Deeply colored and black-appearing Roman glass: a continued research

Highlights

• We present a summary of the present knowledge on black and deeply colored Roman glass.

• Over 400 black Roman glass finds were analyzed by means of SEM–EDX and LA-ICP-MS.

• The chronology and regional distribution of black Roman glass are reconstructed.

• Fe and Mn were added to available glass to obtain a macroscopically black appearance.

Abstract

In the context of archaeological and historical assessment of Roman black-appearing glass, the chemical and physical characterization of a large collection of samples originating from various areas of the Roman Empire has been gathered over the past years to (i) verify whether a minor segment of the overall Roman glass production can help in determining possible diachronic changes in Roman imperial glass production (1st century AD – 5th century AD) and (ii) reveal regional compositional differences. In this paper, the latest results on the chemical composition of an additional 44 black-appearing Roman glass samples are presented, together with general conclusions based upon the entire compositional dataset of over 400 analyzed black glass samples. The results show that the Roman black glass is obtained through several glass compositions with a specific chronological, geographical and typological distribution.

Introduction

Glassblowing, introduced halfway through the 1st century BC, contributed to turning glass from an exclusive material into an affordable common commodity. As a consequence, the glass market expanded and large amounts of glass were produced and distributed in Rome and its provinces. In this context, the diachronic investigation of black glass, a minor but empire-wide distributed product provides an interesting opportunity to explore the dynamics of the two different steps of glass production, namely glassmaking – the production of raw glass by melting the raw materials (primary production) – and glassworking – the remelting of raw glass to produce consumer goods (secondary production).

In ancient times, as nowadays, the technological level and practical conditions required to control glassmaking were significantly more complex than those needed for glassworking (Stern and Schlick-Nolte, 1994, Stern, 1999). Primary workshops needed to master the glassmaking knowledge to mix and melt vast volumes of raw materials to produce workable raw glass. The large slabs of glass made in the primary centers were broken into chunks and shipped along various trade routes to the secondary workshops (Henderson, 1985, Fontaine and Foy, 2007; Nenna, 2007) where glassworkers only needed to re-melt small amounts of the pre-fabricated raw glass to manufacture finished products.

Hence, it has been stated that during the Roman imperial period, glass objects were manufactured in secondary glass workshops all over the Empire (Stern and Schlick-Nolte, 1994, Stern, 1999) by re-melting and shaping imported raw glass. Archaeological findings identified the Syro-Palestinian coast as key area for primary glass production (Foy, 2000, Nenna, 2007, Jackson et al., 2009). The primary workshops had to be close to the supplies of raw materials such as lime-rich coastal sand and natron. The absence of reporting a separate addition of lime in Pliny the Elder's Naturalis Historia implicitly suggests the presence of lime in Roman glassmaking sand.

Hitherto, no clear evidence has yet been found to confirm the existence of primary glass production in the Western part of the Roman Empire. Nonetheless Pliny the Elder stated in his Naturalis Historia that glass was made from raw materials on the Italian Peninsula near Cumae where the Volturno river enters the Tyrrhenian Sea, on the Iberian Peninsula and in Gaul (Silvestri et al., 2006, Foster and Jackson, 2009). Although a work in progress, promising information in diagnosing sands from the Western Mediterranean suitable for glassmaking has been provided by isotopic ratio analysis (Degryse and Schneider, 2008, Degryse et al., 2010).

Some scholars have also suggested that late Roman glassworkers in the western part of the Roman Empire accumulated enough know-how to produce raw glass locally by means of imported natron (Wedepohl and Baumann, 2000, Shortland et al., 2006). At present, the main viewpoint assumes a predominance of large primary glass centers in the eastern Mediterranean and Empire-wide secondary workshops (Foy et al., 2000, Freestone et al., 2005, Nenna, 2007). However, a combined occurrence of regional primary production in the West and large scale distribution of raw glass from the South-Eastern Mediterranean must be taken into consideration (Picon and Vichy, 2003).

Black glass represents only a very small fraction of the entire Roman glass production; the interest such idiosyncratic material represents is based on its link to regional consumption and periodical fashion (Cosyns, 2011). Its changeable period- and location-specific distribution makes black glass a useful category within the archaeological assemblage of small glass finds.

Black glass is mainly encountered in two types of artifacts: vessels and jewelry. The production of vessels in black appearing glass was empire-wide and, particularly in the Western half, fashionable a first time from about 30 to 70–80 AD (Cosyns and Fontaine, 2009). The reappearance of black-appearing vessels from 170–180 to 230–250 AD was only a regional phenomenon within the North-Western provinces Gallia Belgica, Germania Inferior and Germania Superior (Cosyns and Hanut, 2005, Cosyns et al., 2006, Cosyns, 2011). A third re-emergence of black glass vessels took place in the course of the 4th–5th century AD and was limited to the South-Eastern Mediterranean (Cosyns, 2011).

Black glass jewelry was manufactured throughout the entire imperial period, but became especially common in the entire Empire from the second half of the 2nd century AD onwards (Cosyns, 2011). Glass bangles, finger-rings, beads, pendants, hair pins, and gems gradually disappeared during the 5th century AD in the Western part, where only black glass beads decorated with applied trails in opaque glass remained in use up to the mid-6th century AD (Cosyns, 2011). In the Eastern part of the Empire, instead, a continuity of a wider range of jewelry in black glass can be observed, also during the early Byzantine era (Cosyns, 2011).

In previous papers (Van der Linden et al., 2009, Cagno et al., 2013) on black appearing and deeply colored glass, the most abundant and widespread type of coloration was found to be the deep green one. Two different compositions could be distinguished in this class: a low iron group (1–2% Fe₂O₃), which is associated with the Early Roman imperial period up to about 150 AD, and a high iron group (4–10% Fe₂O₃) that is characteristic for the period after 150 AD (Van der Linden et al., 2009). Moreover, it was possible to identify the presence of vegetable ash as added flux to the early Roman low iron black glass and of the use in the Late Roman period of High Iron, Manganese, Titanium glass (HIMT) glass (Cagno et al., 2013).

The results of both papers support the hypothesis that in a first phase, the craftsmen's knowledge in the Western provinces was limited to the production of objects by re-melting imported deeply colored glass in bars or chunks, in a similar fashion as was the case for ‘naturally’ colored or decolorized glass. In a later phase, from about the mid-2nd century AD, the glassmakers' expertise improved by acquiring the independent know-how of coloring the (remelted) glass (Van der Linden et al., 2009).

In this paper, next to discussing the analysis results of an additional relevant set of 44 samples and evaluating the complete set of over 400 glass samples, we also present a reflection of the opportunities that a minor segment of the Roman glass production can provide in order to better understand the history of Roman glass consumption.