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Journal of Medieval Iberian Studies
ISSN: 1754-6559 (Print) 1754-6567 (Online) Journal homepage: http://www.tandfonline.com/loi/ribs20
Recipes and experimentation? The transmission of
glassmaking techniques in Medieval Iberia
David J. Govantes-Edwards, Chloë N. Duckworth & Ricardo Córdoba
To cite this article: David J. Govantes-Edwards, Chloë N. Duckworth & Ricardo Córdoba (2016):
Recipes and experimentation? The transmission of glassmaking techniques in Medieval Iberia,
Journal of Medieval Iberian Studies, DOI: 10.1080/17546559.2016.1209779
To link to this article: http://dx.doi.org/10.1080/17546559.2016.1209779
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Recipes and experimentation? The transmission of
glassmaking techniques in Medieval Iberia
David J. Govantes-Edwards
a
, Chloë N. Duckworth
b
and Ricardo Córdoba
c
a
Facultad de Filosofía, Universidad Nacional de Educación a Distancia, Madrid, Spain;
b
School of Archaeology
and Ancient History, University of Leicester, Leicester, UK;
c
Departamento de Ciencias de la Antigüedad y la
Edad Media, Universidad de Córdoba, Córdoba, Spain
ABSTRACT
This paper explores the potential of a combined historical and
archaeological approach to the study of glass production in the
Iberian Peninsula in the Late Middle Ages. The historical study of
technical recipes compiled during the Late Middle Ages and the
Early Modern period and the compositional analysis of
archaeological material offer different, but equally valuable,
insights into glass production. The methodologies, potential and
limitations of these techniques are summarised, and their
combined use explored, with reference to a fifteenth-century
letter written by one Cristóforo de Soto Mayor, its experimental
reconstruction, and the comparison of these results with real
archaeological datasets.
ARTICLE HISTORY
Received 29 February 2016
Accepted 3 July 2016
KEYWORDS
Glassmaking; technical
recipes; archaeometry;
Iberian Peninsula
Introduction
The study of technology is a recurrent topic for both historians and archaeologists, and
recent decades have witnessed a considerable increase in the theoretical and methodologi-
cal complexity with which the issue is approached. Within this general context, glass pro-
duction has received much attention, but the Iberian Peninsula remains somewhat behind
other regions in the Mediterranean world in this regard.
1
That special attention is paid to
glass production in the Eastern Mediterranean and the Near East during the second mil-
lennium BC, the region and period in which glass was invented, seems to make sense, but
the perpetuation of this research bias for other periods (for example, the Late Middle Ages)
in which glass was pretty much universally known, circulated and produced is question-
able. At any rate, as we shall see in more detail below, the study of glass production in the
Iberian Peninsula in the Middle Ages is still in its infancy, especially concerning the tech-
nological aspect.
This state of affairs is a little perplexing, since the Iberian Peninsula stands as a unique
example of direct, and often peaceful contact between two very different cultural horizons:
Western Christianity and Islam. That the direct transfer of complex technological pro-
cesses occurred between al-Andalus and the Christian kingdoms to the north is a well-
known fact, for example with ceramic lustrewares; originally developed in the Nasrid
© 2016 Informa UK Limited, trading as Taylor & Francis Group
CONTACT David J. Govantes-Edwards dgovantes2@alumno.uned.es
1
Duckworth et al., “El vidrio andalusí”.
JOURNAL OF MEDIEVAL IBERIAN STUDIES, 2016
http://dx.doi.org/10.1080/17546559.2016.1209779
kingdom, the production of ‘Malaga lustrewares’was eventually adopted in Christian
Aragon.
2
There is, however, still a lot to be unpicked in this regard. Technological inter-
action in a particular field is hardly ever a one-off event involving the wholesale adoption
of a certain practice; rather, it is a much more complex process full of twists, counter-twists
and individual acts of adaptation, invention and rejection, especially when such different
approaches to production, identity, power and religion as Islam and the medieval Chris-
tian West are involved (and this is without even mentioning the presence of a very signifi-
cant third factor in the equation: the Jewish contribution to the history of Medieval Iberia).
In order to contribute towards solving this historiographical anomaly, we propose to
examine the potential of combining two very specific strands of evidence –chemical com-
position and technical recipes –as means of determining the technological practices
involved in glass production. As we shall see in detail below, compositional evidence
for glass from the Islamic period/areas is becoming increasingly available, but the infor-
mation provided by Arabic written sources is close to non-existent. Conversely, the
written record for the Christian period/areas is much richer, including some highly
detailed documents, but the compositional evidence is limited, to our knowledge, to
church window glass (addressed below). The really interesting thing here is that, as far
as glass production is concerned (as well as for so much else), what we have just called
Islamic and Christian “period/areas”are conventions; the reality was much more fluid
and frontiers much more permeable. This we already know on the basis of a few significant
pieces of evidence. Firstly, we know that after the end of the Christian conquest of
Granada, in 1492, a type of Islamic glass known as Castril continued in some areas of
the former Nasrid kingdom until the seventeenth century and even later.
3
Secondly and
crucially, the use of plant ashes in glassmaking may have been introduced to the
Iberian Peninsula by the Muslims, but seems to have been adopted at a relatively early
stage in the Christian territories, as shown by Thomas Glick.
4
The methodologies involved in the study of these different strands of evidence are, as
may be imagined, nothing alike, but they make good partners, as we hope to demonstrate,
yielding a result which can be much more than the sum of its parts. As well as allowing
both the historian and the archaeologist to take their conclusions much further, they
open an interesting path towards more general, and probably also more fruitful, fields
of enquiry, such as the perspective on technological practice and exchange as a social
feature of different, but interlocking, human groups.
A (very) brief introduction to glass in history
The earliest glassmaking industry developed in the mid-second millennium BC in Meso-
potamia, and spread from there to Egypt and the eastern Mediterranean. Written
responses to the new material reflect an interest in its properties, particularly that of
colour.
5
Colour was understood as a key, inherent material property, and the ability of
2
García Porras, “El azul en la producción”,26–7.
3
Frothingham, Spanish Glass,52–9.
4
“In 1189 the monastery of Poblet granted to the glassblower Guillem the right to gather glasswort in return for tithe and
two hundred pounds of sheet glass paid annually.”Glick, Islamic and Christian Spain, 241.
5
The earliest known recipes thought to pertain to glass are strongly framed around the creation of various colours. See
Oppenheim et al., Glass and Glassmaking.
2D. J. GOVANTES-EDWARDS ET AL.
glass to fully assume a whole range of different colours may well have been a factor in its
high value at this early stage.
6
The production of a particular colour of glass, or the elim-
ination of colour from glass, remains a significant theme in the corpus of early glass recipes
known to us.
Why the focus on colour? We can suggest several explanations for this. In its earliest
iteration, glass was seen as a form of precious stone: its colour and potential for poly-
chromy were central to its value. In the Roman period, following the invention of glass-
blowing, the material became quotidian for the first time in its history, and was now
able to take on “practical”functions, for example its use in storage and transport contain-
ers, as a drinking vessel (which, unlike metal, did not add its own taste to the contents),
and in lighting. In these contexts, the key factor was not the colour of the glass, but its
translucency. From a textual perspective, it is likely that much of our knowledge of
Roman glassmaking is a response to the relatively rapid expansion in the use of glass as
a material, including its mass production and recycling. A fair proportion of the texts
describing glass production date from the first century AD, around the time that this
expansion took place.
7
At the same time, the production of luxury glass continued, and
the expense of this glassware was often highlighted in the choice of colourants employed
and the use of time-consuming production techniques: the classic example combining
both is the famous fourth-century Lycurgus Cup.
8
Nonetheless, and perhaps because of
the importance of translucency in this period, we have few references in the Roman
period to colouring glass. By the medieval period, with the expansion of the use of
window glass and enamelled decoration, polychromy in glass is again a subject of detailed
consideration, and texts such as the so-called Bologna Manuscript, written in the fifteenth
century,
9
include expositions of the different mechanisms by which glass can be coloured.
An early example of a new direction for glassmaking texts is provided by the twelfth-
century book on glassmaking by Theophilus Presbyter, who has been identified with
Roger of Helmarshausen: in addition to a description of the production of different
colours of glass by varying the redox atmosphere
10
in the furnace and by recycling
Roman glasses, it focuses in detail on furnace construction and the production of
certain types of glass object (windows, flasks, rings).
11
The description of furnaces was
later followed in such treatises as Agricola’sDe re metallica and Vannoccio Biringuccio’s
Pirotechnia (both sixteenth century in date).
12
Interestingly, the latter text includes a long
consideration of the beauty of different colours of glass, but offers no insights into how
these are produced.
6
Duckworth, “Imitation, artificiality and creation”.
7
See for example: Pliny, Natural History 36.106, 109, 110, 190–9, 37.29; Josephus, Jewish War 2.189–90; Strabo, Geography
16.2.25; Martial, Epigrams 1.41.1–5, 12.74, 94; Statius Silvae 1.6.70–4.
8
Freestone et al., “The Lycurgus Cup”.
9
Merrifield, Original Treatises. The treatise was found in the nineteenth century in the convent of San Salvatore, in Bologna,
but the authors do not know its current whereabouts.
10
In simple terms, the redox conditions refer to the potential of the atmosphere for either reduction (loss of electrons) or
oxidation (gain of electrons) of the materials being heated. In practical terms, redox atmospheres are varied by altering
the amount of oxygen and carbon, so that a highly oxidising atmosphere is one with an excess of O
2
, whereas a highly
reducing atmosphere has an excess of CO (carbon monoxide).
11
Theophilus Presbyter, On Divers Arts,47–74.
12
Agricola, De re metallica; Biringuccio, Pirotechnia.
JOURNALOFMEDIEVALIBERIANSTUDIES 3
A (very) brief introduction to the chemistry of glass
Before we proceed any further it may be a good idea to make a very brief introduction to
the chemistry of glass, so the reader can follow the arguments below. The three main com-
ponents are a glass former, chiefly silica (SiO
2
), an alkali flux, mostly soda (Na
2
O) or
potash (K
2
O), and a stabiliser, namely lime (CaO). The silica, which is the principal struc-
tural component of the glass, can be extracted from sand or from crushed quartz pebbles,
which are a particularly pure source of silica. The alkalis (soda and potash), for their part,
act as fluxes, lowering the melting and working temperatures, and can be extracted from
mineral sources, for example “natron”, or from vegetal sources, such as halophytic plant
ashes for soda and other vegetal ashes (e.g. beech wood) for potash. Finally, the lime,
which stabilises the structure and prevents its dissolution in water, can be extracted
from different sources such as shell (often present in the glassmaking sand), limestone,
bone, or the same plant or tree ashes used as a source of alkali. Glasses are typically pro-
duced at temperatures in excess of 1100°C, though temperature varies along with time of
heating, composition of glass, and the grain size of the raw ingredients.
Most ingredients are added to the mix in the form of carbonates, but the carbon burns
off during production, leaving the glass as a network of oxides. It can be seen that some
combinations of raw ingredients (e.g. beach sand + plant ash) will provide all three necess-
ary ingredients (former, alkali, stabiliser), whereas others (e.g. quartz pebbles + mineral
soda) will not, and the glass will either fail, or will be unstable and may not survive in
the material record. Impurities can have negative consequences on colour and working
properties, so another factor relevant to glassmakers would be the combination of raw
ingredients in such a way as to minimise impurities.
Technical recipes in the context of medieval glass making in the Iberian
Peninsula
Technical recipes are a sui generis source of historical information. These texts describe the
steps taken in different technical processes and indicate the various raw materials and
equipment that need to be used to obtain a certain product. However, the category “tech-
nical recipe”is a construction in a very Latourian way; the category is rather broad and
flexible, and can include a wide variety of texts written for very different purposes and
for very different audiences. We cannot approach St. Isidore of Seville’sEtymologies,a
seventh-century encyclopaedic compilation of general knowledge which includes some
technical references to glass,
13
in the same way in which we read Theophilus’sOn
Divers Arts, a twelfth-century specialised technical treatise on painting, glassmaking and
metalwork,
14
and aimed at the craftsman, Hieronymus Münzer’sItinerarium Hispanicum,
a geographical description of late fifteenth-century Spain in which, again, some technical
references to glassmaking are made in passim,
15
or, a commercial agreement in which
transactions involving the raw materials used in glass production are detailed.
16
In
general, all historical sources must be considered critically, that is, taking into account
13
Isidore of Seville, The Etymologies, 328.
14
Theophilus, On Divers Arts.
15
Pfandl, Itinerarium Hispanicum.
16
Glick, Islamic and Christian Spain, 241. Otte, Sevilla y sus mercaderes, 88-9.
4D. J. GOVANTES-EDWARDS ET AL.
their original aim, their transmission, and any other factor that may affect their value and
scope as historical evidence,
17
but this is especially relevant when dealing with a con-
structed textual category, such as “technical recipes”.
Even when recipes are part of a technical treatise written or compiled by someone who
understood the technologies being recorded, we should not assume that they will be com-
plete and systematic, as anyone who has studied them in detail knows. These recipes can
be surprisingly erratic and vague for a kind of text which, from our own perspective,
should be the epitome of orderliness and completeness. The reasons for this are multiple.
The most obvious is perhaps the possibility of copyists and compilers making mistakes,
which is especially plausible when said copyists and compilers had no knowledge of the
technical procedures described in the text.
18
However, the incompleteness could very well be intentional, which is not to say delib-
erately misleading (at least not always). If we send a colleague an email explaining how to
operate a new piece of software, we do not open our email with the instruction: “turn the
computer on”; that much is assumed. If a recipe was put down in writing by an expert
craftsman and addressed to a peer, the standard procedures would be likewise assumed
and not committed to writing for being obvious to them, if not to the non-expert. On
the other hand, we must remember that information is not a free item, and that technical
knowledge is sometimes guarded jealously. History is full of examples of crafts that were
kept secret, sometimes for long periods of time, for their value as instruments of power, the
most obvious being, perhaps, writing itself.
19
A splendid example of this is provided by the
potters of Muel (Zaragoza, Spain), who in the late sixteenth century gave to Henrique
Cock, a member of Philip II’s retinue, the instructions for making lustre-wares, which
include the use of “un poco de alambre”, that is, “a little copper”.
20
What is unclear
from the manuscript (but becomes clear upon scientific investigation) is that copper is
the key in this procedure, and unless it is added in exactly the right amount, the instruc-
tions are simply worthless. In effect, the potters from Muel –possibly quite deliberately –
told Henrique Cock nothing of any use.
21
A key factor to consider here, therefore, is the
purpose behind putting the recipe down in writing in the first place. Let us assume, for the
moment, the idea that the glassmaking texts under discussion were written with the
purpose of allowing a craftsperson to follow and reproduce the recipe. We would argue
that, in spite of all the care taken to include relevant details such as weights and types
of raw material, it is all but impossible to translate the act of making glass into a text in
such a way that it could be followed “from scratch”. For example, experimental archaeol-
ogy and ethnography tell us about the central importance of furnace design to the success
of a glass production recipe. The temperatures involved in glass production are extremely
high, and different temperatures are required for different processes, so furnace design,
fuel type and temperature control are vital factors in success. Yet prior to the sixteenth
century, few recipes even mention the type of furnace to be used. The texts, clearly,
were not designed with the aim of describing the entire chaîne opératoire of glass pro-
duction. Prior knowledge is required: recipes and other texts are aimed at augmenting,
17
Martinón-Torres, “Why Should Archaeologists”, 26; Córdoba, “Un recetario técnico”,8.
18
Martinón-Torres, “Why Should Archaeologists”, 27; Smith and Hawthorne, “Mappae Clavicula”, 15.
19
Goody, The Logic of Writing.
20
Morel-Fatio and Rodríguez, Viaje de Felipe II, 31.
21
Pérez-Arantegui and Pardos, “Lustre recipes”, 164.
JOURNALOFMEDIEVALIBERIANSTUDIES 5
and perhaps adjusting this. Indeed, there is a great difference between the sort of knowl-
edge which can be transmitted via text, and the sort of knowledge which must be acquired
through praxis. There are also social hierarchical associations with different types of
knowledge, and those with the knowledge of controlling a furnace atmosphere –which
is dirty, and learnt by praxis –would almost always have lower status than those with
the knowledge of, for example, which minerals to use to create a desired colouring effect.
Another factor which was significant in the production of many past glassmaking
recipes is their role as elements of alchemical treatises.
22
The aims of such texts vary,
but one theme which recurs throughout the history of glass production is that of
control over the natural world. Recipes were not necessarily designed to transmit knowl-
edge as much as to demonstrate the possession of that knowledge by the owner of the com-
pendium, and the library in which it was housed. Alchemy itself is the ultimate quest for
control over the natural world, and the changing of a material’s inherent qualities was a
significant step in this.
Turning to the specific matter of glassmaking in the Iberian Peninsula during the
Middle Ages, what recipes do we have? The answer, unfortunately, is not many (see
Table 1). Perhaps it is best to begin by mentioning two alchemical compendia, Alfonso
X’sLapidario, compiled in the thirteenth century, and a work known as Sedacina, com-
piled by a Guillaume Sedacer in Catalonia in the late fourteenth century. In these compen-
dia, glass is presented by virtue of its alchemical properties; the value of these texts as a
source for the history of technology is thus limited.
23
The prologue of the Lapidario spur-
iously claims that at least some of the recipes are based on Aristotle, via the transmission of
Arabic scholars (specifically, the mysterious Abolays),
24
but it must be kept in mind that
this work was being compiled during the apogee of the so-called School of Translators of
Toledo, which was in the process of translating most of Aristotle’s works into Latin from
Arabic, and which was to have a deep and lasting effect on Western science and philos-
ophy.
25
In any case, the attribution of work to earlier, even mythological personages is
a recognised difficulty in alchemical texts.
26
A completely different kind of text is manuscript H-490 from l’École de Médicine of
Montpellier University, written sometime between 1460 and 1480, which includes one
recipe on how to prepare glazing and three on how to colour glass. It is a nice example
of some of the typical features and problems of technical recipes during this period.
First it is a miscellaneous compilation of texts, most of which deal with medical and bota-
nical issues.
27
Moreover, only recipe number 31 gives a very schematic account on how to
prepare glass from scratch while the others simply seem to assume that whoever is inter-
ested in dyeing glass red or green does not need telling how to make glass in the first place.
Concerning transmission, recipe number 33 in manuscript H-490 is, according to the text,
based on Albertus Magnus, Thomas Aquinas’s tutor and famous in the Middle Ages for
the scope of his scientific curiosity (and his Aristotelian leanings!). We have not had
the opportunity of examining Albertus’s vast bibliography in full, but suffice it to say
22
Barthélemy, La Sedacina ou l’oeuvre au crible; Neri, The Art of Glass.
23
Rodriguez, Lapidario; Barthélemy, La Sedacina ou l’oeuvre au crible.
24
Amasuno, “En torno a las fuentes”.
25
Martínez Lorca, Maestros de occidente.
26
Hill, “Arabic alchemy”.
27
Córdoba, “Un recetario técnico”,9.
6D. J. GOVANTES-EDWARDS ET AL.
that this glass recipe is not to be found in Of the Virtues of Herbs, Stones and Certain
Beasts,The Book of Minerals or On the Causes of the Properties of the Elements,
28
which
are the three works by Albertus Magnus in which this sort of information is likely to be
expected. This means that either the attribution of the recipe to Albertus Magnus is spur-
ious or that the recipe is in one of Albertus’s works which are not explicitly concerned with
the natural sciences or the properties of materials. As we can see, the routes of medieval
transmission can be circuitous indeed, which does not always make for a straightforward
historical record.
We also have the document known as Epistola Abbreviatoria, a letter dated to the late
fifteenth or the early sixteenth century, in which a Christóforo de Soto Mayor addresses his
friend Juan de Alcalá and gives him the recipe to produce three qualities of glass.
29
Finally,
there is the German geographer Hieronymus Münzer’s (also spelled Müntzer) account of
his travels through the Iberian Peninsula in 1494–1495. Interestingly, apart from a very
schematic description of the technique to make glass (which betrays his knowledge of
Table 1. Summary of texts related to glassmaking in the Iberian Peninsula.
Author and
common name of
source
Type of text and
language Date
Location(s) to which
the text pertains
Provenance of technical
information
Don Christóforo of
Soto Mayor,
Epistola
Abbreviatora
Letter, Latin Early sixteenth
century
Galician writing to
friend in “Alcalá”.
Refers also to Bishop
of Córdoba.
Refers to how glass “used to”
be made (Lat. Olim). Source
(s) unknown.
Guillaume Sedacer,
“Sedacina”
Alchemical treatise,
Latin
Written
sometime
between
1378 and
1382
Sedacer was
Catalonian, and
spent time in
Catalonia and
southern France.
Other medieval treatises, e.g.
Theophilus’sDe diversis
artibus. Cites lists of sources
in the Arabic tradition. Links
to alchemical conclusions
original.
Alfonso X (El Sabio),
Lapidario
Astrological and
mineralogical
treatise, translated
to Castellano
original c.1250,
re-worked
1276–9
Castile (Alfonso X) Prologue claims that the text
originates in the writings of
Aristotle. Probably owes
much to Arabic Lapidaries.
Juan Celaya (?)
“Montpellier
manuscript”H490
Compilation of texts,
mostly medical and
botanical.
Castellano
1460–80 The only recipe which makes
reference to origin of
information refers it back to
Albertus Magnus, but this
has not been confirmed.
Hieronymous
Münzer,
Itinerarium
Hispanicum
1494–5 Description of
glassmaking in
Murcia by German
humanist from
Nuremberg
Isidore of Seville,
Etymologies
The section on glass
production is based upon
Pliny’sNatural History, which
refers specifically to the
production of glass in the
Iberian Peninsula, though
this is not .
Pedro Gil, Historia
natural de
Catalunya
c. 1600 Catalonia
28
Best and Brightman, The Book of Secrets; Wyckoff, Albertus Magnus; Albert the Great. On the Causes.
29
Whitehouse, “The Epistola Abbreviatoria”, 355–7.
JOURNALOFMEDIEVALIBERIANSTUDIES 7
the plant ashes used for this purpose in Germany), Münzer also described the abundance
of a plant called kali,sosam or zozam in the vicinity of Alhama de Murcia, a glassmaking
centre.
30
In conclusion, we are currently aware of just a few recipes (see Table 1 for summary);
and more significantly, we have none from the Islamic regions of Iberia. This is in conso-
nance with a general paucity of written records concerning the production of glass in al-
Andalus. The only direct reference we know of is by al-Maqqari, a seventeenth-century
historian, who quotes Ibn Sa’id (thirteenth century), who mentions the excellence of
glass productions from Almeria, Murcia and Malaga, and makes one passing reference
to the use of “stones”to make glass.
31
That no Andalusi accounts of glassmaking have
reached us is indeed unfortunate, for glass technology in other Islamic regions was
described in some detail, for example by al-Jabir (eighth–ninth centuries), Almass’oudi
(ninth–tenth centuries) and al-Biruni (tenth–eleventh centuries).
32
Methodology: compositional analysis
A variety of analytical techniques exist with which the chemical composition of glass may
be measured. Most of these rely upon generating and firing high-energy waves or particles
(e.g. electrons, X-rays) at the sample, and measuring one or more of the resultant emis-
sions (by mass, wavelength, or energy). These results are compared with a suite of stan-
dards of known composition, and used to determine the chemical make-up of the
sample. In addition to techniques designed to characterise the bulk chemical composition
of a material, it is possible to measure various isotope markers of the geological origin of
the raw ingredients, which can be very useful in the definition of the provenance of specific
ingredients, such as lead or sand.
33
These techniques are able to offer insight into the technologies involved in the pro-
duction of a glass item. However, there are also important limitations to compositional
analysis, which have essentially to do with the nuances of the archaeological material
on which they depend, and of which we shall only mention the most common. To
begin with, glass is fragile, and it does not always survive in recognizable conditions in
the archaeological record, so the amount of evidence available for sampling and analysis
is limited. Moreover, all archaeological materials, including glass, are subject to deposi-
tional and post-depositional processes which can affect their reliability as evidence; only
items found in well-excavated and “closed”archaeological contexts should be used for
analysis, as decontextualised analytical results are of little value. In addition, in the past
as today, goods could move about quite a bit, for example through trade, and nothing
guarantees that a piece found in a particular geographical and chronological context is
representative of technological practice in said context. In this regard, context is vital,
and all analytical programmes should consider as large as possible a population of
samples to ensure maximum representativeness; however, due to sampling issues and
analytical costs the ideal is rarely achievable. To make contextualisation even harder,
30
Pfandl, Itinerarium Hispanicum,34–5.
31
Jiménez, “El vidrio andalusí”, 117; de Gayangos, Mohammedan Dynasties, 148 (Vol I); 311 (Vol II).
32
Henderson, Ancient Glass, 263–5; Young and Latham, Religion, Learning and Science, 328–41, 405–23.
33
For detailed coverage of the most common analytical techniques available to archaeologists, see Pollard and Heron,
Archaeological Chemistry.
8D. J. GOVANTES-EDWARDS ET AL.
glass production facilities are very elusive; most known examples in Spain were located in
urban and suburban areas, and their archaeological exploration more often than not
depends on urban rescue archaeology, which takes place in a context of financial
and time pressure, and thus rarely provides the most adequate tools for specialised
research. This means that items found in non-production contexts (e.g. consumption,
domestic, religious) are seldom open to direct comparison to production remains from
the same period and region. Finally, glass is not only recyclable, but we know that it
was recycled on a vast scale in the past. A good illustration for this is the eleventh-
century Serçe Limani shipwreck, the remains of a westbound ship found off the
Turkish coast. This ship was loaded with a cargo of glass intended for further working
which, significantly, included not only chunks of “raw”glass, but also glass cullet, that
is, broken glass items, presumably to be re-melted and re-shaped at the cargo’s destina-
tion(s).
34
Recycling has the effect of muddying the outcome of compositional analysis,
for example by offering results that appear somewhere between the ranges of known com-
positional groups.
In Spain, little has been done concerning the compositional analysis of medieval glass.
The only quantitative studies published to date that concern Andalusi glass are, to our
knowledge, a batch of material from the excavation of an eleventh- to twelfth-century
glass workshop in Murcia and a group of samples from different domestic contexts
in Córdoba, between the ninth and the twelfth centuries.
35
Another group of
samples, in this case from Malaga (twelfth–fourteenth centuries), has also been
analysed by the authors of this paper and others, but is awaiting publication. In addition
to this, the other field in which analytical work has been carried out concerns church
window glass.
36
The reconstruction of a glassmaking recipe: Don Christóforo’sEpistola
Abbreviatora
In order to make an in-depth exploration of the way in which we can combine the evi-
dence of a written recipe with that of archaeological and archaeometric analyses, we
focused on the text written by Don Christóforo. This incomplete text, summarised by
David Whitehouse,
37
was written by one Don Christóforo of Soto Mayor (Galicia) to
his friend, Juan of Alcalá, around the first quarter of the sixteenth century.
38
It is
unclear which “Alcalá”Juan was from –there are a good few in Spain –but we believe
its contents to be of potential relevance to southern Spain (and thus the lands which
were under Muslim rule for the longest) as Christóforo mentions that he has made
requests to the bishop of Córdoba for books about glassmaking.
A further significant fact about the letter is that Christóforo claims he is describing tra-
ditional, rather than contemporary, glassmaking. This naturally raises some questions
about the origin of his information: was it copied, or based on personal observation?
34
Bass et al., Serçe Limani,4.
35
Carmona et al., “Islamic glasses”, 439–45; Duckworth et al., “Electron Microprobe Analysis”,27–50.
36
Window glass is a specialism in itself, and going into details about these compositional studies is well beyond the scope of
this paper. For results and discussion, see Bazzochi, Las vidrieras góticas; Carmona et al., “Vidrios y grisallas”; Alonso et al.,
“Un vidrio medieval”.
37
Whitehouse, “The Epistola Abbreviatoria”.
38
On dating the text, see Whitehouse, “The Epistola Abbreviatoria”, 355.
JOURNALOFMEDIEVALIBERIANSTUDIES 9
The part of the letter which is of the most relevance to us is that which reports on the
making of raw glass. Three different qualities are described: infima (the lowest quality),
christallina (middling quality) and physica (the best quality).
The key stages in the recipe for infima glass are as follows:
1. Take three parts of sosa or salicornia (which glassmakers call barrilla).
39
2. Pound one part of sand or finely ground pebbles.
3. Mix them, add ordinary water and form the mixture into “loaves”.
4. Dry these in the furnace and cool them.
5. Melt them in pots, leaving them for at least one night, or [preferably] 24 hours.
6. With an iron spoon, continuously remove the scum that rises to the surface and is
known in the vernacular as anitron salt.
7. You will know that the glass is ready when it adheres to the tip of an iron rod and
remains clear.
8. Then for every 100 pounds of the melt, add one ounce of manganese (which glass-
makers call tinta spiritorum).
9. Mix it well and immediately the melt will become violet –wait until the colour settles to
the bottom, and you have white [colourless] glass ready to receive any other colouring
agent.
The recipe for the christallina glass is similar, but uses two parts of clear, white ground
quartz pebbles rather than one part of sand, and only uses the sosa plants. The physica
glass has a different ratio again: one to one of sosa plant ashes to ground pebbles.
Making the glass
We followed the recipe for infima glass, using modern washed silica sand and the ashes of
halophytic plants from locations in Almería and Murcia. Table 2 summarises the ingredients.
The plants were mainly gathered from sites in Murcia, although some plants from
Almería were also included. The majority of the ashes came from Alhama de Murcia,
which was famous as a centre of glassmaking, and was also mentioned by Münzer in
the Itinerarium Hispanicum, as noted above. Samples of the gathered plants were dried
and ashed at 650°C. No further treatment was applied, save for that suggested by Chris-
tóforo himself and outlined below.
The experiment was conducted using a modern electric furnace, made by Vecstar Ltd.
and housed in the School of Archaeology and Ancient History, University of Leicester. The
furnace has a maximum temperature of 1300°C and can be programmed to raise tempera-
ture incrementally. The glass was fired in this furnace to 1150°C under an oxidising atmos-
phere, according to the temperature chart shown in Figure 1.
Because our experimental setup did not allow us to stir the glass during production, we
were unable to follow this part of the recipe. In fact, the outcome demonstrates that it is
probably a crucial part, as we were left with just a small fraction of glass beneath a mass of
bubbly, frit-like material, opaque white in colour (Figure 2). Frit is the term for a sintered
39
The term barrilla is a generic name used to refer to a group of plants rich in sodium, the ashes of which were profusely
used in glassmaking. The term originates in the Iberian Peninsula but became widely recognised throughout Europe due
to the good reputation of barilla ashes from at least the seventeenth century.
10 D. J. GOVANTES-EDWARDS ET AL.
(semi-fused) fusion of the glassmaking ingredients, and in some glass production oper-
ations it is a key initial stage in glassmaking, with the frit being crushed and reheated
to form true glass.
Initially, we suspected that the frit-like formation was a scum resulting from the indis-
criminate use of untreated plant ashes: if plant ashes high in sodium chloride (NaCl) as
well as those rich in sodium carbonate (NaCaO
3
) were employed, we could expect the
chlorides to form an immiscible scum on the surface, as the solubility of chlorine in
glass is highly limited.
40
Another possibility, however, is that the glass simply had insuffi-
cient reaction time: it was not held at temperature for very long, and as Don Christóforo
offers little information on the operation of the furnaces, we cannot be certain that either
our maximum temperature or soak time was an accurate reconstruction of those used in
the fifteenth and sixteenth centuries.
Samples of the frit-like material formed by the experiments and the small amount of
successful glass produced were analysed using a Horiba XGT-7000 micro X-ray fluor-
escence spectrometer (School of Archaeology and Ancient History, University of Leice-
ster). The sample was analysed under full vacuum, with an acquisition time of 300
seconds, 15 kV accelerating voltage and 1 mA current, and beam diameters of 100 µm
and 1.2 mm. The points selected for analysis are shown in Figure 3.
Table 2. plants used for the experiment. Plants from several locations were combined due to limited
availability (the plants were originally gathered by us for quantitative analysis, which is yet to take
place). The species identifications are tentative only, as we have not yet consulted a botanist.
Weight of dried plants Gathered at (location) Geographical coordinates of sample Species
10.8 g San Pedro de Pinatar, Murcia 37°49′35.22′′N
0°45′50.75′′W
Sarcoconia perennis
75.4 g Alhama de Murcia, Murcia 37°46′34.34′N
1°27′2.34′′W
Halogeton sativus
19.6 g San Pedro de Pinatar, Murcia 37°49′38.45′′N
0°45′53.69′′W
Sarcoconia perennis
21.2 g Cabo de Gata, Almería 36°44′26.38′′N
2°12′31.91′′W
Salicornia europaea
8.8 g San Pedro de Pinatar, Murcia 37°49′35.22′′N
0°45′50.75′′W
Salicornia europaea
Figure 1. Time-temperature chart for the firing. The glass was held at a temperature of between 1146°
C and 1152°C for an hour, before being allowed to cool within the furnace.
40
See for example Tanimoto and Rehren, “Interactions between silicate and salt melts”, 2567.
JOURNAL OF MEDIEVAL IBERIAN STUDIES 11
The results of µ-XRF analysis are semi-quantitative only. The results for the most con-
sistent glassy phase analysed are reported in Table 3.
Figure 2. (a) Halophytic plants prior to ashing; (b) the sand; (c) plants after ashing; (d) “loaf”of plant
ashes, sand and water; (e,f) the loaf after firing; (g,h) the vitreous product after the second firing.
12 D. J. GOVANTES-EDWARDS ET AL.
Silicon, as to be expected, is the most abundant element detected. It derives from the
sand and is the glass former. Also likely to have come from the sand are aluminium
and iron, and possibly the small amount of titanium and the trace of manganese. Probably,
the rest of the identified elements largely, if not entirely, derived from the plant ashes.
These include the alkalis (sodium and potassium) and calcium (a crucial ingredient in
soda-silica glasses as it acts as a stabiliser), demonstrating that the plants ashed were suit-
able choices for glass production insofar as they contained the two necessary ingredients
not found in the sand (alkali and lime, CaO). Interestingly, the results for all areas analysed
included a significant proportion of chlorine, apparently close to its limit of solubility in
soda-silica glasses, though the poor accuracy and precision of the analytical technique
means that we should interpret all reported results as broadly as possible.
Discussion: Don Christóforo’s text and glass production in al-Andalus
In order to investigate whether the combined use of a glassmaking recipe with archaeolo-
gical and archaeometric approaches can shed any light on the earlier glass production
industry of al-Andalus, we now offer some comparisons between the results presented
Figure 3. Close-up image of the analysed cross-section (left) showing the presence of glassy phases
within the crystalline mass of the fired batch, and the three points (marked with an x) selected for
analysis.
Table 3. Elements, with mass percentages normalised to 100 by the XRF software. Results are rounded
to whole numbers for any reading >1 wt%, and listed as ‘trace’for any reading <1 wt%. Note that in
reality the elements, with the exception of chlorine, are bonded in the glass with oxygen.
Element Read mass % (normalised to 100) Probable raw ingredient source
Na (sodium) 14 Plant ashes
Mg (magnesium) 5 Plant ashes
Al (aluminium) 2 Sand
Si (silicon) 66 Sand
P (phosphorus) trace Plant ashes
S (sulphur) trace Plant ashes
Cl (chlorine) 2 Plant ashes
K (potassium) 3 Plant ashes
Ca (calcium) 7 Plant ashes
Ti (titanium) trace Sand
Mn (manganese) trace Sand, plant ashes
Fe (iron) trace Sand
JOURNAL OF MEDIEVAL IBERIAN STUDIES 13
above, the information provided in the text itself, and the first results of our compositional
research into medieval glass in Cordoba and Malaga.
Despite using orange-coloured sand for our experiments rather than the purest white
quartz pebbles suggested for the higher qualities of glass, the small fragments of glass
which we produced were clear and colourless in appearance, while the crystalline, frit-
like material which formed the majority of the product was white in appearance. This indi-
cates that the plant ashes were relatively free of the sorts of impurities which produce
unwanted colouring effects in the glass.
We might thus assume that the need to decolourise the glass with manganese, as
described in Christóforo’s description, arose from the use of a relatively impure sand
source. An alternative explanation is that the repeated use of an iron spoon at high temp-
erature, in order to remove the scum from the surface of the glass melt, would have intro-
duced a significant iron impurity to the glass. The two explanations are not mutually
exclusive, and both sand and tools are potential contributors of iron to glass. The fact
that even glasses made from the purest silica sources described by Don Cristóforo were
subjected to decolouring may support this view.
In this vein, it is interesting to note that relatively high iron was a feature of ninth- to
twelfth-century Islamic glasses from Cordoba.
41
Initially this may seem to support the idea
of contamination, but the iron in these glasses was strongly correlated with alumina (R
2
=
0.89), suggesting a common source for the two. The most likely source of both iron and
alumina is the glassmaking sand, which could contain iron impurities and aluminosili-
cates, rather than the glassmaking tools. The use of iron tools in glassmaking may well
affect the chemical composition of the glasses, but it is unlikely to do so in a regular
way: sample COR18 from Cordoba, which has higher iron by comparison with its
alumina content, is an example of a glass which may have been contaminated by metal
tools. Other possible sources for iron and alumina contamination include the refractory
materials (such as the crucibles) used during production, which could contribute a
small amount depending on the acidity of the glass and the properties of the crucible itself.
The presence of an excess of sodium chloride in our glasses could have been easily
solved by pre-treating the plant ashes, in order to remove the soluble sodium chloride,
but this may not have been common practice at the time. Pre-treatment of ashes such
as boiling to remove chlorides is mentioned in other historical texts,
42
but Don Cristóforo
himself does not mention it, which could imply that it was not done, or that the ashes were
procured by the glassmakers in a pre-treated state. In favour of the former explanation, we
might note that Münzer makes no mention of pre-treatment either, and he was present in
Alhama de Murcia itself, where the glassmaking plants grew and were gathered. Further-
more, the very need to remove a surface scum from the glass melt using a metal tool seems
to imply the presence of chlorides, and may be an argument against the pre-treatment of
the ashes at this time. This may have interesting implications for the export of the Spanish
plant ashes (barrilla), and the perceived difference in quality between ashes from the East
Mediterranean, preferred by the Venetians, and those from the Iberian Peninsula.
It is also interesting to note that the different qualities of glass described in the text seem
to specify a different ratio of silica to sosa: infima glass is three parts sosa and salicornia to
41
Duckworth et al., “Electron microprobe analysis”, 42.
42
E.g. Biringuccio, The Pirotechnia, 127.
14 D. J. GOVANTES-EDWARDS ET AL.
one part sand or quartz pebbles; christillana is three parts sosa to two parts quartz pebbles;
physica is one part sosa to one part quartz. Given that we cannot be certain about the
origin of the recipe, it is worth considering that there may have been a substantial differ-
ence in the quality of the plant ashes used for the various qualities of glass. This is already
apparent in the fact that the salicornia plants are not used for the production of christillana
or physica, the two finer qualities of glass. Was there also a difference between the plant
ashes used for these two qualities? It is possible that Don Cristóforo was unaware of
this, if so. Another possibility is that the one to one ratio of plant ashes and quartz
pebbles in the physica glass produces a clearer glass (lower in impurities derived from
the plant ashes), but one that is less ‘fast’; i.e. that must be worked at a higher temperature
due to its lower quantity of temperature-reducing alkali. Interestingly, Don Cristóforo’s
assertion that the finer grades of glass include more silica echoes that of Münzer, who
notes that, “If you wish to make white glass like crystal, it is necessary to add more fine
white sand …”.
43
This seems to confirm the practice, but it is also possible –given the
many similarities between the texts –that Don Cristóforo was aware of Münzer’s text
and used it as a source in his letter. The most effective way to investigate this in the
future would be to analyse a number of contemporary glasses, in order to test whether
there is indeed a lower soda content in the highest-quality glasses, which would
confirm the different ratio of plant ashes to silica suggested by the texts.
The use of lead in glass production may also provide a relevant angle of investigation,
particularly in terms of the transfer of technological practices. Don Cristóforo recounts the
use of lead in the production of smalti, which David Whitehouse, in reporting the text,
considers as enamel (coloured glass applied to the surface of glass or other materials),
but also coloured glasses used for other purposes (e.g. making jewellery by flame-
working).
44
One pound of lead is added to ten pounds of the best quality physica glass.
He also advocates the addition of the ashes of tartar, meaning potassium bitartrate (KC
4-
H
5
O
6
), which forms as a by-product of wine-making. By adding both lead and an alkali
(potassium), the coloured glass or enamel maker would lower the melting temperature
of the material, which would presumably aid in its application as enamel or use in
flame-working (lead can also combine with other oxides to produce vibrantly coloured
glasses).
An understanding that lead “softens”glass by lowering its melting temperature may be
traced back to at least the Roman period, when the outer coating of cameo glass vessels was
made in a high-lead, white glass, which was easier to carve by comparison with the blue
glass beneath.
45
Perhaps more relevantly, there may yet prove to be some association of the
medieval addition of lead to glass with the Iberian Peninsula, particularly in the tenth and
eleventh centuries.
46
In this case, we can also trace some link between the glass and glazed
ceramic industries: the use of lead in glasses appears at around the same time as the devel-
opment of Iberian glazed ceramic production using lead (for example, at Madinat al-
Zahra). Later, the development of tin-glazes, so crucial to the production of Iberian lustre-
ware, was followed by the development of tin as a glass colourant: Don Cristóforo himself
43
Whitehouse, “The ‘Epistola Abbreviatora’”, 357.
44
Whitehouse, “The ‘Epistola Abbreviatora’”, 356.
45
Bimson and Freestone, “The Portland vase and other Roman cameo glasses”, 58.
46
Duckworth et al., “Electron microprobe analysis”,32–7; Duckworth et al., “Non-destructive µXRF analysis”,9–10.
JOURNAL OF MEDIEVAL IBERIAN STUDIES 15
explains how the addition of tin oxide produces an opaque white glass of superior
quality.
47
The primary production of glass in crucibles rather than larger structures, as described
by Don Cristóforo, is also significant when considering which of our known glass pro-
duction remains could have been used for primary glassmaking.
48
Did the production
of glass from Don Cristóforo’s“loaves”begin earlier than the fifteenth century, and
what is the technological precedent of the pot furnace in the Iberian Peninsula? It
would certainly have been possible to make glass in some of the archaeologically identified
installations, such as the twelfth- to thirteenth-century furnace from Belluga (Murcia).
49
Glass production facilities can be difficult to interpret, as there is often very little in the
way of by-products; the skimming off of surface scum from the glass-melt as described
in the text may be one sign of primary glass production to watch out for in future research.
There is no mention of glass re-melting or recycling in Don Cristóforo’s text, although
it is explicitly mentioned in other sources: Theophilus describes the collection and re-
melting of antique glass as a means of colouring newly made glasses,
50
and Agricola
describes the re-melting of cast-off fragments from the glass-blowing process.
51
On the
other hand, the archaeological and compositional evidence for earlier centuries provide
very strong indications of recycling. For example, the results of chemical analysis of
glasses from ninth- to twelfth-century Malaga indicate that glasses of a mineral-alkali,
“Roman”-type composition were recycled together with plant ash glasses in south-east
Spain.
52
From a technical perspective, the addition of small amounts of preformed glass
to the raw ingredients would help to ensure their fusion. The practice is not mentioned
by Münzer, either, so –assuming for the moment that the two texts were unrelated –
we may argue that it was less significant in the fifteenth-century glass industry than it
had been previously. By Don Cristóforo’s time, it was no longer possible to gather and
reuse vast quantities of Roman glass as had been practised extensively in Late Antiquity
and the early medieval period. Indeed, in the sixteenth century Biringuccio describes
antique glass as a rare curiosity, apparently mistaking corrosion patterns for the myster-
ious art of Roman glassmakers: “At present I have near me a fragile piece of antique glass
of square form …in which a very beautiful leaf pattern has been worked like tarsia, and it
has such a distribution of colours that I cannot understand how the artificer made it so
beautifully and marvellously …”.
53
The need to produce larger quantities of primary
glass to meet the demand for window and vessel glass in the medieval period may well
have put paid to the large-scale practice of recycling.
Final thoughts
The comparison between the evidence of glass production recipes and the archaeological
and compositional evidence may raise more questions than it answers. But this is not to
say it is without value: indeed, the value of the process may be judged by the interest of the
47
Whitehouse, “The ‘Epistola Abbreviatora’”, 356.
48
Duckworth and Govantes Edwards, “Medieval glass furnaces”.
49
Córdoba, “Technology, craft and industry”, 110.
50
Theophilus, On Divers Arts, 59.
51
Agricola, De re metallica, 592.
52
Duckworth and Govantes-Edwards, “La produccion de vidrio”.
53
Biringuccio, The Pirotechnia, 131.
16 D. J. GOVANTES-EDWARDS ET AL.
questions raised. Don Cristóforo’s text is an excellent example of the importance of recog-
nising assumed knowledge in technological treatises. In this case, the assumed knowledge
is that of furnace types and constructions, without which the production methods
involved will be difficult to ascertain. It is likely that these areas were dealt with by indi-
viduals who occupied a lower position in the professional hierarchy.
Two other key questions emerge from the comparison, and both will need to be
explored in greater depth in future research. First, what relationship was there between
the development of glassmaking in al-Andalus and the later southern European use of
plant ashes in glass production: did the Iberian industry have an influence on the
Italian here, too, as it did in glazed ceramic production?
54
Second, what are the antece-
dents of the sorts of furnaces indicated here, in which glass production takes place in cru-
cibles, rather than in the large tanks known from Late Antiquity? Glass recycling may
provide an important temporal link between the larger- and smaller-scale glass making,
with the technology of adding alkalis to recycled glasses, for example, being eventually
developed in the Islamic world into a technology of producing primary glasses using
plant ashes and silica.
We expect to address these questions in future, with a continued programme of
research. Although this was a small-scale study, in which we have not tested all of the pro-
duction parameters, the combination of methodological approaches employed here
emphasises the value of approaching these various forms of evidence from a truly inter-
disciplinary perspective, enabling us to develop –and eventually to answer –new research
questions. These will feed into our understanding of trade, daily life, production and con-
sumption in the Iberian Peninsula and beyond.
Acknowledgements
The work involved in the production of this paper has benefited from different sources of funding,
for which we are most grateful. The work concerning medieval recipes was carried out within the
framework of the project “El conocimiento científico y técnico en la Península Ibérica (siglos XIII-
XVI): producción, difusión y aplicaciones”HAR 2012-37357 (Ministerio de Economía y Competi-
tividad), directed by author 3. The experimental work was carried out within the framework of
project “Addressing the Invisible: Recycling Glass and Technological Practice in the 1
st
Millenium
AD”, British Academy Post-Doctoral Fellowship in the School of Archaeology and Ancient History,
University of Leicester, awarded to author 2.
Notes on contributors
David J. Govantes-Edwards is a professional archaeologist and student of philosophy; his research
focuses on the archaeological perspectives on technology from a theoretical point of view as well as
several aspects of the archaeology of al-Andalus. He is also directly involved in a number of research
projects, which include the Priniatikos Pyrgos Excavation Project (Crete, Greece), the Al-Andalus
Glass Project (Spain) and the Landscapes of Construction Project (Easter Island, Chile).
Chloë N. Duckworth, after being awarded her PhD (AHRC-funded) at the University of Notting-
ham, established an interdisciplinary research project and network (The al-Andalus Glass Project)
exploring glasses and glazed ceramics from the Muslim and Christian periods in the Iberian Penin-
sula. She has been involved in numerous archaeological projects in the UK, Iran, North Africa,
54
Komaroff, “Color, precious metal, and fire”, 47.
JOURNAL OF MEDIEVAL IBERIAN STUDIES 17
Greece and Spain. Most recently, she was awarded a British Academy postdoctoral research fellow-
ship, which focuses on experimental and “big data”approaches to glass recycling in the first mil-
lennium AD.
Ricardo Córdoba de la Llave, Professor at the University of Córdoba (Spain), focuses his research
on the way technical knowledge was transmitted during the Middle Ages, especially via the writing
of technical recipe books. In this regard, he has directed several international projects which include
the foremost specialists throughout Europe. He is also a central member of the al-Andalus Glass
Project.
ORCiD
Ricardo Córdoba http://orcid.org/0000-0003-0186-7290
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Bibliography of relevant technical treatises
Agricola, Georgius. De re metallica. Trans. Herbert Clark Hoover and Lou Henry Hoover.
New York: Dover Publications, 1950.
Albertus Magnus. On the Causes of the Properties of the Elements. Trans. Irven Resnick. Milwaukee:
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Alfonso X Rey de Castilla. Lapidario.
Barthélemy, Pascale. La Sedacina ou l’oeuvre au crible. L’alchimie de Guillaume Sedacer, carme
catalan de la fin du XIV
e
siècle. Paris: S.É.H.A., 2002.
Biringuccio, Vannoccio. The Pirotechnia of Vannocio Biringuccio. The Classic Sixteenth-Century
Treatise on Metals and Metallurgy. Trans. and ed. Cyril Stanley Smith and Martha Teach
Gnudi. New York: Dover Publications, 1990.
Celaya, J. [?] Manuscript H-490. l’École de Medicine, Montpellier University.
Isidore of Seville. The Etymologies. Trans. Stephen Barney and Muriel Hall. Cambridge: Cambridge
University Press, 2006.
Neri, Antonio, The Art of Glass. Ed. Christopher Merrett and Michael Cable. Sheffield: Society of
Glass Technology, 2001.
Pliny. Natural History.
Theophilus Presbyter. On Divers Arts. The Foremost Medieval Treatise on Painting, Glassmaking
and Metalwork. Translated from the Latin with Introduction and Notes by John G.
Hawthorne and Cyril Stanley Smith. New York: Dover Publications, 1979.
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