An early Byzantine alkali glazing tradition? Discussion of P. Armstrong (2020). The earliest glazed ceramics in constantinople: A regional or international phenomenon? Journal of archaeological science: Reports, 29, 102,078

The suggestion by Armstrong (2020) that the ceramics from the fifth century monastery of St. Lot, Jordan, represent evidence for an early Byzantine alkali glazing tradition is based upon a misinterpretation of an earlier study by Freestone et al. (2001). The St. Lot glazes were unintentional and formed as a result of the reaction of the kiln vapour with the clay ceramic. Evidence for an early Byzantine alkali glazing technology is called into question

Glass production in Late Antiquity and the Early Islamic period: a geochemical perspective

First millennium AD glass production was divided between a relatively small number of workshops that made raw glass and a large number of secondary workshops that fabricated vessels. Glass compositions reflect the primary glassmaking source. For most of the period, Egyptian mineral soda was fused with lime-bearing siliceous sand to produce soda-lime-silica glass. The location of the Belus glassmaking sand, which is known from the classical literature, is located on that part of the Levantine coast where iron contents are lowest. Sr-87/Sr-86 of primary glass from workshops in the Levantine region is close to that of modern seawater, and confirms the use of beach sand, which contained shell. Heavy mineral assemblages of Levantine beach sands are dominated by hornblende, hence the primary glasses are characterized by very similar trace element signatures. Glasses believed on archaeological grounds to have been made in other regions, for example in inland Egypt, may have higher Sr-87/Sr-86, reflecting terrigenous sources of lime, and have different trace element signatures. Compositional data for glasses from as far away as Britain suggest origins of the glass material in the Eastern Mediterranean. Recycling of old glass may be recognized by the presence of elevated transition metals. The use of plant ash as a flux became dominant practice in the ninth century and preliminary data for plant ash glasses from the early Islamic world indicate that primary production centres may be separated using strontium and oxygen isotopes as well as by major and trace elements

Compositional analysis of archaeological glasses

At CoDaWork'03 we presented work on the analysis of archaeological glass composi- tional data. Such data typically consist of geochemical compositions involving 10-12 variables and approximates completely compositional data if the main component, sil- ica, is included. We suggested that what has been termed `crude' principal component analysis (PCA) of standardized data often identi ed interpretable pattern in the data more readily than analyses based on log-ratio transformed data (LRA). The funda- mental problem is that, in LRA, minor oxides with high relative variation, that may not be structure carrying, can dominate an analysis and obscure pattern associated with variables present at higher absolute levels. We investigate this further using sub- compositional data relating to archaeological glasses found on Israeli sites. A simple model for glass-making is that it is based on a `recipe' consisting of two `ingredients', sand and a source of soda. Our analysis focuses on the sub-composition of components associated with the sand source. A `crude' PCA of standardized data shows two clear compositional groups that can be interpreted in terms of di erent recipes being used at di erent periods, re ected in absolute di erences in the composition. LRA analysis can be undertaken either by normalizing the data or de ning a `residual'. In either case, after some `tuning', these groups are recovered. The results from the normalized LRA are di erently interpreted as showing that the source of sand used to make the glass di ered. These results are complementary. One relates to the recipe used. The other relates to the composition (and presumed sources) of one of the ingredients. It seems to be axiomatic in some expositions of LRA that statistical analysis of compositional data should focus on relative variation via the use of ratios. Our analysis suggests that absolute di erences can also be informativeGeologische Vereinigung; Institut d’Estadística de Catalunya; International Association for Mathematical Geology; Patronat de l’Escola Politècnica Superior de la Universitat de Girona; Fundació privada: Girona, Universitat i Futur; Càtedra Lluís Santaló d’Aplicacions de la Matemàtica; Consell Social de la Universitat de Girona; Ministerio de Ciencia i Tecnología

Ancient glass: from kaleidoscope to crystal ball

Research over the last few decades has greatly enhanced our understanding of the production and distribution of glass across time and space, resulting in an almost kaleidoscopically colourful and complex picture. We now recognise several major ‘families’ of glass composition, including plant-ash based glass in Late Bronze Age Egypt and Mesopotamia, and the Islamic World; mineral natron glass in the Greek, Roman and Byzantine Empires; mineral-based lead- and lead–barium glass in Han period China and medieval Europe; and wood-ash and ash-lime glass in medieval Europe. Other glass groups include a peculiar granite-based glass in medieval Nigeria, and probably mineral-based glass in Bronze Age southern Europe. However, despite two centuries of research, we know very little about the actual production locations and technologies for most of these glass groups, and how and where glass making was invented. The early literature reflects the comparatively limited number of individuals and research groups working on glass; only recently there is a significant broadening of the research community and expansion and refinement of the data base. This enables us now to take stock of our current understanding and identify major lacunae and areas where additional work may make the most significant contributions to our understanding of the complex picture. Hopefully this will help moving from the traditional descriptive and often fragmented opportunistic data-gathering phase (asking ‘what’, ‘where’ and ‘when’) to a more interpretative period looking with fresh eyes at the ‘why’ and ‘how’ of compositional and technical developments. This opening of the research field includes addressing the relationship of the different glass industries to the societies that used glass, and how they organised its production and distribution. A major overarching issue remains the question of the initial invention of glass, and how the idea as well as the material itself spread. Major debates should ask whether there were multiple inventions of glass making; how best to identify and interpret long-distance trade; how to ensure data compatibility and quality; and how to integrate different types of data, from archaeology through craftsmanship and typology to chemistry and optical properties

Using handheld pXRF to study medieval stained glass: A methodology using trace elements

The surfaces of 30 pieces of glass from panel 3b of the Great East Window of York Minster (1405-1408 CE) were analyzed by handheld portable X-ray fluorescence (pXRF) and small samples from the same pieces were analyzed by electron microprobe (EPMA). Comparison of the two methods reveals significant divergences which are not systematic, particularly for elements lighter than Ti. Rather than a problem with pXRF calibration or correction software, the non-systematic error is attributable to the presence of a thin surface layer of weathered glass. Analysis of the depths of X-ray generation indicate that virtually all X-rays characteristic of Ca and K are generated within the top 50 µm of the glass. However, for heavier elements such as Rb, Sr and Zr, most emitted X-rays are generated below 100 µm. Using pXRF data for the heavier elements, it is possible to replicate the compositional groupings identified by quantitative EPMA. White glass in the window is likely to have originated in England, while colored glasses were probably obtained from the Continent. The alkali contents of the green and yellow glasses appear to have been manipulated to generate their colors. Glass which is medieval in technology but not original to the panel was identified. In particular, zirconium proved a useful indicator of glassmaking regions, and rubidium and strontium were more sensitive to differences between batches, which has interesting implications for future work

Cross-craft interactions between metal and glass working: slag additions to early Anglo-Saxon red glass

Opaque red glass has been extensively studied over the years, but its compositional complexity and variability means that the way in which it was manufactured is still not fully understood. Previous studies have suggested the use of metallurgical by-products in its manufacture, but until now the evidence has been limited. SEM-EDS analysis of glass beads from the early Anglo-Saxon cemetery complex at Eriswell, southeast England, has provided further insights into the production and technology of opaque red glass, which could only have been possible through invasive sampling. The matrix of the red glasses contains angular particles of slag, the main phases of which typically correspond to either fayalite (Fe2SiO4) or kirschsteinite (CaFeSiO4), orthosilicate (olivine-type) minerals characteristic of some copper- and iron-smelting slags. This material appears to have been added in part as a reducing agent, to promote the precipitation of sub-micrometer particles of the colorant phase, copper metal. Its use represents a sophisticated, if empirical, understanding of materials and can only have resulted through deliberate experimentation with metallurgical by-products by early glass workers. Slag also seems to have been added as a source of iron to colour ‘black’ glass. The compositions of the opaque red glasses appear to be strongly paralleled by Merovingian beads from northern Europe and Anglo-Saxon beads from elsewhere in England, suggesting that this technology is likely to have been quite widespread

Compositional identification of 6th c. AD glass from the Lower Danube

A group of finds (vessels, raw glass chunks, window panes) from three sites in present-day Bulgaria was selected as representative of the circulation and usage of glass in the Lower Danube region during the 6th c. AD. In total, 79 samples were analysed by EPMA and/or LA-ICP-MS techniques. The data quality was assessed for each analytical run according to the measurement of reference glasses and to pairs of results obtained from representative samples of archaeological glass analysed by both techniques. Combining EPMA and LA-ICP-MS data allowed a sufficiently consistent and unified set of primary results to be formed. As already suggested in an earlier preliminary paper, only a single glass composition was found to dominate the 6th c. contexts in the region. The current study recognises this 6th c. glass from the Lower Danube as identical with the so called 'Serie 2.1.' defined by D. Foy and co-workers (2003) in various assemblages in Southern France and North Africa. The major, minor and trace oxide evidence presented here indicates that this is a distinct primary glass composition, with an iron-rich sub-group tentatively differentiated within the main group. Accordingly, an attempt is made to situate it relative to the other main primary compositions in the region. The proposed interpretation is that the 6th c. glass should not be linked to the HIMT glass despite the nominal similarity between them due to their elevated iron oxide, manganese, and titania concentrations. Instead, a possible link between the geochemical characteristics of the 6th c. glass and an earlier group of manganese decolourised glass, equivalent to 'Serie 3.2.' outlined by D. Foy and co-workers (2003) is suggested. This may imply the use of sand from a broadly identical geological area, hence it is possible that both the 6th c. glass and the manganese decolourised composition are likely to share a common origin

Effects of alloy composition in traditional Japanese shakudo patination

Japanese craftspeople have dominated the art of patinating copper-alloys since the 15th century, using precise alloy compositions and complicated patination processes in different hot solutions to create a variety of colours on swords fittings such as tsuba. While this complex tradition is increasingly popular in the East, the reasons behind the choices made by craftspeople in the selection of the components of the alloys and are still not fully understood. This paper investigates the effect of different alloying elements (tin, gold, and silver) on the resulting patina. Experimental results are compared with optical and compositional analyses on historical Japanese artefacts, confirming the effects of the different alloying elements on the patina characteristics and colour. The absence of tin and the presence of gold limit the growth of an oxide layer and promote the formation of a thin patina characterised by a smooth appearance without visible grains. Therefore, a limited thickness of the patinas is a key aspect for the production of the desired colour and appearance of the patinas. The first colorimetric analysis on historical Japanese artefacts demonstrates the influence of gold, silver and tin in the final patina colour, validating the observations in the experimental replicas

The spectrometric analysis of Iron Age glass beads from Novo Mesto, Slovenia

This paper presents the results of spectrometric analysis of Iron Age glass from Novo Mesto, Lower Carniola, Slovenia. Several different glass types were detected in the assemblage. The results indicate that raw glass was imported to Novo Mesto from eastern Mediterranean centres and corroborate the existence of long-distance trade during the first millennium BCE

The introduction of celadon production in North China: Technological characteristics and diversity of the earliest wares

Celadon, technically a stoneware with a lime-rich glaze, had been produced in South China for more than two millennia before it was first made in the North in the second half of the sixth century. It appears to have been an immediate precursor to white porcelain, which was first produced by northern kilns. The compositions and microstructures of early northern celadons from kilns, residential sites and tombs in Shandong, Hebei and Henan provinces, and dated 550s-618 CE, have been determined by SEM-EDS. The majority of the vessels were made using a low-iron kaolinitic clay, with high alumina (20–29%), as anticipated for northern clays. A small number of celadon vessels from a kiln at Caocun, which produced mainly lead-glazed wares, have lower alumina contents and appear to have originated in the South. It seems possible that these imported vessels were being used by the potters as models on which Caocun wares were based. Consistent differences in major element composition are observed between the products of kilns at Anyang, Xing, Luoyang and Zhaili. Unlike southern celadon glazes, which were prepared as two-component mixtures of vegetal ash and body clay, the northern celadon glazes are three-component, and typically contained an additional siliceous component, probably loess. An exception is the glazes of the Xing celadons, which present no evidence for loess but which are rich in Na2O. The source of the soda is unclear, common salt and albitic feldspar are discussed as possibilities. Based upon micromorphological characteristics such as the relative size and abundance of remnant quartz and the extent of observable mullite, as well as the position of the glazes in the CaO-Al2O3-SiO2 phase diagram, the Xing bodies are more mature and they appear to have been fired to higher temperatures than the products of other kilns. These results suggest that celadon technology was not directly transferred to the North from the South, but that the northern potters adopted their own strategies to make high-fired glazes. Furthermore, each kiln appears to have had its own preferred recipe, to suit the available raw materials. The products of Xing kiln were exceptional and it appears that here the trajectory towards white porcelain was already apparent, perhaps reflecting the creativity of the Xing potters who were among the first to make a successful white porcelain