Ancient Monuments Laboratory
Report 21/95
INVESTIGATION OF POSSIBLE
GLASS FRAGMENTS FROM SITE 2 ,
WESTHAMPNETT, SUSSEX
C Mortimer
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England.
Ancient Monuments Laboratory Report 21/95
INVESTIGATION OF POSSIBLE
GLASS FRAGMENTS FROM SITE 2,
WESTHAMPNETT, SUSSEX
Catherine Mortimer
Summary
Energy-dispersive X-ray analysis was carried out on tiny fragments of 'glassy'
material recovered by sieving Iron Age, Romano-British and Anglo-Saxon
contexts. Four samples were found to have soda glass compositions comparable
with Roman material. Five samples are comparable to late medieval and postmedieval 'forest glass', but could have been made accidentally. Three samples
of accidentally-formed glasses (fuel ash slags) were also analysed.
Author's address :Catherine Mortimer
Ancient Monuments Laboratory
English Heritage
23 Savile Row
London
WlX lAB
©
Historic Buildings and Monuments Commission for England
Investigation of possible glass fragments from Site 2, Westhampnett, Sussex
Catherine Mortimer
During the course of post-excavation work at this multi-phase site, small fragments of 'glassy'
material were recovered through the sieving oflate Iron Age (first century BC), Romano-British
and Anglo-Saxon contexts (1mm and 2mm residues). Glass artefacts are relatively rare in Iron
Age contexts and so these samples, although small, merited further investigation.
More than half the samples were discovered to be natural quartz, in the form of large
grains, usually rounded and sometimes slightly tinted (pink or brown).
Of the remaining samples, two major types of material were noted, one a black shiny
substance, without any porosity visible (found in 24 contexts) and the other translucent glasses,
of various colours- green, blue and near-colourless (22 contexts). One dark blue glass bead was
identified (context 20246) and one of the pieces of glass was said to show tooling marks (context
20323). Fragments of fuel ash slag were noted in seven contexts.
Glassy material may be formed accidentally, in any high-temperature environment where
silica can react with alkalis. This obviously includes cremation pyres, where silicate matter (from
the soil and the ashes) and alkaline materials (from the wood ash) are together at a high
temperature. Elevated levels of iron are also likely to be present and could cause dark glassy
slags. It was considered that the only way to distinguish between accidentally formed glasses and
deliberately formed glasses was by their chemical composition and for this reason quantitative
analysis was carried out on 13 samples from Iron Age and Roman contexts, using SEM-EDX
(scanning electron microscopy-energy dispersive X-ray analysis). The results of this analysis are
shown in Table 1.
Four of the translucent coloured samples (1 to 4) have compositions which conform
closely with known Roman soda-glass compositions (eg Heyworth et a/1990; Turner 1956, Table
IV) but which can be contrasted with known medieval soda glasses, for example, by virtue of the
high magnesium contents in medieval soda glasses (eg Barrera an9 Velde 1989; Mortimer 1991).
Despite these comments, it should be noted that glass cannot be unequivocably dated by its
composition; a very large range of compositions are known to have been used during the medieval
period.
The compositions of samples 1 and 2 are very similar and they may be fragments from the
same vessel. Similarly, the compositions of samples 3 and 4 are very similar to each other and
may be from another vessel. Another sample (5) is also comparable to these soda-glass
compositions except it has lower sodium levels and rather high lime levels.
Five samples of green or dark green glass (6 to 10) have compositions which closely
resemble late medieval and early post-medieval 'forest' glass of the high-lime, low-alkali type (eg
Mortimer 1991; 1993). This glass type was used to make windows and vessels during these
periods but was not known in the Iron Age or Roman periods.
It is possible that the 'forest' glass compositions were formed accidentally in a pyre but I
think that both the soda glasses and the 'forest' glasses are likely to be deliberately formed glasses.
Their presence in Iron Age and Roman contexts therefore requires explanation.
Three samples have compositions which are not paralleled in ancient glasses. One of these
(11) was identified visually as being fuel ash slag (grey-green and vesicular) and this has an
alumino-silicate composition, with traces of iron. The other two samples are dark green or 'black'
(12 and 13) and are rather more glass-like in appearance than the fuel ash slag, but they have very
high manganese, iron and aluminium contents and low alkali contents. These are likely to be
accidentally formed glasses and certainly do not conform to what is known about coloured glass
bead compositions of the Iron Age (eg Henderson 1987).
These analyses were earned out on a small selection from the material available at the site.
It is thought likely that the other glassy material retrieved by sieving is of a comparable nature,
but without quantitative analysis, it is difficult to be sure. A listing of identifications is provided
in the appendix, on the basis of the work carried out here.
References
Barrera J and Velde B 1989 'A study of french medieval glass composition' Archeologie
Medievale 29, 81-130.
Henderson J 1987 'Glass' and 'Glassworking' in Cunliffe B W Hengistbury Head, Dorset, Volume
1: the prehistoric and Roman settlement, 3500BC- AD 500; 160-3 and 180-6.
Heyworth M P, Baxter M J and Cool H 1990 'Compositional Analysis of Roman Glass from
Colchester, Essex' Ancient Monuments Laboratory Report no 53/90
Mortimer C 1991 Analysis ofmedieval and post-medieval glass from the City ofLondon, Ancient
Monuments Laboratory Report no 135/91
Mortimer C 1993 Analysis of window glass from Chastleton House, Oxfordshire' Ancient
Monuments Laboratory Report no 117/93
Turner W E S 1956 'Studies in Ancient Glasses and Glassmaking Processes. Part IV. The
Chemical Composition of Ancient Glasses' Journal of the Society of Glass Technology 40; 162186.
Appendix
Contexts with 'black glassy' material (accidentally-formed glass)
20070,20094,20096,20100,20150,20156,20164,20180,20190,20238,20243,20247,20281,
20339 (lots), 20343, 20347, 20349, 20383, 20390, 20391, 20462, 20621, 20756, 20783
Contexts with glass
transparent pale blue glass, 20150, 20740, 20792
transparent pale green glass: 20323, 20371, 20679, 20771, 20074, 20079, 20129
transparent dark green: 20241 (with irridescence), 20726
near-colourless glass 20058, 20139, 20164, 20281, 20206, 20238, 20336, 20390,
20458,20594
Contexts with fuel ash slag
20129,20352,20485,20649,20775,20070,20074
2
Table: SEM-EDX analysis
Sample no.
Context
I
2
3
4
5
Small Find no. Description
20740
29293 trans!. colourless
20792
29321 trans!. colourless
20241
20150
20206
trans!. blue
trans!. blue/green
trans!. colourless
6
7
8
9*
10
20074
20079
20129
20241
20679
29043 !green 'glass'
29046 Igreen
29070 Igreen
II
20070
20074
20726
27050 lgrey-green 'FAS'
29043 dark green 'slag'
27385 dark green
12
13
dark green
green/brown
A.I,o,
MgO
Na,O
SiO,
15.7
16.5
18.8
19.4
10.9
0.9
1.1
2
2
1.8
1.8
0.5
I
P,O,
I
71.7
70.9
69.1
68.9
69.5
0.5
0.5
0.5
0.5
0.4
4.2
3.2
2.4
3
5.7
4.7
3.9
1.5
1.9
4.4
64.3
60.2
64.3
52.2
59
1.2
3
3.6
2.5
0.7
0.4
nd
nd
nd
0.3
42.9
8.9
8
54.1
61.6
59.2
0.9
1.4
0.4
2.5
1.1
0.7
1.2
nd
I
1.1
.
Cl
S
nd
0.3
0.5
0.3
0.7
na
na
na
nd
CaO
KO
nd
nd
nd
nd
nd
na
na
0.5
na
nd
nd
na
na
na
na
0.3
nd
TiO
3
nd
nd
nd
na
na
nd
nd
1.5
1.6
0.9
1.4
1.9
0.7
0.7
0.7
0.7
2.6
2.5
6.7
5.2
1.4
19.5
21.6
18.8
18.1
24.6
I
nd
0.6
0.4
14.2
20
Samples analysed three times each, using SEM-EDX on default settings. Average normalised weight percentages are given
for twelve oxides and elements.
na =not analysed for; nd =not detected (below 0.2%).
0.3
0.4
0.7
0.9
nd
nd
-----------
* This sample suffered from poor sample preparation and was analysed only once.
nd
nd
0.3
0.4
0.4
0.3
7
7.3
6.2
6.1
16.1
1.3
Fe20 3
MnO
na
nd
0.3
0.4
0.9
na
na
I
nd
nd
2
3.1
2.5
10.6
7.1