Characterising copper-based metals in Britain in the first millennium AD: a preliminary quantification of metal flow and recycling.
Pollard, A.M. ; Bray, Peter ; Gosden, Chris 等
[ILLUSTRATION OMITTED]
In an essay studying the evidence for Bronze Age metalworking in
the Middle Volga of Russia, Peterson (2009: 194) wrote:
Although discussions of ancient Eurasian metalwork have frequently
assumed a direct correlation between the element composition of the
metal in objects and their sources, practices such as alloying and
recycling can alter the element profiles of copper and bronze to
the point that the origin of the materials found in individual
pieces is hopelessly obscured. Recycling also has significant
socio-economic implications, as it changes the production cycle
from one that is initiated with mining to another in which
metalworkers may operate more independently from miners and
smelters.
This position is in stark contrast to the more optimistic views
about provenancing ancient metal artefacts that came to dominate
archaeometallurgy for a large part of the previous century. It would
appear that, in most quarters, this optimism regarding the suitability
of trace elements, and subsequently lead isotopes, for determining the
provenance of copper (Cu) has now given way to a realisation that these
enquiries are potentially complicated, and perhaps even futile at times
when recycling and re-alloying are common. We present here a new method
of examining the chemical data that we believe has the potential to
reveal the extent of such practices and to point towards a way of
partially disentangling such information. It is based on the concept of
a dynamic flow of metal in circulation over both space and time, which
manifests itself as a particular set of objects with particular forms at
any given time, rather than taking a static view of both form and
composition as fixed properties. The composition of this flow of metal
is influenced by the coming and going of new sources of metal, but it is
also affected by the recycling and re-alloying of metal objects already
in circulation. Thus, the chemical and isotopic analyses of a set of
objects are merely a 'snapshot' of the composition of the
metal in circulation at that time. We see chemical composition as an
indicator of the 'life history' of the metal from which a
particular object is made. We do not initially attach importance to the
exact values of the composition of an object. Instead, we initially use
the presence or absence of trace elements to classify the category of
metal in circulation, and only then use the distribution of particular
elements within and between these categories to document this history
more closely.
To illustrate this methodology, we present a very broad preliminary
interpretation of the copper in circulation in Britain from the late
Iron Age to the beginning of the medieval period, with a particular
focus on the transition from the end of Roman occupation into the
Anglo-Saxon period. To do this, we have used four large datasets
covering the period: Caple (1986), Blades (1995), Dungworth (1995) and
Bayley and Butcher (2004). These datasets were selected because of the
large number of multi-period analyses that they present (c. 5340
samples), and because they appear to be internally and mutually
consistent. A more detailed discussion of the data is given in the
online supplementary material. For the purposes of this preliminary
overview, we have focused on a restricted time period, from the early
Iron Age to the end of the late Saxon period (AD 1066), although most of
the data fall into the first millennium AD. We consider two primary
aspects of the data--the varying proportions of alloying metals--tin
(Sn), lead (Pb) and zinc (Zn) used over time, and, for Blades's
data only, the trace elements--arsenic (As), antimony (Sb), nickel (Ni)
and gold (Ag)--present in the copper.
Classification of alloys
Traditionally, archaeological copper alloy types are classified by
the amounts of each alloying element (zinc, tin and lead) present,
displayed either as a triangular diagram (plotting only the normalised
values of these three elements), or as a binary plot (usually tin vs
zinc), with the term 'leaded' being added if the lead
concentration exceeds a specified value. For example, Table 1 shows the
definitions specified by Bayley and Butcher (2004: 14), based on a
combination of zinc: tin ratios and absolute values for tin or zinc.
Using this, a 'brass' has to have more than 8% zinc and also
four times more zinc than tin. If, in addition, it has more than 4% but
less than 8% lead, it would be a '(leaded) brass': if the lead
content is greater than 8%, it is then a 'leaded brass'. Other
classification schemes, if not identical, are very similar.
We have taken a completely different approach to categorising the
alloy type, which is indicative of a fundamentally different
philosophical approach to understanding the life histories of the
objects analysed. Previous classifications have assumed that alloy
composition is mostly deliberate and targeted at producing alloys with
approximately modern specifications. For example, an object is only
called a 'brass' if it contains more than 8% zinc or a
'bronze' if it has greater than 3% tin--in other words, if the
level of the alloying element exceeds a value that is considered to have
an effect on the physical properties of the object and could therefore
be expected to be a conscious addition. Our approach is radically
different, in that we do not want to impose any assumptions about past
practice, which may have ranged from 'designed recipe' to
'randomness' caused by cycles of recycling and mixing. We wish
to categorise the chemical results impartially, with the view that the
concentration of particular alloying elements in a specific object
depends on the life history of the object. By looking at assemblages of
objects as snapshots of the overall metal flow, we aim to infer the
metallurgical and social processes at work. We acknowledge that some
objects will always have been made from 'primary' metal, with
'deliberate' levels of alloying, but note that such objects
are easily recognised in our scheme, as shown below.
We have considered the presence of tin, lead and zinc as
significant if they are above a cut-off value, which is set at 1%--much
lower than previous studies but high enough to exclude natural
contributions from the copper ore itself. Our classification system is
shown in Table 2, and is intended to have the effect of allowing the
visibility of the mixed nature of copper alloys (i.e. the simultaneous
presence of several alloying elements at levels below those that could
be regarded as having a significant effect), rather than focusing on
those objects that might have 'designed' compositions. It does
mean, however, that when we refer, for example, to an alloy as being
leaded brass, it may contain as little as =1% of both zinc and lead, and
it therefore would not be a brass alloy in any modern sense.
Change of alloy composition over time
Table 3 shows the ubiquity (percentage of the assemblage composed
of that particular alloy) of each alloy type for all of the assembled
data. The definitions of alloy type are based on the presence of each
alloying element above 1%, as listed in Table 2. The chronological
classifications used are those given in the original publications, but
for Bayley and Butcher's Roman brooch data we have attempted to
allocate the types to a more specific chronological phase, as described
in the online supplementary material.
A number of features stand out from Table 3, most strikingly at the
end of the Iron Age and at the end of the Roman period into the early
Saxon. The Early Iron Age and Iron Age data of Dungworth show that the
metal in circulation consists primarily of bronze, at about 73%
ubiquity. Brass is almost completely absent, as is any ternary alloy
containing zinc i.e. leaded brass, gunmetal and leaded gunmetal.
Dungworth's Late Iron Age data shows a marked shift to a more
'Roman' pattern--characterised by the presence of brass and
some of the ternary and quaternary alloys containing zinc (leaded brass,
gunmetal and leaded gunmetal). Bayley and Butcher's Late Iron Age
samples (of which there are only 10), are somewhat different, in that
they consist primarily of bronze, with no brass, but with a combined 40%
of gunmetal and leaded gunmetal, indicating that alloys containing zinc
were available at this time. The early Saxon period (Blades's)
material is very similar to that of the C4AD (fourth century AD Roman,
Dungworth), but there is a complete change in the metal supply during
the middle Saxon period (Blades's data), as indicated by a rapid
drop in the ubiquity of leaded gunmetal (from 75-25%), a rise in leaded
bronze (from 18-42%) and the re-appearance of brass and leaded brass (up
to 9% and 7% respectively).
In order to investigate diachronic changes more carefully, we have
created a time series' of alloy types by combining the data into
the following sequence using the chronological intervals defined in
Table 4. Some of the trends identified above are now more clearly seen
by plotting the ubiquity of a single alloy category over time (figure
1a-e). Figure 1a shows that bronze was the dominant alloy in the Iron
Age, accounting for more than 70% of the objects in the assemblages, the
remainder being mostly leaded bronze. Bronze declined sharply in the
first century AD, with brass (figure 1b) suddenly appearing in 30% of
the objects from the late Iron Age and first century AD (CIAD). This too
declined in the second century AD and was reduced to virtually nothing
by the early Saxon period. Leaded bronze (Figure 1c) dominated the Early
Iron Age, and was always a significant component of the assemblage; yet,
apart from a 'spike' in the post-conquest Roman brooches, it
did not dominate again until the middle and late Saxon periods. The
ternary copper-tin-zinc alloy (Cu-Sn-Zn) gunmetal (Figure 1d) also first
appeared at the Late Iron Age and Roman margin (first century AD), as
did leaded gunmetal (Figure 1e) but less significantly. Leaded gunmetal,
then, shows a steady increase in ubiquity from the late Iron Age through
the Roman period, peaking at over 70% in the early Saxon period, whereas
gunmetal peaked during the first century AD and declined for the rest of
the millennium.
Some of the irregularity of the profiles shown in Figure 1 may well
be due to overlap and uncertainty between the chronological phases;
another factor may be that the four sources in the sample contain
objects of different function and technology from different regions of
Britain. Smythe (1931-1937), for example, in his study of 250
metallographic sections of Roman copper alloy objects from northern
Britain, concluded that 61% had been cast and 39% wrought. We take some
comfort from the analysis shown in Table S4 (supplementary material),
which shows our alloy reclassification of the Roman objects from each
published source, indicating that the ubiquity of each alloy type is
approximately consistent across the data. The trends in Figure 1 become
even clearer if we restrict the data to the better dated material
through the Roman period (i.e. using Dungworth's categories of
C1AD-C4AD, and dropping his less chronologically precise EROM, MROM and
LROM, as well as the Bayley & Butcher Roman brooches and
Blades's Roman group), as shown in Figure 2 (containing 1486
samples). This reveals a monotonie decline in the ubiquity of brass from
the first century AD, accompanied by a strong and monotonie increase in
the ubiquity of leaded gunmetal, peaking significantly in the early
Saxon period and followed by a rapid fall. Gunmetal peaked in the second
century AD, after the decline of brass had begun, but then dropped away
in parallel to the decrease in brass. We interpret the brass signal as
the injection of new metal stock (in the form of brass) in or just
before the first century AD. The peak in the abundance of the ternary
alloy gunmetal (Cu-Zn-Sn) in the second century AD we take to indicate
the subsequent practice of mixing this new brass with the local bronze,
as previously suggested by Dungworth (1997). We also take the relentless
rise in the presence of the quaternary alloy leaded gunmetal as a clear
indication of the increasingly widespread practice of mixing and
recycling metal in general, given that the most likely route to the
production of gunmetal and leaded gunmetal is the mixing of copper
alloys containing both tin (bronze or leaded bronze) and zinc (primarily
brass). We contend, therefore, that the recycling of metal
(conservatively indicated by the ubiquity of leaded gunmetal) was a
practice that steadily increased throughout the Roman occupation of
Britain and continued, using the same metal stock, into Blades's
early Saxon phase (AD 430-650), as evidenced by the peak in the ubiquity
of leaded gunmetal at this time. By our interpretation, at least 70% of
the copper alloy objects in circulation contained some proportion of
recycled metal. Whether this indicates only the recycling of Roman metal
in the early Saxon period, or a continuing flow of fresh metal that was
rapidly mixed with recycled stock, is more difficult to say, and is
discussed further below. It has long been recognised that the curation
and recycling of Roman metalwork, both ferrous and non-ferrous, took
place in the fifth and sixth centuries (e.g. Fleming 2012), but this is
the first time that we have had any clear indication of the scale at
which such recycling may have taken place.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
It is possible to combine the data shown in Figure 2 to emphasise
the significance of all alloy types containing zinc during the Roman and
early Saxon periods. This is done by combining the ubiquities of
brass+leaded brass+gunmetal+leaded gunmetal into a 'zinc'
group, and copper+leaded copper+bronze+leaded bronze into a no
zinc' group, as shown in Figure 3. This emphasises the importance
and prevalence of alloys containing zinc throughout the Roman period and
continuing into the early Saxon period, but it is not a direct measure
of the amount of zinc in circulation, for using our definitions any
alloy with >1% zinc would be counted as a zinc-containing alloy.
Figure 4 shows the actual distribution of the zinc content in Roman
brass, Roman gunmetal and Roman leaded gunmetal, compared to early Saxon
leaded gunmetal. This shows that brass (i.e. using our definition, a
copper-zinc alloy, with tin and lead <1%) was remarkably consistent
in terms of zinc content throughout the whole Roman period (although of
course the majority of the brass objects belong to the first century
AD), with a typical zinc content of 15-22%. The zinc distribution in
Roman gunmetal (Cu-Zn-Sn) shifted to a broader and lower peak between
about 10 and 20% zinc, with a few samples having <5% zinc. This is
consistent with the idea that this alloy was made by mixing brass with
bronze, with the consequence that the zinc levels were diluted. Roman
leaded gunmetal (Cu-Zn-Sn-Pb), on the other hand, has a very different
pattern, with the majority of samples having <5% zinc, and a
'tail' in the zinc distribution going up to c. 12%. The early
Saxon pattern for zinc in leaded gunmetal is virtually identical to
this, and, as shown in Table 3, the early Saxon period was dominated by
this alloy type (c. 75%), and the almost complete absence of any other
alloy containing zinc. We are led to conclude not only that the dominant
presence of leaded gunmetal is indicative of the use of highly recycled
metal stock, but also that the nature of such metal in the early Saxon
period is indicative of a large-scale continuity of Roman metal in
circulation.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
It has long been known that the zinc content of Roman copper alloys
declined after the first century AD. For example, from the analysis of
Roman brass coins, Caley (1964) noticed that the zinc content declined
from the late first century BC to the early third century AD (when
production of brass coins ceased). He suggested that brass production
started in the late first century BC but stopped shortly after, and that
brass coins of the late first century AD onwards were made from recycled
brass. This 'zinc decline' (Dungworth 1995: 148; 1997: 907)
was thought to be a consequence of the volatility of zinc, which meant
that the zinc content of the brass declined after each re-melting (Caley
1964: 99). Figure 4 shows a somewhat more complex picture. The zinc
content of brass (copper with zinc =1%) seems to be constant at 15-22%
throughout the Roman period in Britain, although the ubiquity of such
alloys was very low after the second century. This suggests that the
brass that has survived is largely primary metal. The 'zinc
decline' was due to the increasing ubiquity of other
zinc-containing alloys (gunmetal and leaded gunmetal), with a
significantly lower zinc content. Although the loss of zinc through
volatilisation is undoubtedly a factor to be considered, we suggest that
the primary cause of Caley's 'zinc decline' was most
likely the mixing of brass with other alloy types.
Trace elements: allocation of an object to a 'copper
category'
The picture of recycling suggested by our analysis of the main
copper alloy types is further supported by analysis of the trace
elements in the copper objects. We have described elsewhere the
philosophy behind our approach to classifying copper alloy objects into
one of 16 'copper categories' using the presence or absence of
four key trace elements in the copper itself--arsenic, antimony, gold
and nickel (Bray & Pollard 2012; Pollard & Bray 2014). There are
two stages to our interpretation, the first of which is a broad
categorisation step, based upon the presence or absence of these four
diagnostic elements. The second considers the distribution of the
elements within and between the copper categories. For example, whether
the arsenic is broadly distributed or heavily skewed, mono- or
multi-modal and so on, can be associated with metallurgical practice. It
is important to appreciate immediately that 'copper category'
is not synonymous with 'copper source'--several sources
(mines) may have produced the same 'category' of copper, and a
single source may have produced more than one copper type, depending on
ore geochemistry, ore pre-treatment, smelting conditions, refining
processes and so on. Our system is intended to look at the complete
biography of the metal that composes the objects, rather than focus
simply on the 'birth' of the metal. In other words, we see a
separation between the unit of metal that makes up an object (which may
persist in broad association for centuries), and the form of the object
itself (which could be a relatively short event before re-casting).
To allocate an object to category, we first mathematically strip
out the alloying elements (tin, lead and zinc), and re-normalise the
remaining elements to 100%, to give what we describe as the composition
of the copper base'. We realise that this is somewhat artificial,
but it is a necessary step to avoid 'dilution effects' on the
trace elements when significant levels of alloying elements have been
added. It also assumes that the trace elements of interest are primarily
associated with the copper rather than the alloying elements. This is
reasonable in the case of antimony and nickel, but more difficult with
gold (which is often associated with lead), and perhaps a matter of
definition with arsenic. We deal with the lead/gold issue by checking
that objects in the 'Ag present' categories do not all contain
high lead in the data under study. With regard to arsenic (and possibly
also antimony), we can broadly check that it is an incidental component
of the copper alloy (and therefore associated with the copper) through
the overall arsenic distribution pattern. In cases where it is a
deliberate alloying element, we see dramatic peaks at high percentages
in the distribution plots, which are different to the gradually
declining curves at low abundance that are associated with incidental
presence. At the levels encountered here, we can, however, safely ignore
this issue.
Having calculated a 'raw copper composition' for each
object, we can now assign each object to a 'copper category'
(CC) based on the presence or absence of the four trace elements
arsenic, antimony, gold and nickel. We simply allocate the metal
composition to one of 16 bins (denoted as copper category 1-16), based
on the presence or absence (Y/N) of each trace element, and ordered as
listed in Table 5. Thus, a metal with arsenic but nothing else would be
YNNN (assigned as Group 2 in our numbering system), whereas
Ag--only' would be NNYN (Group 4). Again, an arbitrary decision is
needed about when 'presence' becomes 'absence'. This
can be determined by a close examination of the data to estimate minimum
detectable levels (mdl) for each element, but in mixed data (i.e. data
from different methods of analysis and different dates of determination)
it is often only possible to use an arbitrary cut-off, which, as in the
work reported here, we typically set at 0.1%. It is possible to check
the sensitivity of the outcomes to this arbitrary choice by varying the
cut-off value and looking for patterns of stability in the distributions
of copper categories. For a more complete analysis, we would then
continue by using mapping options within a GIS to determine the extent,
movement and timing of the circulation of particular copper categories.
It is important to note that this method can be applied
independently, without any prior knowledge of copper sources, and is
thus a powerful means of testing previous assumptions. While there is
undoubtedly information about source within these descriptions, we do
not make any assumptions about allocating a particular copper category
to a specific copper source, known or unknown. A single category may
contain copper from one or more discrete geological sources--all we can
say is that these sources must have the same characteristic pattern of
trace elements. For example, the As-Sb-Ag metal of Group 12 (YYYN) is
associated with Bronze Age mines in Ross Island of County Kerry,
Ireland, but also with the 'Osenring' metal type from
continental Europe. Mapping and chronology can, however, help to
distinguish the metal in circulation from geographically discrete
sources.
Change of copper categories over time
We have, as yet, only been able to apply our methodology for trace
elements to the data published by Blades (1995), as none of the other
studies (Caple 1986; Dungworth 1995; Bayley & Butcher 2004) reported
the full suite of trace elements. This means that in the Roman period we
are currently restricted to the 94 samples analysed by Blades and simply
identified chronologically as 'Roman'. It does, however, allow
us to look at the continuity of copper supply from the end of the Roman
period into the medieval era. Table 6 shows the allocation of all of
Blades's data into copper category by period.
The Roman copper objects are all made of copper categories
containing no nickel, and only CC6, a mere 1.1% of the assemblage,
contains arsenic. The Roman assemblage is dominated by copper of CC3
(NYNN, or copper which contains only antimony as an impurity above
0.1%), with a contribution from CC7 (NYYN, copper with antimony and
gold). Our work with Bronze Age copper (Bray 2009; Bray & Pollard
2012) suggests that in Britain this combination of copper types, both
containing antimony, is uniquely Roman, and as yet we do not know where
it came from, or whether it represents partially refined copper
containing both arsenic and antimony, from which the arsenic has been
removed. This seems a plausible scenario due to the higher vulnerability
of arsenic to loss through simple oxygen refining (Bray & Pollard
2012). Copper categories CC9-16, which contain various combinations of a
wider range of trace elements than those found in Roman copper, first
appeared in the early Saxon period, but all in small proportions of the
assemblage.
[FIGURE 5 OMITTED]
Figure 5 shows the ubiquity of the main copper types (1, 3, 7 and
12) identified in Britain from the Roman period to the medieval era. CC3
(NYNN) dominated the metal stock in the Roman period (>60% of the
objects analysed), and declined from then until the mid-Saxon period (c.
15%), after which it experienced a revival in the late Saxon period
(>40%). CC7 (NYYN) rose from 20% of the assemblage in the Roman
period to nearly 50% in the early Saxon period, and then declined. CC12
(YYYN) was absent in the Roman period, but rose to c. 15% of the metal
stock by the mid-Saxon period. CC1 (NNNN, a copper base containing no
other trace elements) was generally between 5 and 15% of the assemblage
throughout the entire sequence. Overall, we see the continuing dominance
from the Roman to the middle Saxon period of two copper categories, CC3
and CC7, which are rarely seen in any other British archaeological
contexts. When combined with the overall alloying patterns discussed
earlier, it seems clear that this is caused by the reuse and recycling
of earlier Roman metal, and therefore also continuity in technological
practice. The simplest explanation for the decline of CC3 (antimony
only) in the early Saxon phase, combined with a rise in both CC7
(antimony+gold) and CC12 (arsenic, antimony and gold), is that we are
seeing the arrival of new metal alongside the re-use of Roman metal,
which consisted of CC12. The recycling or refining of CC12 would result
in CC7, which has had the arsenic driven off, and thus metal consisting
of CC3 represents predominantly recycled Roman material, whereas CC7
could represent either refined or recycled CC12. We note that if CC3 and
CC7 were to be mixed, the resulting copper would still be classified as
CC7.
[FIGURE 6 OMITTED]
The CC12 (YYYN; copper with arsenic, antimony and gold, but no
nickel) that appeared in the middle Saxon period but then declined into
the early medieval period is crucial to understanding the relationship
between English and continental patterns of copper use. It may indicate
a brief importation of large volumes of fresh, relatively unaltered
copper-alloy material reaching England from continental Europe during
the middle Saxon period. This supply quickly closed with the events of
the late Saxon period, and this particular copper base only became
prevalent again in the later medieval period. It is tempting to link the
influx of a fresh metal supply in the middle Saxon period to renewed
trade and cultural contacts with the Franks and, through them, the
Mediterranean world. It is apparent from the commercially developed
parts of eastern England during the eighth and ninth centuries, at sites
such as Flixborough and Brandon, that at least some high-status
communities were able to discard reusable metalwork on a lavish scale
(Evans & Loveluck 2009).
We can shed a little more light on the changes to metal supply that
happened between the early and middle Saxon periods by combining
information on copper category with alloy type. Focusing on zinc, Figure
6 shows the zinc distributions in all early and middle Saxon leaded
gunmetals to be similar. However, if we divide the middle Saxon
gunmetals into those made from CC7 copper and those from CC12, we see
that although both are dominated by values <5% zinc, a significant
proportion (>20%) of the CC12 copper samples contain around 11 %
zinc. We have shown above (Table 6, Figure 5) that CC12 copper (YYYN) is
a new addition to the metal supply in the middle Saxon period. We
suggest, therefore, that we are seeing an injection of fresh metal
supply into Britain during the middle Saxon period, consisting of CC12
copper alloyed with c. 11% zinc.
Conclusions
All of the authors of the data that we have used accepted in their
original publications that recycling was a necessary part of the
picture. For example, Dungworth (1995), in the abstract of his
dissertation, stated that:
It is clear that copper alloys were recycled and that some care was
taken over the ways in which this occurred. The lack of low zinc brasses
shows that this alloy was rarely recycled on its own. If brass was
recycled then it was always mixed with some bronze. (Dungworth 1995: i).
We would argue that greater clarity can be given to this discussion
by changing the way in which we define copper alloy types, using a
system that moves away from an assumption that alloy design is
principally deliberate to an impartial categorisation in which the
concentration of particular alloying elements in a specific object
depends on the life history of the object. If this is done, not only can
we see that recycling was significant during and after the Roman
occupation, but we can also suggest that recycling was not necessarily
as well controlled as is suggested by Dungworth and others.
Under our definitions, brass in Britain was primarily an alloy of
the first century AD (and the Late Iron Age), but continued in
circulation in declining ubiquity (as a pure copper-zinc alloy) until
the start of the middle Saxon period, accompanied by increasing amounts
of other zinc-containing alloys (gunmetal and leaded gunmetal),
indicating that some of the brass was being mixed with bronze and leaded
bronze to create these mixed alloys. We suggest that, in a system that
contains zinc-bearing alloys, the proportion of leaded gunmetal (copper
with more than 1% of tin, lead and zinc respectively) gives a minimum
estimate for the frequency of recycled metal in circulation. Yet we do
recognise that alloy design, for example the deliberate addition of lead
to increase fluidity, is also a part of this story.
By combining data from various sources to give a long time-depth
and a substantial number of analyses, we can see a number of major
transition points in the pattern of copper alloy circulation in
Britain--specifically, at the Late Iron Age/Roman transition, and,
significantly, at the early to middle Saxon transition, indicating
continuity from the late Roman fourth century AD into the early Saxon
period. These transitions are evidenced by large changes in the patterns
of alloy types represented in the assemblages, and also, where we have
been able to examine it, in the categories of copper (defined by the
trace elements arsenic, antimony, gold and nickel) in circulation. Taken
together, these changes suggest new injections of copper into the metal
supply at these times. This echoes the model of 'metal
systems' proposed by Caple, in which he saw the metal in
circulation as made up of the metal stock, to which is periodically
added "freshly made metal [...] coming in the forms of ready formed
objects and/or fresh metal to be fabricated in a variety of forms and
states, to add to the existing metal stock" (Caple 1986: 530). We
endorse this model, and suggest that our combination of redefined alloy
types with copper categories can help to illuminate and quantify this
system with increased clarity.
We accept, of course, that this work represents merely the starting
point for a much more detailed analysis of the metal circulation system.
It is essential to consider the data broken down into more specific
typological, technological, chronological and geographical categories.
It is also necessary to combine the metallurgical data with other forms
of archaeological information--to see if, for example, the appearance of
CC12 copper in the middle Saxon period reflects the closer continental
links that are similarly mirrored in new forms of ornamental metalwork
and the reintroduction of wheel-turned pottery and silver coinage. Such
work requires the careful re-analysis of more good quality analytical
data.
doi: 10.15184/aqy.2015.20
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Received: 7 January 2014; Accepted: 27 March 2014; Revised: 25
April 2014
A.M. Pollard, Peter Bray, Chris Gosden, Andrew Wilson & Helena
Hamerow *
* School of Archaeology, University of Oxford, 36 Beaumont Street,
Oxford OX1 2PG, UK
Table 1. Copper alloy classifications according
to Bayley and Butcher (2004: 14).
Zinc: tin ratio Absolute values
Brass Zn>4Sn Zn = 8%
Brass/gunmetal 2.5Sn<Zn<4Sn Zn = 8% or Sn = 3%
Gunmetal 0.67Sn<Zn<2.5Sn Sn = 3%
Bronze/gunmetal 0.33Sn<Zn<0.67Sn Sn = 3%
Bronze Sn = 3Zn Sn = 3%
Copper Zn<3% and Sn<3%
Copper/brass 3% = Zn<8% and Sn<3%
Leaded alloys Pb>8%
(Leaded) alloys 8% = Pb = 4%
Table 2. Copper alloy classifications used in this paper.
Symbol Name Definition
C Copper Pb, Sn, Zn each <1%
LC Leaded copper Pb > 1%; Sn, Zn both <1%
B Bronze Sn >1%; Pb, Zn both <1%
LB Leaded bronze Sn, Pb both >1%; Zn <1%
BR Brass Zn >1%; Sn, Pb both <1%
LBR Leaded brass Zn, Pb both >1%; Sn <l%
G Gunmetal Zn, Sn both >1%; Pb <l%
LG Leaded gunmetal Zn, Sn, Pb all >1%
Table 3. Ubiquity of copper alloy types in the three datasets by
date; data recalculated from Blades (1995), Dungworth (1995) and
Bayley & Butcher (2004).
Leaded
Copper copper Bronze
Bayley & Butcher Late Iron Age 0 0 50 *
Bayley & Butcher AD 1-70 0 0.3 4.6
Bayley & Butcher AD 43-70 0 0 2.2
Bayley & Butcher AD 70-170 0.3 0 3
Dungworth Early Iron Age 0 0 72.9 *
Dungworth Iron Age 0 0 73.2 *
Dungworth Late Iron Age 2.8 0 27.8 **
Dungworth first century AD 3.3 0 14
Dungworth second century AD 1.2 0 18
Dungworth third century AD 0.7 0 15
Dungworth fourth century AD 0 0 11.5
Dungworth EROM 0 0 16.4
Dungworth MROM 0 0 12.5
Dungworth LROM 1.7 1.1 13.2
Dungworth ROMN 0.4 0 13.6
Blades Roman 1.1 1.1 6.4
Blades Early Saxon 0.3 0 2.4
Blades Middle Saxon 4 0 8.1
Blades Late Saxon 4.1 1.4 6.8
Caple Roman 8.3 0 1.4
Caple Saxon 0 3.8 3.8
Leaded Leaded
bronze Brass brass
Bayley & Butcher Late Iron Age 10 0 0
Bayley & Butcher AD 1-70 22.3 * 19.3 1.1
Bayley & Butcher AD 43-70 7.7 27.5
Bayley & Butcher AD 70-170 49 ** 0.7 0.7
Dungworth Early Iron Age 27.1 * 0 0
Dungworth Iron Age 21.4 1.8 0
Dungworth Late Iron Age 11.1 27.8 0
Dungworth first century AD 14 30.5 1.2
Dungworth second century AD 18.4 13.1 0.4
Dungworth third century AD 22.4 4.8 0
Dungworth fourth century AD 24.4 1.3 0
Dungworth EROM 20.1 4.5 1.5
Dungworth MROM 21.9 6.3 3.1
Dungworth LROM 28.7 2.3 3.4
Dungworth ROMN 23.4 6.4 1.7
Blades Roman 33 6.4 3.2
Blades Early Saxon 18.3 0.8 0.5
Blades Middle Saxon 41.6 9.4 6.7
Blades Late Saxon 47.9 6.8 12.3
Caple Roman 15.3 8.3 1.4
Caple Saxon 28.8 5.8 19.2
Leaded
Gunmetal gunmetal Total
Bayley & Butcher Late Iron Age 30 * 10 10
Bayley & Butcher AD 1-70 34.8 * 17.7 368
Bayley & Butcher AD 43-70 42.9 * 19.8 91
Bayley & Butcher AD 70-170 11.1 35.1 * 296
Dungworth Early Iron Age 0 0 59
Dungworth Iron Age 1.8 1.8 56
Dungworth Late Iron Age 19.4 11.1 36
Dungworth first century AD 20.6 * 16.5 243
Dungworth second century AD 24.6 * 24.2 * 244
Dungworth third century AD 16.3 40.8 ** 147
Dungworth fourth century AD 12.8 50 ** 78
Dungworth EROM 23.9 * 33.6 * 134
Dungworth MROM 3.1 53.1 ** 32
Dungworth LROM 13.2 36.2 * 174
Dungworth ROMN 11.1 43.4 ** 235
Blades Roman 9.6 39.4 * 94
Blades Early Saxon 2.9 74.8 ** 377
Blades Middle Saxon 5.4 24.8 * 149
Blades Late Saxon 2.7 17.8 * 73
Caple Roman 26.4 * 38.9 * 72
Caple Saxon 9.6 28.8 * 52
Colour Key: 10-20% 20-40% * >40% **
Table 4. Chronological intervals and date ranges used.
Interval Date range
Late Bronze Age/Early Iron Age As Bayley & Butcher
Early Iron Age As Bayley & Butcher
Iron Age As Bayley & Butcher
Late (Roman) Iron Age As Bayley & Butcher
First century AD Roman, post AD 42
Second century AD Roman
Third century. AD Roman
Fourth century AD Roman
Early Roman First-second century AD
Middle Roman Second-third century AD
Late Roman Third-fourth century AD
Roman First-fourth century AD
Early Saxon AD 430-650
Mid Saxon AD 650-850
Late Saxon AD 850-1066
Early medieval Eleventh-thirteenth century
Late medieval Thirteenth-fifteenth century
Post-medieval Post-AD 1600
Table 5. Copper categories used in this study, classified by presence/
absence of non-alloy trace elements, in the order arsenic (As),
antimony (Sb), silver (Ag) and nickel (Ni).
Copper Category As Sb Ag Ni
1 N N N N
2 Y N N N
3 N Y N N
4 N N Y N
5 N N N Y
6 Y Y N N
7 N Y Y N
8 N N Y Y
9 Y N Y N
10 N Y N Y
11 Y N N Y
12 Y Y Y N
13 N Y Y Y
14 Y Y N Y
15 Y N Y Y
16 Y Y Y Y
Table 6. Percentages of artefacts of a particular period in
each copper category; data recalculated from Blades (1995).
CC 1 CC 2 CC 3 CC 4 CC 5 CC 6
Roman 12.8 * 0.0 62.8 ** 4.3 0.0 1.1
Early Saxon 3.7 0.3 28.9 * 5.0 0.3 2.1
Middle Saxon 8.7 8.7 14.1 * 2.0 2.7 7.4
Late Saxon 13.7 * 2.7 41.1 ** 5.5 0.0 5.5
Early medieval 13.8 * 1.7 25.9 * 1.7 1.7 27.6 *
Late medieval 9.6 1.1 7.0 10.3 * 5.1 20.2 *
Post-medieval 9.9 2.4 4.7 8.0 24.1 * 1.9
CC 7 CC 8 CC 9 CC 10 CC 11 CC 12
Roman 19.1 * 0.0 0.0 0.0 0.0 0.0
Early Saxon 48.3 ** 0.0 0.5 1.1 1.3 4.0
Middle Saxon 34.2 ** 0.7 2.0 0.0 1.3 14.1 *
Late Saxon 17.8 * 0.0 0.0 0.0 0.0 11.0 *
Early medieval 12.1 * 0.0 0.0 0.0 1.7 5.2
Late medieval 6.6 0.4 0.7 1.5 0.7 23.5 *
Post-medieval 4.7 1.9 0.5 1.9 5.2 13.2 *
CC 13 CC 14 CC 15 CC 16
Roman 0.0 0.0 0.0 0.0
Early Saxon 0.5 1.6 0.3 2.1
Middle Saxon 0.0 1.3 0.7 2.0
Late Saxon 0.0 0.0 0.0 2.7
Early medieval 0.0 0.0 0.0 8.6 *
Late medieval 0.0 2.6 4.0 6.6 *
Post-medieval 0.9 2.8 10.4 * 7.5 *
Colour key: >30% ** 10-20% * 5-10%
