two different explanations can be suggested for the greater compositional stability of the cylindrical cups. If manufacture from raw materials was taking place in the north-western provinces, colourless glass would no longer have been a new and exotic product by the late second century. It is possible that the glassmakers of the north-western provinces could have devised a more standard recipe for their glass. If, by contrast, the glass blowers were working with imported raw glass, greater stability could have come about through a small number of glassmaking factories becoming the dominant sources of supply.

Which of these explanations, if either, is preferred depends on how one believes the glass workers of the first and second centuries were working. Evidence for either is slight. In Britain, for example, the only evidence for fritting glass comes from Coppergate, York (Bayley 1987, 249). (This material was originally published as being of Viking date, but is now thought to be associated with a second-century episode of glassworking.) Equally, however, no glass 'ingots' have been found unless the triangular block of blue/green glass from Culver Street, Colchester, is part of one (Crummy 1992, 1 18).

Many glass data sets that have been studied in the past have involved small sample sizes, and statistical analyses have focused on the mean composition within archaeologically defined groups. The advent of methods such as ICPS has the potential to produce large data sets. both in terms of sample size and the number of variables measured. The analyses of the data in Heyworth er al. (1990) and this paper have produced results that suggest that the analysis of compositional variation within typological groups is a potentially illuminating waq of looking at the data that complements. and may even be more informative than, the inspection of mean compositions.

Compositional variability in colourless Roman vessel glass 141

ACKNOWLEDGEMENTS

We would like to thank the following for their support: English Heritage (the Historic Buildings and Monuments Commission for England) (for funding the original research reported in Heyworth et al. 1990); Dr J. N. Walsh (for supervising the ICPS analysis); Colchester Archaeological Trust (for allowing analysis of the glass fragments); and SERC (for funding a research studentship for C.J. at the University of Bradford). Comments by Stuart Fleming and Ian Freestone on an earlier draft were much appreciated.

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