Archaeometry 47, 4 (2005) 763– 780. Printed in Singapore
Archaeometry
ARCH
0003-813X
©
Oxford,
4November
47University
UK
2005
of
Oxford,
2005
ORIGINAL
Making
C.
M. Jackson
colourless
ARTICLE
glass
in the Roman period
Blackwell
Publishing,
Ltd.
MAKING COLOURLESS GLASS IN THE ROMAN
PERIOD *
C. M. JACKSON
Department of Archaeology, University of Sheffield, Northgate House, West Street, Sheffield S1 4ET, UK
This paper discusses the compositional analysis of Roman colourless glass from three sites
in Britain. The findings suggest that two broad compositional groups can be identified on
the basis of the choice of the initial raw materials selected for glass production, in particular
the sand. The largest of these groups is inherently different from the naturally coloured,
blue–green glasses of the same period, while the other group is compositionally similar.
Further subgroups are apparent on the basis of the decolorizers used. These glass groups
are explored in the light of the current theories concerning the organization of glass
production in the Roman world.
KEYWORDS: BRITAIN, ROMAN, COLOURLESS GLASS, ANTIMONY, MANGANESE, SAND
*Received 18 August 2004; accepted 14 December 2004.
INTRODUCTION
Roman glass is recognized as a masterful feat of technology in terms of both manufacture and
design. The most striking visible sign is the control over colour, where colorizers and decolorizers are used skilfully, in conjunction with the control of furnace parameters, to provide a
wide variety of hues. This study argues that colourless glasses are a good medium through
which to show the skill of the Roman glassmaker and illustrate the influence of the choice of raw
materials upon the final glass composition. These choices are explored in the light of current
theories relating to diachronic changes in the nature of colourless glass manufacture and the
organization of glass production in the Roman world.
Colourless glass is known from the early periods in glass-making history (Bimson and Freestone
1988), but its popularity increased during the Roman period. From the late first century ad and
into the second century it was produced in volume for high-quality tablewares, its popularity
only declining in the late third century (Price and Cottam 1998, 16). In fact, Pliny specifically
mentions it, suggesting that ‘the most highly valued glass is colourless and transparent, as
closely as possible resembling rock-crystal . . .’ (NH XXXVI, 200). Such glass forms, which
are clear and ‘sparkle’, can be found in high-quality vessels such as facet-cut beakers and
dietrata. However, the statement also implies that colourless glasses could be found in a
number of different qualities, from the truly colourless to those that have a slight blue or green
tint. This range of hues within colourless glass has been noted by other authors (e.g., Sayre
1963) and is explored in this paper.
The greatest proportion of Roman glass is blue–green (Price and Cottam 1998, 15) owing
to iron impurities introduced to the glass from the raw materials (Bamford 1977, 79). A
*Received 18 August 2004; accepted 14 December 2004.
© University of Oxford, 2005
764
C. M. Jackson
colourless glass can be produced either by selecting raw materials that are low in iron or by
the addition of a decolorizer to the glass.
The selection of high-purity raw materials
Roman glasses are produced from silica sand and natron. Natron, or trona, is a crystalline
mineral composed predominantly of sodium compounds with few impurities. The iron impurity
found in blue–green Roman glasses is therefore assumed to derive from the sand. Although it has
been suggested that decolorizing could have been achieved by calcining the iron-containing
raw materials before melting, thus oxidizing the iron to its more colourless state (Stern 1990,
37), it is more likely that low-iron, high-purity sands were selected for the manufacture of colourless glasses. In fact, Pliny specifically mentions sand from the River Volturno for colourless
glass production (NH XXXVI, 192–5).
The use of manganese and antimony as decolorizers in Roman glasses
An alternative way of making colourless glass is to add a ‘decolorizer’. Antimony and
manganese decolorize glass by oxidizing the iron, although the relationship between the iron,
manganese and other compounds in the glass is a complex one (Sellner et al. 1979; Freestone
et al. 1990). Antimony, a stronger decolorizer than manganese, also acts as a fining agent in
glass by removing dissolved gases (Bamford 1977, 80; Weyl 1981, 118), so producing a more
brilliant glass. The relative amount of either oxidizing agent will depend upon the concentration of iron in the glass-making raw materials and equilibrium between the two (or more) elements (Sanderson and Hutchings 1987, 103).
Manganese and antimony are derived from minerals. The purest commonly found manganesecontaining mineral is pyrolusite (MnO2) (Green and Hart 1987, 276). Others include psilomilane
(∼ 75% MnO, 15% BaO, 5% K2O and 1% CaO), diallogite (between 70% and 90% manganese, 2–3% CaO, 2–7% MgCO3 and up to 11% FeO), wad (which can contain up to 50%
iron oxide) (Greg and Lettsom 1977 [1858], 286; Davy 1815, 117) and rhodochrosite (up to
50% MnCO3) (Mondadori 1983, 92). Many of these would have required processing before
use. Of the antimony-containing minerals, stibnite (Sb2S3) is the best known. Bindheimite
(Pb2(Sb,Bi)2O6(O.OH)) could also have been used, introducing, at the same time, small amounts
of lead (Mass et al. 1998; Biek and Bayley 1979, 9). It is difficult to suggest which minerals
were used in glass manufacture from the interpretation of chemical analyses of the glasses
because of the complex nature of the glass, the uncertainty as to whether the minerals were
processed before use and the inclusion of cullet.
Some authors suggest that both decolorizers could have been added accidentally as contaminants
of the raw materials (Newton 1980, 175; Newton 1985, 98; Gebhard 1989, 167). This is unlikely
in the case of antimony, which is not usually present above a few parts per million in commonly occurring geological materials. However, it may be possible in the case of manganese,
which has been recorded in soils and sands from Egypt (Turner 1956, 48T) and at levels
approaching 0.5% in sands from the River Volturno, a source noted by Pliny for the production
of colourless glass (Brill 1999, 475). Other authors note the presence of manganese in glasses, but
suggest that its inclusion was a result of recycling or the use of glassmaker’s soap to clarify
the glass (Velde and Gendron 1980, 185; Mirti et al. 1993, 234; Velde and Hochuli-Gysel 1997).
In 1961, Sayre and Smith proposed a model for the use of decolorizers in Roman colourless
glasses. They found that colourless glasses from the Syrian coast are characterized by the
Making colourless glass in the Roman period
765
increasing use of manganese oxide (MnO) rather than antimony oxide (Sb2O5), in concentrations in the order of 1% MnO towards the end of the Roman period. This model has since been
used widely to characterize the use of decolorizers in colourless glasses throughout the Roman
Empire (Velde and Gendron 1980, 185; Freestone et al. 1990, Mirti et al. 1993; Fleming 1999,
141). Later work by Sayre (1963), Sayre and Smith (1967) and Smith (1971, 616) showed that
in Italy and northern Europe, glasses were generally decolorized with either antimony or antimony/manganese until the end of the third century, when an increase in manganese is
observed. These observations assume the deliberate addition of decolorizers, above 0.2% in
each case (Sayre 1963, fig. 1, 265).
Models of production
The use of high-purity sands and/or the addition of decolorizers in the production of colourless glass have implications for the organization of production. Current theories, relating to
late Roman glass production, are centred around two models (Jackson et al. 2003a). The first
assumes large primary manufacturing centres near raw material sources, with a host of
secondary working centres throughout the Empire (Nenna et al. 2000; Freestone et al. 2002).
The second model proposes local glass-making and -working centres (Wedepohl et al. 2003), to
which raw materials were locally available and/or imported. Both models have been projected
to earlier periods, although the archaeological and scientific evidence for either is difficult to
interpret (Baxter et al. 2005). Assuming that glass production sites were located near to raw
material sources, the use of high-quality sands from a small number of sources may indicate
that a few (large) glass-making centres specialized in colourless glass production. This may be
indicative of centralized production. The use of impure raw materials, with the addition of a
decolorizer, implies that colourless glass could be produced using the same raw materials as
coloured glasses and therefore could be produced at the same centres. This may be more indicative of dispersed production. Of course, the assumption in both of these cases is that ‘pure’
sands are only found at a small number of locations, and that impure sands, used to manufacture coloured glasses, were found widely and used in glass manufacture.
Therefore, colourless glass could have been produced in a number of ways relating to the
choice of primary raw materials, decolorizers and the production technology. Using compositional analysis, this paper will investigate these technological choices, to determine the nature
of the raw materials used and the selection of decolorizers, in a group of first- to fourthcentury ad glasses from Britain. The implications arising from this, relating to the organization of production and possible distribution of glasses to Britain, will be explored.
THE GLASS
The glass derives from three Romano-British sites spanning the first to fourth centuries ad
(see the Appendix) (Jackson et al. 1990; Jackson 1992, 1994). The largest group, excavated
from 16–22 Coppergate, York, between 1976 and 1981, consists of vessel and window glass,
predominantly second- to third-century, but spanning the first to fourth centuries (Jackson
1992). The glass from Blue Boar Lane, Leicester (third-century), excavated in the 1970s by
J. Wacher, and that from Hartshill near Mancetter (second-century), excavated by K. Hartley,
again in the 1970s (both unpublished; noted in Price and Cool 1991) are the remains of glassblowing waste. These two assemblages consist of drips and trails and, in the case of Mancetter,
some heat-affected vessel fragments that appear to be waste glass (cullet).
C. M. Jackson
766
EXPERIMENTAL
The glass was analysed by inductively coupled plasma spectroscopy (ICPS) at Royal Holloway,
University of London, Egham. The instrumentation and sample preparation protocol are
described in Jackson et al. (2003b), and are adapted from the method for the analysis of silicates
detailed in Thompson and Walsh (1983). In addition to the samples, four European Science
Foundation glass standards (which would best mimic the archaeological glasses to be analysed: 76-C-144, 76-C-150, 76-C-151 and 77-C-33), two blanks (acids and diluent only to
check the purity of the reagents, contamination from any of the equipment used and to assess
background levels), two internal rock standards (KC10 and KC11, used for calibration and
drift monitoring) and two duplicate ‘unknowns’ (to check for reproducibility within the run)
were analysed. Independent measurements on the glass standards provided data for accuracy
and precision, as well as affording a check on inter-batch variability.
Synthetic sodium nitrate was prepared from 0–30% Na2O to reflect the sample matrix,
as this element was outside the concentration of the rock standard. Antimony and lead ICP
stock solutions were independently prepared at levels similar to those expected in the unknown
samples, to analyse for these elements, which were outside the concentrations of the synthetic
rock standards and/or absent in the glass standards. Separate calibration lines were computed
for each run and the data corrected: the line for soda generally formed a quadratic, and this is
reflected in the levels of accuracy and precision. Silica cannot be measured using this method,
because heating with hydrofluoric acid removes it by evaporation as silicon tetrafluoride (SiF4).
Evaluation of the results
The analysis of blanks showed no contamination above minimum detectable levels. The precision and accuracy, defined by the uncertainty in the estimate of concentration as derived from
that value (Boumans 1987, 162) and the comparison between the measured concentration and
the published value respectively, can be seen in Table 1. Accuracy and precision could not be
calculated for lead, as it was not present in the standard. Those elements that could be determined with precision and accuracy, and that occur in the archaeological samples in concentrations above their minimum detectable levels (Floyd et al. 1980), are given in Table 1 and are
used in the discussion below.
THE COMPOSITIONAL CHARACTERISTICS OF COLOURLESS GLASS
The base glass composition
The compositional groups described below are shown in Figure 1: primarily, the colourless
glasses fall into two broad groups, on the basis of those oxides and elements associated with
the primary glass-making raw materials. The first and larger group (Group 1) is compositionally distinct from the more common blue–green glasses, which tend to characterize Roman
glass assemblages (Fig. 2). It has lower concentrations of alumina (< 2%, cf., 2.5%), iron
oxide (0.3%, cf., 0.5%) and phosphorus pentoxide (0.04%, cf., 0.10%), and is termed low
iron-alumina glass. A positive correlation between iron and phosphorus, alumina and titanium
suggests that these oxides originate in the same glass-forming raw materials. These glasses are
also slightly lower in calcium oxide and potash, and are depleted in barium, zinc, vanadium
and copper.
Making colourless glass in the Roman period
Table 1
Al2O3
Fe2O3
MgO
CaO
Na2O
K 2O
TiO2
P 2O 5
MnO
Pb
Sb
Ba
Cu
Li
Ni
Sr
V
Y
Zn
767
Minimum detectable levels, accuracy and precision for ICPS analysis
Spectral
line
(nm)
Minimum
detectable
level (ppm)
Accuracy (%),
inter-day
(one day)
Accuracy (%),
inter-day
( five days)
Precision (%),
intra-day
(n = 25)
Precision (%),
inter-day
(n = 131)
308.21
259.94
383.83
315.89
588.99
766.49
335.05
178.22
257.61
220.35
206.83
455.4
324.75
670.78
231.6
407.77
290.88
371.03
213.86
21.67
69.96
26.12
530.37*
296.51*
72.22
67.89
16.27
6.97
4.95
84.7
0.049
0.62
0
5.29
0.077
2.04
0.31
0.86
1.4
6.8
2.3
0.6
−0.9
−0.9
3.6
6.3
−7.1
−1.7
2.4
−1.3
−2.9
−7.3
−7.9
0.9
6.3
−7.1
3
2.5
2.9
2.6
4.8
5.3
3.5
2.9
6.7
4.4
4.4
4.6
3.8
7.8
7.6
4.5
2.9
6.7
−0.8
−4.1
−3.7
−3.7
4.6
2.1
4.8
14.3
−2.5
8.4
−0.4
−5.6
−3.7
0.4
−1.4
1.4
7.1
−2.5
2.3
4
5
5
2
3
3
3
3
4.4
6
5
7
4
6
4
3
5
*Quadratic equation used for calculation.
Accuracy and precision could not be calculated for Pb because it was not present in the standard.
Co and Sc were measured but found to be below the MDL in the glasses.
Cr, Mo, Nb and Zr were found to be either imprecise, inaccurate or both and were discounted from further analysis.
Figure 1
Compositional groups of Roman colourless glass.
The second group (Fig. 1, Group 2) falls within the compositional range of the more common blue–green glasses, but has slightly lower mean levels of iron oxide and alumina, and
marginally lower calcium oxide than the blue–green glasses (Fig. 2 and Table 2). These are
described here as high-iron alumina glasses (in comparison with Group 1 above).
768
C. M. Jackson
Figure 2 An illustration of the iron and alumina content of colourless and nearly colourless glasses. The ellipse
shows the range of blue–green glass compositions from the same sites, taken from Jackson (1992).
Decolorizers
Four subgroups can be identified on the basis of the decolorizers used (Figs 1 and 3). The first
two of these fall into the low iron-alumina group (Group 1). One is predominantly decolorized
using antimony (Group 1a), while the other, consisting here of only one example (York
14108), contains very little of either decolorizer (Group 1b). This latter sample is decolorized by
using low-impurity raw materials and careful control of the redox conditions within the furnace
(Stern 1990).
The third group (Group 2a) contains approximately equal quantities of both antimony and
manganese oxides (∼ 0.4%). The majority of these glasses fall into the high iron-alumina
group (Group 2), although some are transitional between Groups 1 and 2 (e.g., cylindrical
cups 10781 and 13995 from York; see the Appendix). The fourth group of glasses (Group 2b)
has high levels of manganese oxide (> 0.8%) and all fall within the high iron-alumina composition (Group 2).
In summary, the glasses that are low in iron and alumina (Group 1) appear to be decolorized
predominantly using antimony, while those of the high iron-alumina composition (Group 2)
contain antimony and manganese, or are predominantly decolorized using manganese. The
differences between the groups are also noticeable visually. The glasses in Group 1 are more
clearly colourless, they do not have a green tinge when viewed on the cut edge and they ‘sparkle’ in the light. The glasses in Group 2 are weakly coloured, with a green tinge on the cut surface. These glasses are therefore ‘nearly colourless’.
DISCUSSION
A comparison of the colourless glass analysed here with published groups
The trends observed in the data presented can be seen in other groups of Roman glasses,
although few publications specifically note the glass colour. The most prevalent examples are
Table 2
All glasses
%
Al2O3
Fe2O3
MgO
CaO
Na2O
K 2O
TiO2
P 2O 5
MnO
PbO
Sb2O5
ppm
Ba
Cu
Li
Ni
Sr
V
Y
Zn
1.93 ± 0.13
0.35 ± 0.07
0.45 ± 0.09
5.69 ± 0.53
19.34 ± 0.57
0.52 ± 0.08
0.07 ± 0.01
0.04 ± 0.01
0.07 ± 0.09
0.04 ± 0.07
0.53 ± 0.15
152
18
11
12
394
10
7
22
±
±
±
±
±
±
±
±
19
18
5
3
59
3
1
5
Leicester, glassworking waste
(third century)
Group 2,
nearly
colourless
(n = 19)
York, domestic assemblage
( first to fourth centuries)
Group 1,
colourless
(n = 5)
Group 2,
nearly
colourless
(n = 3)
Blue–green*
(n = 66)
Group 1,
colourless
(n = 53)
Group 2,
nearly
colourless
(n = 8)
Mancetter, glassworking waste
(second century)
Blue–
green*
(n = 70)
2.30
0.55
0.60
6.29
18.85
0.70
0.10
0.10
0.58
0.05
0.32
±
±
±
±
±
±
±
±
±
±
±
0.12
0.13
0.10
0.72
1.39
0.11
0.02
0.03
0.36
0.07
0.18
1.83
0.34
0.39
5.13
19.3
0.48
0.07
0.04
0.02
0.06
0.61
±
±
±
±
±
±
±
±
±
±
±
0.05
0.05
0.07
0.83
0.6
0
0.01
0
0.01
0.11
0.08
2.34
0.49
0.56
6.11
19.6
0.74
0.10
0.11
0.35
0.05
0.46
±
±
±
±
±
±
±
±
±
±
±
0.04
0.12
0.03
0.12
0.12
0.06
0.01
0.01
0.02
0.01
0.01
2.37
0.69
0.55
6.56
18.3
0.71
0.10
0.12
0.27
0.03
0.35
±
±
±
±
±
±
±
±
±
±
±
0.13
0.17
0.03
0.77
1.2
0.08
0.01
0.01
0.08
0.01
0.16
1.95
0.36
0.47
5.79
19.4
0.52
0.07
0.04
0.09
0.04
0.52
±
±
±
±
±
±
±
±
±
±
±
0.13
0.09
0.10
0.53
0.6
0.09
0.02
0.01
0.15
0.07
0.16
2.35
0.55
0.67
6.52
18.6
0.73
0.10
0.07
0.70
0.06
0.19
±
±
±
±
±
±
±
±
±
±
±
0.29
0.15
0.10
1.22
1.47
0.17
0.02
0.02
0.41
0.11
0.18
2.55
0.54
0.60
7.07
18.1
0.78
0.09
0.13
0.50
0.03
0.19
±
±
±
±
±
±
±
±
±
±
±
0.28
0.19
0.15
0.74
1.07
0.21
0.02
0.05
0.26
0.03
0.15
241
88
18
18
432
20
8
33
±
±
±
±
±
±
±
±
92
79
4
4
66
5
1
11
144
7
14
10
347
9
6
21
±
±
±
±
±
±
±
±
6
1
1
2
73
1
1
1
208
113
17
17
404
18
8
34
±
±
±
±
±
±
±
±
6
28
1
1
7
1
0
4
206
82
20
16
398
16
8
35
±
±
±
±
±
±
±
±
15
74
11
2
28
2
1
11
155
19
11
12
404
10
7
22
±
±
±
±
±
±
±
±
22
18
5
3
67
4
1
5
301
99
17
16
427
19
8
33
±
±
±
±
±
±
±
±
150
118
8
4
24
6
1
16
247
88
13
17
432
18
8
29
±
±
±
±
±
±
±
±
4
91
8
3
52
4
1
12
Group 1,
colourless
(n = 1)
1.78
0.34
0.38
5.78
19.4
0.53
0.07
0.05
0.02
0.01
0.49
138
7
15
11
454
8
6
17
Group 2,
nearly
colourless
(n = 8)
Blue–green*
(n = 87)
2.31
0.52
0.54
6.26
18.2
0.68
0.10
0.12
0.56
0.03
0.47
±
±
±
±
±
±
±
±
±
±
±
0.06
0.19
0.04
0.57
1.8
0.04
0.02
0.02
0.34
0.02
0.36
2.45
0.48
0.53
7.08
17.5
0.70
0.08
0.14
0.43
0.03
0.16
±
±
±
±
±
±
±
±
±
±
±
0.11
0.09
0.05
0.66
1.05
0.15
0.02
0.02
0.17
0.02
0.15
223
69
17
20
419
20
8
34
±
±
±
±
±
±
±
±
27
45
2
5
35
4
<1
8
230
59
16
17
424
17
8
26
±
±
±
±
±
±
±
±
21
51
3
3
27
4
1
5
Making colourless glass in the Roman period
Group 1,
colourless
(n = 59)
Means and standard deviations for glasses from Leicester, York and Mancetter
*Blue–green glass compositions were taken from Jackson (1992).
769
770
Figure 3
Britain.
C. M. Jackson
The concentration of the decolorizers measured in colourless and nearly colourless Roman glass from
glasses that are similar in composition to Group 1. The analysis of around 170 samples of
Romano-British colourless glass from Colchester and Lincoln, 40 from Binchester and 240
glasses from various sites around Britain also showed a predominance of low iron-alumina
glasses decolorized with antimony (Heyworth et al. 1990; Mortimer and Baxter 1996; Paynter
2004; Baxter et al. 2005). This same pattern was observed in second- to third-century reliefcut colourless vessels from Britain (Boon 1985, 15), in two second- to fourth-century vessels
from Caerleon and four third- to fourth-century vessels from Caerwent (Brill 1999, 118 –9).
Mirti et al. (1993, 233) analysed eight samples of Italian colourless glass and many were low
iron-alumina glasses, although antimony was not analysed for. This pattern is not geographically discrete; it is also seen in contemporary colourless glass from Sedeinga, Sudan (Brill
1991, 11). Brill suggests that it is likely the latter glass was imported from Alexandria, or from
another glass-making centre.
It is not only Roman colourless glass that appears have been produced using low ironcontaining raw materials (Group 1). Low iron-alumina concentrations are seen in Egyptian
glasses dating from around 1500–1300 bc (although these glasses do not appear to contain
either antimony or manganese and so would fall into Group 1b; Bimson and Freestone 1988),
in first- to third-century Iron Age beads from Meare and Glastonbury (Henderson and Warren
1981) and in Saxon cone beakers (Sanderson and Hutchings 1987).
Colourless glasses exhibiting higher levels of iron and manganese (Group 2) are more difficult to trace in the literature. Sayre (1963, 277) cites examples of ‘nearly colourless’ glass from
Italy which are high in manganese and some similar Rhenish examples from the third to fourth
centuries, although a full composition is not presented. Similarly, Stawiarska (2005) also cites
some second- to third-century high-manganese colourless glasses from Poland. There are also
a number of examples from the late third and fourth centuries, given in Foster (2004).
It is increasingly apparent that glasses that appear to be made of low-impurity sands in conjunction with higher levels of antimony are often the more elaborate and ‘high-status’ vessels,
such as the facet-cut beakers and cage cups discussed by Sayre (1963, 279). It has been suggested that these types were fabricated in the Rhineland (Harden 1987, 107), although this
interpretation is based on distributions of vessel styles rather than manufacturing evidence. An
exploration of the use of decolorizers in specific vessel types is given in Jackson et al. (2003a)
and Baxter et al. (2005).
Making colourless glass in the Roman period
771
Glass raw materials—different sands?
The compositional trend of low iron-alumina seen in the largest group of colourless glasses
(Group 1) may be explained in a number of ways. A strict refining procedure of the sands or
subsequent frits could have been practised (Cole 1966, 47), although washing would probably
have been inadequate to remove the undesirable iron-containing fraction or the alumina and
phosphorus-bearing heavy clay minerals. Potash and calcium oxide, associated with feldspar
minerals such as anorthite, anorthoclase and orthoclase, would also have been difficult to
remove by this method. Chemical washing may have had the desired effect, although it is
difficult to suggest which strong acids may have been used. The model that best explains this
compositional patterning is the deliberate selection of specific low-impurity sand sources, as
Pliny documents and as seen in modern glass-making (Turner 1940, 199; Davies and Rees
1945a; Davies and Rees 1945b, 277). This suggestion is reinforced by the low concentrations
of trace elements seen in these glasses, which are indicative of pure silica sands.
The roles of antimony and manganese in decolouration
The analysis of the colourless glasses suggests that in some cases both decolorizers are
present. Either of these in sufficient quantity would have the desired effect and, therefore, the
presence of both is puzzling. The level of the decolorizer used is relative to the amount of iron
and, as antimony is a stronger decolorizer than manganese, lower quantities will render the
glass colourless. Although Sayre (1963) suggests that levels above 0.2% of either would be the
result of intentional addition, Brill (1988) puts this value for manganese at ∼ 0.4%. This level
is upheld here, as concentrations of manganese are often found in the blue–green glasses at
0.5% or higher (Table 2), where it is assumed that manganese was not being deliberately
added as a decolorizer. The use of antimony as the decolorizer in the colourless glasses (Group
1) is supported by a weak to moderate correlation with iron (r = 0.62); a similar relationship
between iron and manganese in these glasses is not evident.
It is suggested, therefore, that where both decolorizers are present in low quantities, manganese is probably entering the glass either unintentionally or for purposes other than as a
decolorizer. Unintentional addition could be explained by manganese-containing raw materials.
Brill (1999, 457) notes manganese concentrations of 0.43% in sand from the mouth of the
River Volturno, which would give a base level of around 0.3% in a natron glass. Alternatively,
Freestone et al. (2005) suggest that the presence of manganese in some high iron-manganese
fourth-century green glasses (HIMT) is to oxidize sulphur and prevent the glass from going
black. As some of the glasses analysed here are also late Roman, they may be part of the same
tradition. They are also high in iron and titanium (e.g., Appendix, York 12628). Of course, the
effects of recycling cannot be discounted.
In summary, it is suggested that for the majority of glasses analysed here, antimony is the
primary decolorizer. This is because the colourless glass often has lower (or similar) values of
manganese to that seen in the blue–green glass, but has relatively higher concentrations of
antimony. The concentration of manganese required to decolorize these glasses (which have
iron concentrations generally below 0.5%) would appear to be > 0.5%, rather than the 0.2%
suggested by Sayre (1963). This may suggest that the manganese was not deliberately added
as a decolorizer in most cases. It is, however, clear that some greenish-tinged glasses are
decolorized using manganese in relatively high proportions (≥ 1%). In these cases, the addition of manganese is deliberate.
772
C. M. Jackson
Recycling
The effects of indiscriminate recycling of cullet may account for the presence of manganese
in some samples. Indeed, elevated levels of decolorizers, colorants and opacifiers, such as
manganese, lead, copper and antimony, are commonly associated with recycling practices
(Jackson 1997; Freestone et al. 2002).
The glasses produced using low iron-alumina sand, decolorized predominantly with antimony (Group 1) have low levels of copper and manganese, which suggests that they were not
produced using recycled material (Jackson 1997). In contrast, the nearly colourless samples
produced using higher iron-alumina sands (Group 2) have comparable levels of copper, manganese and antimony to the blue–green examples, which suggests recycling in both cases.
Recycling may also explain the presence of both decolorizing agents in the blue–green and
nearly colourless glasses.
Chronological patterning
The conclusion that antimony is being used as a decolorizer in the majority of these
Roman glasses, in preference to manganese, can be tested against the earlier work of Sayre
(1963), Sayre and Smith (1967) and Velde and Gendron (1980), who found that trends of
antimony and manganese oxide ratios varied over time in the Roman period. Work by Sayre
(1963) showed that northern European colourless glass from the late first and early second
centuries appears to have been decolorized using manganese, while by the late second century
it was predominantly decolorized using antimony, or both antimony and manganese oxides.
The trend is towards increased relative manganese concentrations from the end of the
third through to the fourth centuries. Thus, in northern Europe the most dominant groups
throughout the second to late third centuries ad appear to be the antimony and manganese
glasses, although the ratios of the two oxides are not consistent between samples from the
same century.
Using the criterion outlined by Sayre (1963, 265), that intentional decolorizers are
present above 0.2%, most of the colourless glass waste from the mid-second-century site at
Mancetter is antimony/manganese decolorized, while that from the third-century site at
Leicester appears to be mainly antimony decolorized (see the Appendix). The glass from York,
spanning the first and fourth centuries, is predominantly antimony decolorized but includes
all types, including those with high manganese. These may fit well into Sayre’s chronological
development of the use of the two oxides in northern Europe, where in the second and third
centuries both antimony and manganese were used (Sayre 1963, 277). However, of the dated
fragments, there are no manganese-decolorized examples from the late first or early second
centuries.
In contrast, if we assume that manganese was only used intentionally to decolorize glasses
at levels above 0.5%, as suggested earlier, then only seven samples, from Mancetter and York,
appear to be intentionally decolorized using manganese. These glasses all contain manganese,
at around 1%, with only traces of antimony. The two samples from Mancetter can be dated
only broadly to the second century, but dated examples from York exist from the fourth century. This evidence cannot wholly support the idea of a move to manganese glasses in the
fourth century; further analyses of later glasses are needed. However, it is interesting to note
that in the fourth century manganese features heavily in HIMT glasses, thought to be produced
in Egypt (Freestone et al. 2005).
Making colourless glass in the Roman period
773
CONCLUSIONS
These results show that the production of colourless glass was not just a highly developed skill
that required knowledge of the nature of decolorizing agents, but that also the raw materials
for the base glass were chosen specifically to enable the decolorizing properties of the
minerals to have a greater effect. The results presented here suggest that antimony was
the preferred decolorizer, especially for earlier glasses, and that a specific high-grade sand
was chosen for the production of the majority of truly colourless glasses found in the
north-west provinces throughout the Roman period. In these glasses, there is no evidence of
recycling.
A smaller subsample of less successfully decolorized glasses shows different compositional
characteristics. These ‘nearly colourless’ glasses are either decolorized with manganese
(around 1% and above), or with manganese and antimony together (∼ 0.5%), where antimony
acts as the stronger decolorizer. Manganese only appears acting as a decolorizer at concentrations above 0.5%; below this level, it was probably not deliberately added for this purpose. In
these cases, the base glass composition, and hence the raw materials used to produce the glass,
are similar in composition to those used for coloured glasses. There is no evidence that refined
or selected sands were used, and many glasses show evidence of recycling. These glasses are
predominantly later types, which tantalisingly upholds Sayre’s suggestion that there was a
move to manganese in the fourth century. However, this change in the technology of production may be linked to the compositional and stylistic changes seen in all Roman glasses at this
time. Both types of colourless glass are seen at the three sites studied.
These findings would indicate that there are at least two broad models of colourless glass
production, which may reflect some diachronic variation. The first is that the majority of
glasses in the second and third centuries were produced using high-purity sands. Sands such
as these, with high levels of strontium, are suggested by Freestone et al. (2002) to derive from
beach sands, typically Mediterranean coastal sands. There is no evidence for recycling in these
compositions. The second is that other colourless glasses, perhaps those that were less elaborate or of lower status, were produced using raw materials similar to those used to produce
coloured glasses. For these latter glasses, mixtures of decolorizers were used, either intentionally or otherwise, and cullet was added to the melt. It is probable that these two (or more)
glass compositions were produced at different centres. It is not possible to deduce from these
analyses whether this fits with models of centralized primary glass production suggested by
Freestone et al. (2002) and Nenna et al. (2000) for late Roman and early post-Roman glasses
or with a more dispersed pattern indicated by Baxter et al. (2005), or both—the picture is
complex. It is clear, however, that high concentrations of low-impurity colourless glasses in
the second and third centuries indicates centralized control over raw materials and processes.
In the later, higher-impurity glasses, decolorized using a mixture of decolorizers and recycled
material, the pattern suggests more dispersed production.
ACKNOWLEDGEMENTS
The Science and Engineering Research Council are thanked for financial assistance (Award
88803864), and Drs J. N. Walsh and S. James of the NERC ICP–AES facility at Royal Holloway, Egham, are thanked for help with chemical analysis. Dr H. E. M. Cool and Professor
M. J. Baxter are acknowledged and thanked for discussions concerning colourless glass. The helpful
comments from the three referees are acknowledged, and Professor I. Freestone is thanked for
774
C. M. Jackson
reading an earlier manuscript. Without the support from Dr S. E. Warren and Professor J. R.
Hunter, this project would not have been instigated or completed.
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appendix
The composition of ‘colourless’ glass from York, Leicester and Mancetter, UK
Ba
Cu
Li Ni
1.82
1.86
1.75
1.88
1.83
2.37
2.34
2.30
2.31
2.22
2.43
2.31
2.24
1.78
2.33
2.32
2.30
1.97
2.00
2.04
0.37
0.41
0.31
0.30
0.30
0.57
0.35
0.55
0.56
0.36
0.44
0.57
0.63
0.34
0.51
0.55
0.56
0.28
0.46
0.42
0.40
0.50
0.35
0.35
0.33
0.55
0.59
0.53
0.56
0.44
0.56
0.52
0.57
0.38
0.56
0.55
0.55
0.37
0.51
0.43
5.59
6.25
5.15
4.45
4.20
6.12
6.22
5.99
6.17
6.30
7.47
6.01
5.47
5.78
6.49
6.10
6.09
5.52
5.60
5.58
0.08
0.08
0.06
0.07
0.07
0.10
0.10
0.11
0.11
0.06
0.07
0.10
0.12
0.07
0.10
0.11
0.11
0.05
0.07
0.07
0.04
0.05
0.04
0.04
0.04
0.11
0.11
0.10
0.12
0.15
0.15
0.11
0.11
0.05
0.12
0.11
0.12
0.03
0.04
0.04
0.02
0.03
0.02
0.02
0.02
0.37
0.35
0.33
0.41
0.96
1.22
0.30
0.40
0.02
0.44
0.36
0.35
0.01
0.06
0.06
0.25
0.01
0.01
0.01
0.01
0.06
0.06
0.04
0.05
0.01
0.01
0.03
0.05
0.01
0.04
0.04
0.04
0.02
0.06
0.02
0.63
0.74
0.57
0.61
0.52
0.47
0.47
0.45
0.47
0.15
0.03
0.41
0.53
0.49
0.37
0.45
0.47
0.64
0.50
0.69
141
140
140
155
146
214
207
203
214
244
280
202
202
138
224
209
207
145
151
163
9
5
6
7
6
132
125
81
124
14
17
54
73
7
68
63
142
13
58
15
14
16
14
13
12
18
17
17
20
19
14
18
19
15
16
17
17
9
14
14
2.06
2.04
0.35
0.25
0.38
0.30
5.70 19.86 0.59 0.05
5.60 19.66 0.63 0.04
0.03
0.02
0.02
0.01
0.27
0.39
0.74
0.88
158
165
1.84
0.26
0.34
5.06 19.15 0.54 0.04
0.03
0.01
0.02
0.65
145
1.83
0.35
0.53
5.80 19.62 0.45 0.06
0.03
0.01
0.02
0.61
133
20.02
19.76
18.91
18.82
18.97
19.74
19.50
19.59
19.49
17.53
14.20
17.76
19.86
19.44
18.92
19.19
18.70
19.61
19.30
20.24
0.51
0.50
0.45
0.50
0.44
0.71
0.70
0.80
0.70
0.63
0.63
0.65
0.68
0.53
0.69
0.74
0.71
0.64
0.53
0.64
10
13
10
11
8
17
17
16
16
26
28
15
19
11
17
16
19
12
14
14
Sr
V
Y Zn
379
446
360
286
266
409
406
396
412
426
497
384
392
454
427
410
400
386
382
358
10
10
8
8
9
17
18
18
18
20
28
15
19
8
20
19
18
7
11
11
7
7
6
6
6
8
8
8
8
8
9
8
8
6
8
8
8
7
7
7
Assigned
group
Context/ Colour Description
SF no.
20
20
22
21
21
32
39
31
32
31
35
29
33
17
27
29
52
19
25
28
1
1
1
1
1
2a
2a
2a
2a
2b
2b
2a
2a
1a
2a
2a
2a
1a
1a
1a
L19
L19
L19
L19
L19
L19
L19
L8
M64
M63
M64
M24
M24
M24
M63
M24
M63
14331
12266
13493
C
C
C
C
C
NC
NC
NC
NC
NC
NC
NC
NC
C
NC
NC
NC
C
C
C
16 12 15 416
10 12 15 533
8 7 17
7 7 16
1a
1a
12461
11172
C
C
8 12 13 359
8 7 15
1a
13955
C
13 12 14 415 11 7 20
1a
12582
C
Water-rounded lump
Pinched fragment
Water-rounded lump
Pinched fragment
Water-rounded lump
Cylindrical moile
Cylindrical moile
Pinched fragment
Trail
Cylindrical moile
Bubbly fragment
Trail
Chip
Chip
Chip
Chip
Fragment
Chunk
Body fragment
Cylindrical cup,
Isings 85b, late
second/third century
Body fragment
Facet-cut beaker,
Isings 21
Cylindrical cup,
Isings 85b, late
second/third century
Chunk
Making colourless glass in the Roman period
Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO PbO Sb2O5
777
778
appendix
Ba
2.07
0.38
0.44
5.95 18.89 0.61 0.07
0.06
0.45
0.02
0.42
212
14 13 18 384 18 7 24
1/2a
10781
C
1.79
0.33
0.39
5.32 19.87 0.47 0.05
0.03
0.01
0.02
0.57
137
12 12 12 364
1a
13634
C
2.09
0.39
0.44
6.04 19.08 0.65 0.07
0.06
0.43
0.02
0.44
215
15 15 19 389 19 8 24
1/2a
10781
C
1.91
2.10
1.94
2.00
1.67
2.03
2.04
2.02
2.08
1.86
0.39
0.44
0.44
0.50
0.36
0.41
0.44
0.42
0.49
0.35
0.51
0.63
0.48
0.66
0.46
0.55
0.58
0.55
0.50
0.39
6.67
5.93
5.88
6.38
6.08
5.97
5.71
5.97
6.72
5.31
0.07
0.07
0.07
0.10
0.07
0.08
0.09
0.08
0.08
0.07
0.03
0.03
0.05
0.04
0.03
0.03
0.03
0.03
0.04
0.03
0.01
0.01
0.14
0.01
0.01
0.01
0.01
0.01
0.04
0.08
0.02
0.02
0.06
0.02
0.02
0.02
0.02
0.02
0.13
0.02
0.61
0.45
0.53
0.45
0.53
0.46
0.50
0.41
0.88
0.61
131 16 14 19 486 11 8 25
157 14 13 12 429 11 7 22
169 105 22 14 392 12 7 32
138 13 12 15 567 13 7 19
127 15 11 13 436 11 7 22
145 11 11 15 409 12 7 18
158 15 11 12 419 11 8 22
144 11 13 13 406 12 7 19
144 13 15 15 579 11 7 19
156 17 14 15 352 11 7 23
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
12287
14130B
14291A
11993
14321A
11984
14264B
11894
11420
8625
C
C
C
C
C
C
C
C
C
C
1.84
0.37
0.43
5.47 19.47 0.51 0.07
0.04
0.13
0.02
0.58
162
19 16 16 356 12 7 25
1a
8207
C
2.05
1.95
0.44
0.34
0.65
0.41
5.91 19.21 0.40 0.08
5.50 20.51 0.48 0.07
0.03
0.03
0.01
0.01
0.02
0.01
0.45
0.61
156
158
12 13 15 424 11 7 16
10 12 13 396 9 7 15
1a
1a
12201
13657
C
C
1.67
2.05
1.98
1.87
0.34
0.43
0.44
0.28
0.46
0.57
0.55
0.32
5.67
6.01
5.95
4.49
0.03
0.04
0.05
0.03
0.01
0.10
0.09
0.01
0.02
0.06
0.08
0.12
0.89
0.48
0.46
0.72
132
164
156
143
11 13 12 394 9 7 16
61 25 13 416 11 7 25
58 18 11 398 10 7 26
15 7 11 286 5 6 16
1a
1a
1a
1a
13443
14311A
14311B
14346B
C
C
C
C
19.61
18.81
18.44
20.03
18.62
19.26
20.38
19.49
19.62
18.72
19.68
19.75
19.84
18.60
0.54
0.44
0.63
0.45
0.47
0.45
0.39
0.42
0.56
0.49
0.52
0.55
0.52
0.44
0.06
0.07
0.08
0.06
Cu
Li Ni
Sr
V
Y Zn
9 7 22
Assigned
group
Context/ Colour Description
SF no.
Cylindrical cup,
Isings 85b, late
second/third century
Cylindrical cup,
Isings 85b, late
second/third century
Cylindrical cup,
Isings 85b, late
second/third century
Body fragment
Chunk
Chunk
Chunk
Body fragment
Chunk
Chunk
Chip
Body fragment
Body fragment
(fourth century?)
Beaker/flask, late
second century
Body fragment
Globular flask, late
second/early third
century
Chunk
Chunk
Chunk
Body fragment
C. M. Jackson
Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO PbO Sb2O5
Ba
Cu
2.13
0.35
0.52
6.54 19.72 0.62 0.07
0.04
0.03
0.00
0.49
140
18
8
8 7 30
1a
12103
C
1.84
1.62
2.07
1.85
1.99
0.22
0.25
0.33
0.29
0.31
0.33
0.43
0.46
0.41
0.43
5.13
5.13
5.43
5.51
5.70
0.05
0.06
0.07
0.06
0.06
0.03
0.03
0.06
0.04
0.05
0.02
0.02
0.16
0.11
0.11
0.00
0.00
0.01
0.01
0.01
0.40
0.74
0.42
0.41
0.39
142
129
172
152
162
8
5
23
17
17
5 9 354 6 7 16
8 10 358 8 6 15
7 11 355 11 7 28
8 9 355 9 7 23
9 10 377 9 7 23
1a
1a
1a
1a
1a
13835
10864
12392
13288
12037
C
C
C
C
C
1.97
1.87
0.29
0.30
0.45
0.48
6.73 19.62 0.52 0.07
6.90 20.62 0.52 0.07
0.04
0.05
0.02
0.09
0.00
0.01
0.05
0.41
138
151
13
19
6
9
9 469 9 8 21
8 406 10 7 28
1b
1a
14108
13871
C
C
1.90
2.01
0.26
0.30
0.41
0.43
5.32 19.83 0.51 0.05
5.59 18.86 0.62 0.06
0.03
0.06
0.02
0.21
0.00
0.01
0.29
0.35
137
174
6 6
24 11
9 359 6 7 19
9 358 10 7 23
1a
1a
13351
13995
C
C
1.76
0.23
0.37
5.60 18.69 0.42 0.06
0.03
0.02
0.00
0.45
137
12
8 384
7 7 18
1a
12457
C
2.10
0.36
0.53
6.14 19.25 0.66 0.08
0.06
0.20
0.03
0.45
176
56 13 10 403 13 7 32
1a
12252
C
1.93
2.01
2.12
1.78
1.79
1.95
0.29
0.34
0.34
0.27
0.24
0.42
0.42
0.45
0.50
0.39
0.36
0.46
5.54
5.43
5.53
5.19
5.16
5.84
19.59
19.94
18.60
19.62
19.06
19.64
0.55
0.55
0.37
0.47
0.36
0.62
0.07
0.06
0.08
0.05
0.06
0.08
0.05
0.04
0.03
0.04
0.03
0.06
0.11
0.06
0.02
0.10
0.02
0.24
0.00
0.04
0.00
0.01
0.00
0.02
0.42
0.52
0.31
0.49
0.35
0.53
156
158
146
149
135
185
14 10 7 362 9
14 10 9 370 8
7 7 11 358 9
16 8 8 334 7
8 6 8 326 6
21 9 12 388 13
1.87
1.83
1.76
2.18
1.92
0.37
0.31
0.32
0.55
0.27
0.49
0.37
0.40
0.64
0.32
5.77
5.51
5.45
6.43
5.59
18.48
19.21
19.37
19.61
18.44
0.59
0.47
0.44
0.79
0.46
0.08
0.06
0.07
0.09
0.05
0.04
0.04
0.03
0.07
0.03
0.03
0.02
0.02
0.26
0.02
0.01
0.01
0.01
0.03
0.24
0.64
0.40
0.53
0.40
0.57
149
141
136
214
146
5 8 10
14 6 10
11 9 11
62 28 13
8 6 10
18.55
19.37
19.48
19.42
19.01
0.48
0.42
0.59
0.49
0.55
Li Ni
6
Sr
8 504
V
Y Zn
Assigned
group
Context/ Colour Description
SF no.
7
7
7
6
7
7
24
21
19
26
18
27
1a
1a
1a
1a
1a
1/2a
14309A
14310A
14320
15522
14308A
9707
C
C
C
C
C
C
400 9 6
345 7 6
370 8 6
430 15 8
401 6 6
18
19
24
40
16
1a
1a
1a
2a
1a
13992
9501
13778
11325
13864
C
C
C
NC
C
Cylindrical bottle,
late second/third
century
Body fragment
Chip
Body fragment
Body fragment
Wheel-cut beaker,
late first/second
century
Body fragment
Cylindrical cup,
Isings 85b, late
second/third century
Body fragment
Cylindrical cup,
Isings 85b, late
second/third century
Body fragment
(fourth century?)
Body fragment
(fourth century?)
Body fragment
Body fragment
Body fragment
Body fragment
Body fragment
Vertical-ribbed jug,
late second/third
century
Wheel-cut bowl
Circular facets
Body fragment
Body fragment
Body fragment
Making colourless glass in the Roman period
Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO PbO Sb2O5
779
780
appendix
Ba
Cu
1.86
0.37
0.45
5.81 18.04 0.46 0.07
0.04
0.02
0.01
0.65
143
22
7 12 403 10 7 36
1a
8442
C
1.99
2.00
0.47
0.41
0.53
0.47
6.03 19.70 0.63 0.09
6.01 19.22 0.51 0.08
0.06
0.04
0.07
0.03
0.03
0.01
0.72
0.61
154
146
14
14
7 13 409 10 7 25
6 8 438 9 7 24
1a
1a
8516
13716
C
C
1.97
2.27
0.33
0.73
0.47
0.77
4.99 17.99 0.49 0.07
5.53 19.99 0.85 0.13
0.03
0.07
0.02
0.92
0.01
0.04
0.25
0.16
150
231
6 6 8 324 8 6 15
84 22 18 394 24 8 38
1a
2b
8674
12628
C
NC
2.23
2.65
2.12
2.16
0.71
0.44
0.51
0.69
0.74
0.59
0.52
0.77
5.95
8.10
6.05
5.64
0.12
0.07
0.08
0.12
0.07
0.08
0.07
0.07
1.00
1.06
0.22
0.87
0.03
0.02
0.30
0.03
0.08
0.08
0.54
0.16
318 85 24
581 25 11
183 350 27
239 124 18
44
18
60
35
2b
2b
2a
2b
10315
12788
9718
13770
NC
NC
NC
NC
2.19
2.25
0.35
0.75
0.66
0.81
5.91 18.92 0.43 0.08
7.66 20.08 0.59 0.13
0.03
0.05
0.02
0.93
0.00
0.01
0.31
0.11
152
229
11 8 10 433 9 7 16
11 13 19 675 28 8 19
2?
2b
14309B
12096
NC
NC
19.76
17.78
19.37
18.85
0.98
0.74
0.67
0.75
C = colourless; NC = nearly colourless.
Context/SF no.: L = Leicester; M = Mancetter. All other samples are from York.
Li Ni
21
16
16
20
Sr
430
461
401
418
V
22
21
15
24
Y Zn
8
9
7
7
Assigned
group
Context/ Colour Description
SF no.
Cylindrical cup,
Isings 85b, late
second/third century
Chip
Cylindrical bottle
neck
Body fragment
Isings 96/106, fourth
century
Body fragment
Body fragment
Body fragment
Body fragment,
fourth century
Body fragment
Body fragment
C. M. Jackson
Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO PbO Sb2O5