Roman blown glass compositions:
following supply patterns in the manufacturing chain
Bruce Velde
Directeur de Recherche Emérite CNRS
Laboratoire de Géologie, CNRS UMR 2385 – ENS (École normale supérieure) Paris (France)
Roman glass objects were produced using a two step process: production of raw glass material in the Near East and production of blown or poured objects in workshops throughout the Empire. An obvious question then concerns the distribution of the raw glass to local manufacturing sites. Was semi-finished glass traded as a bulk commodity to be stocked and redistributed on demand or was it supplied directly on demand to the workshop from the producing region? We consider chemical data indicating a probable production – transport – workshop use path. Data for individual workshops appears to indicate a continued supply of a specific raw glass composition. This suggests a commercial system of supply on demand of a given material. Overall it indicates a centralization of raw glass production that was maintained for 400 years or so. Such a system is unusual in the annals of economic history. In this instance innovation comes from a change in the organization of the production cycle, breaking it into different parts, which allows a significant change in the way things were produced. The efficiency of producing a raw, semi-finished glass distributed to different sites for final production of objects allowed a significant gain in productivity with high standards for the finished product. This innovation changed the use of glass over the Roman Empire.
Les compositions de verre soufflé romain :
des indices révélateurs de la chaîne de fabrication.
Les objets en verre romain étaient produits en deux temps : fabrication du verre brut au Proche-Orient, puis production d'objets soufflés ou moulés dans des ateliers à travers l'Empire. La mise à disposition de verre brut dans les sites de fabrication locale devient une question fondamentale. Le verre semi-fini était-il commercialisé comme une matière première en vrac, stocké et redistribué, ou était-il directement fourni à la demande d'un site de production local ?
Les données d'analyses chimiques sont utilisées comme sources pour repérer une chaîne de fabrication : « lieu de production possible – transport – atelier de transformation ».
En effet, les données réunies pour des ateliers locaux individuels indiquent la fourniture d'une qualité spécifique de verre brut. Ceci signe bien un système commercial de fourniture à la demande pour un matériau précis. De manière générale il révèle une centralisation de la fabrication de verre brut qui s'est maintenue pendant 400 ans environ, un système tout à fait inhabituel dans les annales de l'histoire économique.
Dans un tel contexte, l'innovation apparaît dans le changement d'organisation du cycle de production, le rompant en différents lieux et permettant des changements conséquents dans les manières de faire.
La production d'une matière première brute centralisée et répartie ensuite vers des centres régionaux garantissait des gains de productivité importants ainsi que des standards de qualité élevée. Les transformations vont bouleverser l'usage du verre dans tout l'Empire romain.
Introduction
Over the past decades, a large amount of information has been amassed concerning the composition of Roman glass objects and a reasonable number of studies show compositions for material coming from individual glass workshops. Early suggestions that a raw glass semi-finished product re-worked to be made into blown glass objects of a more or less common source (Velde and Sennequier, 1985) has been confirmed by more recent publications (see La Route du Verre, Maison d'Orient, Lyon, 2000). Sites probably somewhere along the Mediterranean coasts of present-day Egypt, Israel, Syria and Lebanon produced sodic glass from mineral sources of natron coming from a site near Alexandria, and using dune sands along the Levantine coast as a source of silica. The composition seems to have been first used systematically by the Hellenistic Greeks (Dussart and Velde, 1990) as ancient texts attest the use of dune sand near Sidon (Strabon quoted by Toutain, 1927, p. 127). There seems to have been continued use of the same general sources of raw materials until about the 5th century AD (Aerts, 2003; Foy et al. 2000). Exceptions are found in later Roman raw glass productions found in the area near the source of soda, Wadi Natron (Nenna, 2000 and 2005) where, by looking at the averages for the analyses one can deduce that higher amounts of soda in the raw glass materials reflect differences in the production method and lower impurities of alumina and calcium indicate different sand sources.
The raw glasses were furnished to secondary production sites over the Roman Empire and fashioned into different objects ranging from window panes, to bottles or table ware. Raw glass ingots have been found in numerous archaeological contexts (Foy et al., 2000 for example). Given the highly ordered and efficient economic system of the period, one can attempt to investigate how the raw glass, produced for the most part in the Near East, was procured by the end term production agents, the glass blowers. Did they take what was offered to them by local secondary dealers from intermediate warehousing facilities or could they place an "order" for material from a particular production site with a given composition and working properties? The following study attempts to elucidate some of the aspects of supply and use in the glass making and production chain.
Extensive work has been done in trying to establish compositional groups for Roman glass (see Foy et al. 2003 for example). The method used by these authors to establish these groupings is accomplished by multivariate analysis of the chemical data which is a very powerful tool, taking into account simultaneously all of the elements analyzed. However, the size or definition of the compositional ranges used to establish the groups is determined by the operator of the statistical analysis system and the resultant groupings are not necessarily based upon specific functional, archaeological relations which could be established by archaeologists. For example, changes in production method of raw glass over time might form several groups in the analytical analysis result or perhaps one continuous trend in compositions which will not be seen by these statistical methods. Changes in supplier over time might create different compositional groups for the same finished product. For example, an attempt to establish variations for a particular type of glass production, the Frontinian barrel bottles found in northern Europe (notably Germany and France) by Velde and Sennequier (1985), indicated shifts in composition as a function of the region where the objects were found as well as their age. However, no direct production site could be established for all of the bottles and their overall compositional variation is sufficiently large to suggest possible multiple production sites.
A basic question which can be asked is: what does composition mean for a given production or production site? We propose to use simple compositional variation diagrams for objects from several glass blowing workshop sites in order to investigate the possible relations between supply and use of raw glass in manufacturing objects at various sites in the Roman Empire. The compositions of glass fragments from the workshope are compared to variations in glass objects found at different sites in three large Gallo-Romancities in order to give a perspective on the variations that are possible for roamn glass and those seen for a specific production site. Using these local atelier variations we wish to investigate the significance of compositional variation in raw glass related to production at a specific site over the period of activity of the atelier.
