CHAPTER 9
Naturam ars imitata: European Brassmaking
between Craft and Science
THILO REHREN AND MARCOS MARTINÓN-TORRES
ABSTRACT
This chapter presents a summary of analytical work on
medieval brassmaking crucibles, spanning more than half a millennium and
tracing what is believed to be a gradual development of increasing skill and
efficiency of the craftspeople who used these crucibles. This summary is then
contrasted to a similarly diachronic sequence of textual sources concerned
with brassmaking, illuminating the discrepancy between the matter-of-fact
practitioners’ reports and the somewhat befuddled attempts of philosophers
of nature to understand and explain the essence of brass as opposed to
copper. The different strands of knowledge generation and transfer, namely,
observation and apprenticeship versus theoretical consideration and text,
are placed into the changing scholarly environment from the High Middle
Ages to the Renaissance. This exercise provides fresh insight not only into
the development of brassmaking technology but also into the driving, or
otherwise, forces in technological developments in general. The comparison of
archaeological, scientific, and historical evidence is used to demonstrate the
potential of such multisource studies.
Introduction
Brass has a more chequered history than most other alloys do, so much
so that despite more than three millennia of brass use, it finally was
acknowledged as an alloy only less than 500 years ago. For most of its
history, brass caused wonder and stimulated the minds of people trying to
understand what it really was – a process that in some sense continues to
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this day. The reason why it took so long for brass to be recognised as an
alloy has been discussed elsewhere in length (de Ruette 1995; Zacharias
1989); it suffices here to summarise the key points.
The term brass refers to an alloy of copper and zinc. Zinc concentrations
in archaeological brass range from about 5 wt% at the lower end to about
30 wt% at the upper end. Practically, there is no lower limit for the zinc
concentrations in copper: it is a semantic question when we start calling
zinc-containing copper ‘brass’, and when we consider it an impure copper,
or copper with just trace or low concentrations of zinc. Archaeologically,
though, this is more than a linguistic question, as the term brass (rather
than impure copper) strictly speaking implies an intentionally produced
alloy, as opposed to an accidentally produced mixture of metals.
The problem with zinc as a metal is that at temperatures above 907°C
it exists only as a vapour, but most smelting furnaces operate at much
higher temperatures. Thus, metallic zinc on its own was almost unknown
in Antiquity in the West (but see Rehren 1996, and Rehren in Fellmann
1999, and literature therein), ruling out the traditional way of alloying by
co-fusing existing metals, as done with copper and tin to produce bronze.
Instead, brass was produced by a technique called cementation, which
allowed the manufacture of the copper-zinc alloy several centuries before
unalloyed zinc metal could be exploited. In this process, metallic copper,
charcoal, and powdered zinc ore (usually carbonates or oxides) were heated
together in a crucible, so that zinc vapour formed and reacted with the
copper metal, thus producing brass. Significantly, and owing to the high
vapour pressure of zinc already well below its boiling point, this process can
proceed at temperatures as low as 800°C. What is important here is that,
contrary to the more common alloying method, only one metal is seen
to enter the reaction vessel (copper), and a different type of metal is produced (brass), hence the problems in the understanding of the process.
To facilitate the reaction, the crucible had to be closed to force the
zinc vapour into the copper, rather than it escaping with the fumes of the
furnace. Brass with the typical Roman level of approximately 15 to 25 wt%
zinc melts at 1050 to 950°C, which is well above the temperature necessary
for the cementation process but still lower than the temperature required
for smelting copper, of around 1100 to 1200°C. Thus, depending on the
composition of the forming brass and the operating temperatures, the resulting alloy would be either solid or liquid.
The upper zinc limit for cementation brass has been intensively
disputed. Empirically, very few well-dated objects are known with more
than about 32 wt% zinc, although experimental work indicates that
higher zinc concentrations can be achieved by both classical and modern
methods of brass production (Haedecke 1973; Newbury, Notis, and
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Newbury 2005; Ullwer 2001; Welter 2003; Werner 1970; Zwicker et al.
1985). This, however, has no particular implications for the perception of
the material, and we are not addressing this aspect here any further.
This chapter looks at the changes in the perception of brass, as well
as the understanding and explanation of the principles behind its production, taking place in Europe over the last millennium. In contrast to
most papers on medieval and later brass, we do not focus on analytical
data of objects themselves but juxtapose historical texts addressing the
material, its properties and its production, with archaeological evidence
for the latter. In doing so, we hope to identify and explain the transition
from an intuitive process of improving brassmaking practice, based on
tradition and practical replication with modification on the side of the
practitioners, to a period of more conscious developments, based on a
theoretically framed transcendence of the process on the side of scholars,
and possibly involving experimental work. In addition, we attempt an
insight into the evolving relationship between theory and practice in this
arena, and the impact of the Renaissance rationality and empiricism in the
making and understanding of brass.
Here, we are able only to scratch the very surface of this topic, which
is clearly much richer archaeologically and much more complex in its
textual sources than can be dealt with in the space of a few thousand
words. However, we hope to show some of the potential in this subject
for in-depth study.
Premedieval Brassmaking in the Old World
By way of introduction, and as a backdrop against which European brassmaking is to be understood as fundamentally different from the other
alloys inherited from Antiquity, we briefly outline three main phases of
brass use, mainly in Europe, separated by periods for which we have little
or no evidence for brass being made or widely known.
The first phase of brass use covers the Bronze Age and Early Iron
Age of the Near and Middle East. The number of brass artefacts now
known from the eastern Mediterranean and Western Asia is so big that
they can no longer be ignored as flukes of chance (Craddock and Eckstein
2003; Thornton and Ehlers 2003). This archaeological presence broadly
correlates with textual reports in the archaic Greek literature of a particular
and highly prized copper alloy called oreichalkos, or ‘mountain copper’,
now widely believed to be brass (Craddock and Eckstein 2003:217). In
the 1st century C.E., Pliny refers to aurichalcum (literally ‘golden copper’
but phonetically an adaptation of the Greek term) in a way suggesting that
by his time this alloy was no longer being made (Hist. Nat. XXXIV:2–4;
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translated in Rackham 1952). In the absence of any published archaeological evidence for the production of this alloy in pre-Roman times, and
given its typically low and rather variable zinc content, one may assume
that it was produced by smelting a particular zinc-rich copper ore. This
assumption is consistent with the use of a specific term for this metal,
identifying it as a variant of copper but sufficiently different for it to not be
included in the generic chalkos (aes in Latin) used for copper and bronze.
It is also consistent with the archaeological disappearance of this alloy,
possibly because of simple exhaustion of the particular ore deposit(s)
from which it was smelted. At least during the final phase of this period,
in the Iron Age, oreichalkos seems to have enjoyed a reputation as a rare
and desirable material, worthy of use by the best of the time, mostly for
jewellery and ornaments.
The second phase in this history spans from the mid-1st century B.C.E.
up to the late Roman period. It starts with the sudden appearance of
small quantities of brass both in the East and to the north and west of the
Roman empire, followed by a boom of brass use, first in military implements and other state-controlled contexts such as currency, during the late
first century B.C.E. and the first century C.E. (Bayley 1998; Ponting 2002;
Ponting and Segal 1998; Weeks 2004). This gradually develops into a
period of more widely distributed, and literally also diluted, use, when
brass and zinc-containing ternary alloys permeate into general domestic
use. The supply of fresh brass during this later stage of the second phase
seems to have dried up, as no or only very little high-zinc, high-quality
brass is still being used then (Bayley and Butcher 2004; Dungworth 1996,
1997a, 1997b).
There is a considerable body of literature on alloy compositions from
this period (see references above), but only very few instances of brassmaking installations have been published so far. These have in common
that they clearly point toward the conscious production of brass in closed
crucibles using the cementation method, enabling relatively close control
over the resulting alloy composition. Those few case studies reported
so far paint a rather diverse picture, ranging from tiny purpose-made
acorn-shaped (and -sized!) crucibles used in Colonial Ulpia Traiana/
Xanten (Rehren 1999a) to large amphorae-like vessels used for brassmaking in Lugdunum/Lyon (Picon, Le Nezet-Celestin, and Desbat 1995)
and normal, fist-sized, metallurgical crucibles in Roman Britain (Bayley
1998) (Figure 9.1). These vessels are all conspicuously free of slag, suggesting that the process was probably done at a relatively low temperature,
possibly involving only solid copper (but see Craddock and Eckstein
2003). The resulting brass, thus formed in the solid state, would probably
be melted and further refined only when needed for casting. The cementation crucibles were lidded and tightly sealed, and the pressure released
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Figure 9.1 • Formal diversity in Roman brassmaking crucibles; from left to right,
examples of crucibles from Xanten (Germany); Culver Street in Colchester (United
Kingdom) (© Justine Bayley); Palace Street in Canterbury (United Kingdom) (© Justine
Bayley); and Lyon (France) (© Société Française d’Étude de la Céramique Antique en
Gaule). Note the different scale of the Lyon crucible, which would otherwise appear
even larger when compared to the rest; see text for references (all crucibles redrawn
by M. Martinón-Torres).
probably through the porous ceramic fabric and, in some cases, though a
hole in the lids. Remarkable is the wide range in vessel sizes between the
three more or less contemporary published production sites mentioned
above, indicating that a degree of variability existed between different
production centres, while the basic principle was the same for all three.
The systematic and consistent production of brass of a certain quality
clearly demonstrates full mastery of the process during the early part of
this second phase of brass use, while textual references show that brass was
understood as a derivative of copper. Unfortunately, Pliny does not discuss
its production, although he details that ‘Livian’ copper from Gallia/Gaul
is better suited to be treated with cadmea (zinc ore) than that from other
sources and produces a metal almost as good as the traditional oreichalkos
(Hist. Nat. XXXIV:2–4, 100–103; translated in Rackham 1952). Discussion of the reason(s) for the disappearance of freshly made brass during
the later Roman Empire is beyond the scope of this paper (cf Dungworth
1996). Pliny’s failure to report any technical details may indicate that the
practicals of brassmaking were not in the public domain and that it was
a loss of know-how or state-controlled demand rather than exhaustion
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of raw materials that led to its demise. Hence, we have a fair idea of the
production and perception of brass as a relatively cheap copper-related
metal during the Roman period but no feeling whether the Roman or
indigenous brassmakers considered their craft different from that of other
alloy makers, nor why its production in the West seemingly ended well
before the collapse of the Roman Empire.
Medieval and Early Modern Brassmaking in Europe
The third phase of brass use in Europe begins in the High Middle Ages
and continues to this day. For this phase, we have the most comprehensive
archaeological and textual evidence, and the emphasis of this chapter is
very much on the first half of this third phase, that is, roughly from
1000 to 1600 C.E. Brass was again being made by cementation, soon
replacing bronze as the alloy of choice. It is this classical method of brass
production, prevailing in Europe and the Middle East until the Industrial
Revolution, that led to so much confusion about the nature of brass.
As noted above, metallic zinc occurs in this process only ever as a
vapour phase within the reaction vessel, invisible to the metal worker of
the time and unknown to the scholars interested in the materials of
nature. Accordingly, the actual amount of zinc entering the copper was
less well-controlled than in other alloys. Furthermore, this peculiarity
raised serious doubts whether brass really qualified as an alloy or whether
it was just a particular type of copper ‘coloured’ yellow. It is this enigmatic
change in the properties of copper, similar to the change of properties
known from ‘proper’ alloying when making bronze, but not linked to
any known metal, that puzzled scholars for many hundreds of years.
However, this uncertainty did not preclude generations of metal workers
from mastering and perfecting the process, so much so that brass quickly
became the dominant copper alloy of the medieval and the modern
periods, extensively used for everyday life implements as much as for civil
and ecclesiastical works of art – as widely documented in analytical studies
(for example, Cameron 1974; Caple 1995; Hachenberg 2004; 2006;
Mitchiner, Mortimer, and Pollard 1986; Pollard and Heron 1996; de
Ruette 1996; Riederer 1988).
The six centuries of brass use starting around 1000 C.E. are of particular
interest here, since they provide both a unique sequence of archaeological
evidence for brassmaking and a series of highly illuminating texts about
the nature of this particular alloy, written by some of the brightest minds
of their time. In the following text, we summarize the archaeological
evidence for the gradual evolution of brassmaking in Central Europe, from
the late 1st millennium C.E. up to the Renaissance. This summary will
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173
then be contrasted with some of the contemporary sources discussing
the nature of this alloy and its production, with the aim to understand
the changing perception of brass. Eventually, we look at the relationship
between the practical mastery and the theoretical understanding of
brassmaking, in an attempt to judge the degree of mutual dependency of
practitioners and theoreticians in the field.
The Evolution of Brassmaking: Archaeological Evidence
As noted above, the third phase in the history of European brassmaking
starts after the mid-1st millennium C.E., with fresh brass emerging in
Scandinavian and Viking contexts (Eremin, Graham-Campbell, & Wilthen
2002). By the time of the Carolingian revival, brass is the main copper
alloy in use, and it will retain this position to this day. Thus, brass was and
is clearly an alloy of huge economic significance, and its production and
use played a central role in many aspects of the development of the modern
world. Little, however, do we know of the early period of its production,
and the source of the Viking-period brass remains enigmatic.
The first archaeological evidence for brassmaking, from Dortmund in
western Germany, dates to the late 1st millennium C.E., followed by a
series of somewhat later instances excavated across Germany. The evidence from Soest and Schwerte, some 50 km to the east of Dortmund,
dates to the first half of the 2nd millennium C.E.. The latest evidence of
relevance here is from Zwickau in southeast Germany, dating to the late
15th century C.E..
Of interest to us is the evolution of the crucible design over the period
of half a millennium, particularly in size and profile. On the contrary,
we do not address in any details the zinc ores and their preparation, for
which the existing information is even scantier and has been summarised
elsewhere (Craddock and Eckstein 2003). It suffices to remember that
two main types of zinc ores were used: on the one hand, ‘natural’ minerals
in the form of calcined zinc carbonate (ZnCO3) or roasted sphalerite
(ZnS) – generally called calamine, cadmia fossilis, galmey, and so on,
depending on the authors; on the other hand, ‘artificial’ zinc oxiderich crusts scraped as byproducts from the inner walls of furnaces where
zinc-bearing lead ores were smelted – named furnace calamine, cadmia
fornacum, tutty, and others (cf de Ruette 1995).
The Dortmund crucibles are short and cylindrical in shape, about 6
to 8 cm in diameter, and usually less than 10 cm high, corresponding to
a volume of less than half a litre (Figure 9.2). Little if any evidence for a
lid exists for these vessels, suggesting a rather wasteful process with a high
proportion of the zinc vapour escaping from the reaction vessel instead
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Figure 9.2 • Medieval and Renaissance brassmaking crucibles from Germany; from
left to right, in chronological sequence, sketch drawings of crucibles from Dortmund;
Soest/Schwerte; and Zwickau; see text for references.
of being absorbed by the copper to form brass (Rehren et al. 1993). The
walls are around 1 cm thick, their fabric is dense, and, with over 27 wt%
Al2O3 (normalised data after Rehren 1999b; Martinón-Torres 2001),
much more alumina-rich and hence more refractory than the contemporary
local domestic wares.
The Soest and Schwerte vessels, roughly thought to be two to three
centuries more recent, are considerably bigger than the Dortmund
crucibles. They are tubular with a round bottom, an internal diameter
of around 9 cm and a height of at least 15 to 20 cm (Figure 9.2). The
ceramic is visually almost indistinguishable from the earlier Dortmund
crucibles and, although less rich in alumina (19 wt% Al2O3; normalised
data after Rehren 1999b) still sufficiently refractory to serve its purpose. Significantly, though, there are several ceramic disks among the
assemblage from Soest which were tentatively identified as lids. These
would have to some extent limited the zinc loss, even though they probably only loosely fitted the mouths of the crucibles.
In all these cases, the evidence is for a cementation process as in the
Roman examples, but invariably involving slag formation within the
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175
crucibles. This suggests that the operating temperatures were now on the
whole higher than in the Roman period and probably involved melting
the brass in the crucibles (Rehren 1999b).
Another several hundred years later, in the late 15th century C.E.,
brassmaking vessels had apparently moved up further in scale and technical design. At Zwickau, brassmaking vessels were by now holding about
20 kg of metal, having a height of at least 40 cm and a body with a
bulbous profile reaching 28 cm in diameter, resulting in a volume of
approximately 12.5 litres (Martinón-Torres and Rehren 2002) (Figure 9.2).
They are made from a ceramic of only medium alumina content (around
17 wt%), making them less refractory than the earlier vessels from
Dortmund and similar to those from Soest and Schwerte. However, the
mechanical stability of these large vessels was ensured by walls that are
rather thick, at around 2.5 cm, and further covered with an outer layer
of even less refractory ceramic – an established method of improving performance of technical ceramics (Rehren 2003). Furthermore, the openings
of the Zwickau vessels were covered with a sophisticated dome-shaped
lid, itself comprising a smaller opening that was closed with a lump of
clay if and when needed. While a thin film of slag appears inside some of
these vessels, this is a reaction layer between the ceramic fabric, zinc
oxide, and an iron oxide-rich gangue, but it appears that the brass did not
melt during the cementation process, and there is evidence, within the
same assemblage, for the melting and refining of fresh brass in different,
triangular crucibles (Martinón-Torres and Rehren 2002).
Thus, overall we see a growth in crucible volume from the small
cups of Dortmund to the buckets of Zwickau, with the ceramic design
being adapted to match the physical demands of the increased weights
of the charge (Figure 9.2). In addition, the opening of the crucibles and
their lids become much more sophisticated, obviously in an attempt to
minimise loss of zinc vapour while still maintaining access to manipulate
the charge with iron rods (see below). Finally, the later crucibles indicate a
better control of the working temperatures, which would result in a more
economic use of the raw materials and higher-quality brasses. Altogether,
it is tempting to picture a technology being passed on from one generation
of craftspeople to the next, whereby brassmaking improves by experience
and through iteration but no significant innovation or breakthrough is
noticeable.
Scholarly Understanding of Brassmaking? Historical Evidence
The Middle Ages
The earliest detailed description of brassmaking is by Theophilus, dating
to the 12th century C.E. and therefore roughly contemporary to the
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evidence from Soest und Schwerte. In his third book of On divers arts,
he advises:
And when the crucibles are red-hot take some calamine, about which
I spoke above, that has been [calcined and] ground up very fine with
charcoal, and put it into each of the crucibles until they are about one-sixth
full, then fill them up completely with the above-mentioned copper, and
cover them with charcoal . . . Now, when the copper is completely melted,
take a slender, long, bent iron rod with a wooden handle and stir carefully
so that the calamine is alloyed with the copper. Then with long tongs raise
each crucible slightly and move them a little from their position so that they
may not stick to the hearth. Put calamine in them all again as before and
fill them with copper and cover them with charcoal. When it is once more
completely melted, stir again very carefully and remove one crucible with
the tongs and pour everything into little furrows cut in the ground. Then
put the crucible back in its place. (Hawthorne and Smith 1979:143–44)
The chronologically next source, discussing the nature of brass as well
as its production, is the Book of Minerals by Albertus Magnus, written
probably in the early second half of the 13th century. In Chapter 6 of
the fourth book he explains the nature of copper as a result of the combination of the particular qualities and quantities of quicksilver and
sulphur that make up all metals. For him,
Quicksilver is good, not full of dross and dirt, but still not completely
cleansed of extraneous moisture; and… the substance of the Sulphur is full
of dross, burning hot and partly burnt, and in this condition it is mixed
with the Quicksilver, both in substance and in quality. Then undoubtedly
it changes the copper to a red colour; and because neither [the Sulphur
nor the Quicksilver] is sufficiently subtle, they cannot be well mixed. And
this will make copper, which is not at all well mixed, since much dross is
separated from it, and it evaporates greatly in the fire. (Wyckoff 1967:223)
He then goes on to explain how and why the copper is not well mixed
and burns, concluding that: ‘Now, therefore, we understand the material
of copper; it is a metal having rather more Quicksilver than it ought to
have, which has been converted into a red form by mixture with burning
Sulphur’ (Wyckoff 1967:224). Building on this understanding of copper,
he then explains the making of brass as follows:
But those who carry on much work with copper in our region – that is, in
Paris and Cologne and other places where I have been and seen this tested
by experience – convert copper into brass [aurichalcum] by means of a
powder of a stone called calamine. And when this stone evaporates there
still remains a dark lustre, approaching the appearance of gold. And to make
it paler in colour, and so more like the yellow of gold, they mix in a little tin;
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but because of this, brass loses the malleability of copper. And those who
wish to deceive and to produce a lustre like gold ‘bind’ the stone so that it
may remain longer in the copper on the fire, and not evaporate from it so
quickly. And the ‘binding’ [is done] with ‘oil of glass’. They take fragments
of glass, crushed and sprinkled into the crucible on the copper after the
calamina is put in; and then the glass that has been put in floats on the top
of the copper and does not allow the power of the stone to evaporate, but
reflects the vapour of the stone down into the copper. And in this way the
copper is thoroughly purified for a long time and the drossy material in it is
burnt up. But after a while the oil of glass evaporates, and then the power
of the stone evaporates, too; but the brass is made much more brilliant than
it would have been without it . . . But Hermes says that if powdered tutty is
mixed with molten copper – either white tutty or red – it changes the copper
to the colour of gold. What tutty is will be explained in the following book,
where ‘intermediates’ are treated. But it is enough [to say] here that the
burning heat of tutty consumes the earthiness and purges the superfluous
moisture out of the copper; and so then it will be more beautiful. But the
power of tutty, too, evaporates if it stands for a long time on the fire; and
therefore, unless some remedy is used, the tutty will evaporate and the
copper will regain its original colour. (Wyckoff 1967:224–25)
These two texts, separated by only about a century, show two radically
different approaches to the topic. Theophilus gives a very matter-of-fact
description, a recipe, of how to make brass; he does not seem to spend
much thought on what actually happens to the copper in the process.
Clearly, brass is an important material to him, and he spends several paragraphs on the best preparation of the furnace for brassmaking, as well
as on the different qualities of brass: qualities that are of relevance if the
metal subsequently is to be gilded or hammered rather than cast. Based on
this text, any dexterous person can successfully make brass, regardless of
whether they have a degree in science, or philosophy, or are barely literate
enough to read his text. No word is spared on theoretical speculations. Thus
it is not surprising that the contemporary archaeological evidence from
brass workshops is much more easily explained in the light of this text.
Albertus Magnus, in stark contrast, explains in much more detail the
basic constituents that make up the various metals, manifested in their
different degrees and qualities of coolness, humidity, and ‘burning heat’,
and as embodied in the principles for the generation of all metals:
quicksilver (mercury) and sulphur. He then places copper within this theoretical framework, which he carefully developed from the scriptures of
the classical authors, before addressing brass. The relevant sentences here
are an interesting mixture of plain observations of current practice in
Paris and Cologne and attempts to accommodate what he noticed within
his theoretical framework of varying ‘qualities’ of metal that have to be
‘purged’ of inferior aspects.
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It would be quite difficult to follow his description in practice,
though. For instance, no mention is made by him of the crucial ingredient
charcoal, necessary for the zinc ore to be reduced to metal vapour and
without which the whole cementation process would not work. However,
Albertus devotes many words to discuss the addition of crushed glass as a
protective coating for the melt, which, from a present-day perspective, is a
less important aspect of the process. The ensuing discussion demonstrates
some form of practical understanding that the central component zinc is
volatile and should be kept from evaporating. However, the agent that he
describes to evaporate, and that he clearly links to the change in colour
from red copper to yellow brass (and back to red copper through evaporation), is seen more as an immaterial quality, a ‘power’ of tutty, rather
than substantially the tutty itself – which is, of course, zinc oxide.
The apparent discrepancy could not be stronger, but it can be somewhat
explained by considering the backgrounds and aims of both authors.
Theophilus was a Benedictine artisan monk extensively experienced in arts
and crafts, he collected practical recipes as a memory aid, and showed no
concern with explanations. The result was a book that, from a technical
viewpoint, was not superseded ‘until the books by Cennini (1437) on
pain-ting, Månsson (ca 1520) on glass, and Biringuccio (1540) on almost
everything but painting’ (Hawthorne and Smith 1979:xxxi).
Albertus Magnus, widely acknowledged as one of the greatest thinkers
and most prolific writers of the Middle Ages, held high office in the
Dominican Order and later served as a bishop, and was eventually canonised in 1931. From a higher social rank and in a completely different
academic pursuit, what he attempted was an explanation for the nature
and formation of the natural world that could reconcile Aristotle’s four
elements, Avicenna’s sulphur-quicksilver theory, further alchemical
beliefs, and even Christian theology. Returning to brassmaking, his main
objective was not to produce a beautiful metal of appropriate material
properties but to investigate the causes behind its formation and, ideally,
to extract relevant information that would aid the search of metallic
transmutation.
Medieval alchemists sought the ‘tincture’ or ‘elixir’ that, added to base
metals, would give them the quality – that is, the actuality – of gold. It
was in this manner that they sought to imitate natural gold with artifice.
The sensorial qualities of artificial brass did indeed compare reasonably
well those of natural gold, but, to the alchemists’ despair, the yellow
colour of brass (that is, the zinc) evaporated on prolonged melting. In
another text attributed to Albertus, several paragraphs insist that ‘the
alchemical art requires that some means be discovered whereby the spirit
does not escape, is not consumed or burned, but instead penetrates and
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179
is mixed with all parts of the metal, so that it may tint with a permanent
tincture’ (Kibre 1944:305). In these pages, tutia (zinc ore), marcasita
(iron sulphide), and sale armoniaco (ammonium chloride) – all of which
contain volatile elements – are mentioned as potential ‘tinctures’ that
need to be processed and improved.
It is probably within this context that Albertus calls ‘deceivers’ (see
above) those who sprinkle broken glass in the crucibles to prevent
evaporation, since this does not solve the problem of the ‘permanent tincture’. Avoiding colour losses was good enough for a metallurgist, but not
acceptable for a demanding scholar.
All in all, these written sources demarcate two distinct traditions: on
the one hand, those such as Theophilus, concerned with the practicalities;
on the other hand, those pursuing explanations, as was the case of
Albertus Magnus. Not only Theophilus but also Albertus had some firsthand acquaintance with metallurgical practice – probably obtained in the
Rammelsberg some 30 years before writing his book (Wyckoff 1967).
His theoretical speculations bear some technical basis, but craftsmen
and scholars are working in different spheres. Meaningfully, Albertus
disregards Pliny because ‘he does not offer an intelligent explanation of
the causes common to all stones’ (Wyckoff 1967:10), and nothing in The
Book of Minerals suggests that Albertus, the great encyclopedist, ever read
Theophilus.
The Renaissance
The 16th century saw a great expansion in printing technology, together
with other technological and scientific achievements, and a fervent
humanistic interest in understanding nature and recording knowledge. In
this context, several major treatises on mineralogy and metallurgy appeared, where we find detailed accounts on brassmaking. Three crucial
texts must be considered here: Vannoccio Biringuccio’s De la pirotechnia,
first published in 1540, Georg Agricola’s De natura fossilium – notably
the second edition of 1558 – and Lazarus Ercker’s 1580 Beschreibung
aller fürnemisten Mineralischen Ertzt vnnd Berckwercksarten. How do
they differ from the earlier texts?
Biringuccio gives a brief description very similar to Theophilus’s,
although he fails to mention the necessary charcoal charge:
For this process they placed in each one of the vessels 25 pounds of German
rosette copper broken in pieces, and they filled up the rest to within 2 dita
of the rim with a powder of a mineral earth, yellowish in color and very
heavy, that they called calamine. The rest of the empty space in the crucible
they filled with powdered glass . . . Then they applied a melting fire for 24
hours and after this time they found the material entirely fused, and the
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copper, which was red before, had become a smooth and lovely yellow,
almost like 24-carat gold in color. (Smith and Gnudi 1990:72)
His approach to, and understanding of, the matter is clearly expressed
in the introductory paragraph (Smith and Gnudi 1990:70):
Since I have no knowledge other than that gained through my own eyes,
I can tell you as a certainty that just as steel is iron converted by art into
almost another kind of metal, so also brass is copper given a yellow color
by art.
Notably, he describes the furnaces used, and he also mentions that ‘little
fitted clay shutters’ may be used on the crucibles (Smith and Gnudi
1990:72). Later he addresses the nature of this ‘earth that colors copper
into brass’:
I do not know that this earth serves any other purpose than coloring copper, because the mineral matter is of bad elemental mixture and poorly
fixed . . . and not only does it color copper another color, but it increases
its volume so much that the workman covers the cost of the copper and the
expense of coloring it. (Smith and Gnudi 1990:75)
Further, he elucidates:
I believe that in its nature it is of a hot and dry quality like marcasite, as
experience shows, because it does not melt alone by itself but burns and all
its substance goes off in smoke. (Smith and Gnudi 1990:113)
Soon after the publication of De la pirotechnia, in 1546, the first
edition of Agricola’s De natura fossilium came out. Unlike his famous
compendium De re metallica (Hoover and Hoover 1950), the former
volume addressed brass production. This discussion was further expanded
in the second edition, published in 1558 (text on brass translated and
discussed in Martinón-Torres and Rehren 2002). Agricola starts his
exposition by referring to Pliny and the old oreichalkos, before restating an
old alchemical principle: naturam ars imitata (‘art imitates nature’) – it
is thus possible to artificially dye copper. He then presents two alternative
ways of making brass. The first one is again similar to Biringuccio’s,
although he notes that the charge is best arranged in the crucible in
layers, recaptures the possibility of adding crushed glass, and explicitly
acknowledges that either mineral or furnace calamine may be used. He
also insists on the need to carefully adjust the working temperatures. In
the second method, the cementation principle remains the same, but
he provides specific details such as the use of large pots, covered with
THILO REHREN AND MARCOS MARTINÓN-TORRES
181
a perforated lid and externally coated with another clay layer – almost
exactly the description of the archaeological crucibles from Zwickau. It
is also worth noting that he quantifies the amounts of charge to be used
and, particularly, the fact that the copper, ‘transformed into brass . . . has
become much heavier’ (Martinón-Torres and Rehren 2002:102).
A few decades later, Lazarus Ercker provides another very detailed
account of brassmaking. He reports that the calamine is first roasted, then
mixed with twice its volume of charcoal dust and dampened, with water,
urine, or alum solution. After a blow-by-blow recipe of how to charge
and fire the crucibles, he notes that the amount of copper charged in the
crucibles increases in weight over the course of nine hours of firing, from
64 units to 90 units. Again, he judges the quality of the product by its ‘fine
colour’, but all of his discussion centres on mundane issues such as the
comparison between natural calamine from the ore deposits and artificial
calamine collected from the furnaces near Goslar, and the lead content of
the various types (Sisco and Smith 1951:255–56). His record is accurate
and comprehensive, from the practice for the practice and seemingly with
little regard for theoretical speculation and systematic ordering.
∗∗∗
All three accounts have much in common: for example, the use of
colour as a measure for the quality of the metal produced, but also the
recognition of the increase in mass – be it economically quantified by
Biringuccio (‘covers the cost of the copper and the expense of coloring
it’) or directly measured as weight gain by Agricola and Ercker. Some
differences do occur, though these are more subtle than those identified
above between Theophilus and Albertus. Agricola, on the one hand, was a
foremost scholar of his time, with studies of philology, philosophy and
science, and a degree in medicine. As a humanist, he wrote in Latin for a
learned audience and filled his text with references to the classical authors.
Biringuccio and Ercker, on the other hand, were primarily trained as
practitioners – the former as a founder, the latter as an assayer – and
wrote in their vernacular languages. However, they were also exposed to
broader institutional and intellectual arenas: both of them spent their lives
under the patronage of wealthy families and worked alongside scholars
and scientists.
Perhaps most remarkable is the fact that, by and large, these authors
are providing clear and effective metallurgical recipes, but also some observations and thoughts on the nature of the elements involved and
the process taking place. The emphasis certainly is on the practical side,
but, as exemplified in Biringuccio’s lines on the calamine – and many
others not reported here – they show an awareness of existing theoretical
speculations, which they combine with their own observations – such
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as the increase in weight of brass. They are writing in the wake of the
Renaissance: the world is being explored and conquered, information is
published and discussed, knowledge about the natural world is keenly
sought, and, increasingly, practical observations and theoretical abstractions are being linked. The boundaries between the scientist and the
craftsman, between the alchemist and the metallurgist, between theory
and practice, are more and more blurred (Beretta 1997; Long 1991, 2001;
Martinón-Torres and Rehren 2005). This is epitomised by the ‘naturam
ars imitata’, which inspires Agricola as much as it inspired Albertus and
others. Of course, some are still concerned with perfecting the ‘permanent
tincture’ for ‘brass, which is the Philosopher’s Gold, and that is true’ (att.
Alfonso V King of Portugal 1651:69), but they all share a more direct
connection among experiment, abstraction, and practice. To what extent
did this Renaissance empiricism and rationality affect the evolution of
brassmaking?
Interestingly, Ercker not only describes brass production in close
proximity to the smelting area of Goslar. In his Brief Report on the
Rammelsberg, first published in 1565 (Beierlein 1968), he describes
the occurrence of a strange white-metallic material that seeps out of
some of the lead-smelting furnaces. This material, which he says is called
Contrafeth, is most likely metallic zinc, condensing in cracks and crevices
in the furnace walls. He suggests that much more of it could be made if
one only spent some thought on it and laments that the workmen are not
interested in new things, even if they would be useful (‘Such contrafeth
could be made in quantity if effort and thought were put to it; but it is not
valued, and the worker and smelter put no effort into new things, however
much it would help’; translated in Beierlein 1968:252). He goes on to say
that it is not regularly recovered and only ever collected when specifically
requested by someone paying them a tip. Furthermore, he states that
nothing can be made of this metal on its own, because it is too brittle,
and proposes to alloy it with other metals to make it useable. Here we see,
for the first time, the expression of the belief that a new material could
be developed –through experiment and thought – into an economically
interesting commodity: the beginning of industry-based research and
development in the modern sense. It is also during this period that ingots
of metallic zinc begin to arrive from China and India where, contrary to
Europe, the distillation of zinc had been mastered centuries before. This
puzzles scholars such as Andreas Libavius, who describe it as a ‘kind of
tin’ (de Ruette 1995:195).
Eventually, experimentation and active research led to ‘speltering’,
the process in which metallic zinc is alloyed with copper to produce brass.
This possibility seems to have been first realised by the 17th-century
German chymist Johann Glauber, who wrote in 1657:
THILO REHREN AND MARCOS MARTINÓN-TORRES
183
Zink is a volatile mineral, or a half ripe metal when it is drawn out of its ore.
It is much clearer and brighter than tin, yet not so malleable and fluxile as
tin is . . . We have it not much growing in Germany, but great quantity of
it is every year brought us by the merchants out of the East Indies . . . It
is a golden but an unripe mineral, it gives red copper a yellow colour and
turns it into brass, as lapis calaminaris doth; and indeed that same stone
is nothing else but unmeltable zink, and this zink may properly be called
a susile lapis calamnaris (sic); for as much as both of them partake of one
nature (translated in Packe 1689:319).
The realisation was there. However, it would be another 200 years
before the true nature of zinc as a metal was fully understood and its production sufficiently perfected to replace cementation as the basis of
industrial brass production.
Discussion
It is tempting to see in the few examples of archaeological evidence for
brassmaking briefly presented above a chronological trend line from small
and primitive, unlidded vessels to successively larger and better closed
vessels, representing an evolution of increasing productivity and improved
procedures. Minor but significant design details, such as the wall thickness
and the application of an outer wrap of less refractory clay, together with
the better temperature adjustment in Zwickau, also point in this direction.
They could well have been the result of semicontinuous minute changes
in the practice, leading over 500 years to significant advances. This, however, is only the positivistic interpretation of the scenario. As seen from
the three Roman examples mentioned above, it is as well possible that different crucible sizes coexisted (almost) simultaneously, possibly reflecting
different economic settings, scales of production, or simply local technological styles rather than necessarily representing an evolutionary
sequence. Only a much larger and better contextualised archaeological
sample will shed clearer light on this issue.
The textual evidence draws a more complex picture, as so often in this
period. At a first glance, one sees a fault line running through the corpus
of medieval texts addressing brass and brassmaking. On the one side
stand scholars such as Albertus Magnus, who primarily try to understand
fundamentally the material world using theoretical concepts inherited
from the classical authors and further developed by their own thoughts.
On the other side are writers more concerned with description rather than
explanation, who carefully report their observations and convey recipes to
facilitate the arts and crafts, rather than philosophy.
In the Renaissance, the texts suggest a more systematic combination of
practice and abstract thought that enabled a more practical understanding
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of the matter. This required, first, realising that the cementation process
entailed a significant weight increase and not just a colour change. This
observation is a major step forward from the old concept of a ‘power’
that colours copper an alien colour, shifting the balance from the ideas of
hotness, inherent moisture, and so on to acknowledging that a physical
substance is being added to the copper and effects this change. A second
important realisation will be the fact that this extra substance is indeed zinc
and that brass can therefore be made by simply alloying the two metals.
Much needs to be said about the transmission of knowledge, both
over time and between the two sides – practitioners and scholars. It
would appear that the latter owe the former a lot of careful observation
and experimentation; however, practitioners such as Biringuccio and
Agricola explicitly credit ‘the alchemists’ with the invention of brassmaking. Whether this is a reference to a possible ‘Eastern’ origin of the
revived medieval brassmaking or simply a generic attribution for everything enigmatic and artful is another question outside the remit of this
chapter.
Whatever the case, the historical record shows that, in theory, by the
17th century everything was ready for a Renaissance breakthrough in
brassmaking that could have turned it into an even more efficient and
profitable industry – hence Ercker’s encouragement of research and development. What happened in practice? Did these seeds of innovation grow?
To cut the story short, it is enough to remember that a method for the
industrial production of zinc by distillation was first patented only in
the late 1730s (by William Champion in Bristol, England), and still this
proved rudimentary and unprofitable, and in need of much development,
before it could allow speltering as an established brassmaking method (Day
1973, 1991, 1995, 1998; Dungworth and White 2007). Not only for
Britain but also for Bavaria (Priesner 2000) we have a rich historical and
archaeological record evidencing that the industrial production of brass
continued to rely on the traditional cementation method well into the
19th century, that is, over 200 years after the necessary discoveries for
direct alloying had taken place.
There is not enough space to elaborate on the reasons why this
Renaissance wave of experimentation and discovery took so long to affect
daily, industrial practice, that is, what we would now term the relationships
between science and technology. In considering possible explanations,
we are inclined to agree with Johann Glauber: in spite of his excitement
about, and encouragement for, the exploitation of the new metal zinc,
‘which is most excellently excellent (sic)’, he was frustrated that ‘men are
hardly drawn back from an old custom’ (Packe 1689:320). At a practical
level, tradition was clearly stronger than the stimuli for innovation, hence
the slow pace of industrial applied research. Surely, additional reasons
THILO REHREN AND MARCOS MARTINÓN-TORRES
185
for this lie in the particular socioeconomic contexts, the balance between
the perceived costs and benefits, competition between companies, and the
flow of information between scholars and practitioners. As this chapter
has tried to show, the full picture will be reconstructed only through
combined work in archives and libraries, archaeological sites, and
archaeometric laboratories.
Acknowledgments
This chapter grew out of many years of cooperation with many colleagues,
allowing access to excavated finds as well as to their thoughts. We can
mention only some of them, including N. Zieling, H. Brink-Kloke, and
J. Beutmann for their archaeological expertise, and Chr. Bartels and M.
Charlton for their thoughts and comments on ideas developed in this
text. Any errors of judgment remain ours. The underlying analytical work,
published elsewhere, was done first in the Institut für Archäometallurgie
at the Deutsches Bergbau-Museum, Bochum, and later at the Wolfson
Archaeological Science Laboratories at the UCL Institute of Archaeology,
London. We are indebted to these institutions and their staff, particularly
Andreas Ludwig and Kevin Reeves, for their unfailing support. We
acknowledge with gratitude financial support given to MMT by the Barrié
de la Maza Foundation (Spain) and the British Council (UK).
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