Investigation into potential clayey raw material used for Iron Age and Hellenistic pottery production on the island of Hvar and neighboring mainland (Eastern Adriatic, Croatia)

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During the last millennium BCE, the island of Hvar in the region of Central Dalmatia, Croatia, was inhabited by a native population that had lived there since Neolithic times and Greeks whose settlement began in the 4th century BCE. While there are multiple types of informational sources about the two communities, one of the most important resources is their ceramic material culture. This study evaluates the technological aptitude of ceramic artisans predating and postdating Greek colonization by analyzing the choices of plausible raw material. Clay-rich deposits are not common in Central Dalmatia, as it is a karstic landscape dominated by Jurassic and Cretaceous limestone. Geological prospection of the island identified three groups of potential raw material: Eocene flysch, terra rossa soil, and discrete clay layers (DCL). These groups are composed mainly of carbonate minerals with a variable amount of clay minerals, quartz, and feldspars. A multifaceted approach was undertaken to determine which of these groups was optimal for ceramic production and which was also likely used locally. This approach included: granulometric analysis, scanning electron microscopy and electron dispersive spectroscopy, X-ray powder diffraction, X-ray fluorescence, laser ablation inductively coupled mass spectrometry, geostatistical analysis, and stable isotope analysis. Through this approach, it was resolved that the discrete clay layers were most suitable for ceramic manufacture and were likely employed in local production. They had smaller median grain sizes (~6 μm) with up to 76% clay minerals and up to 60% carbonate minerals, limiting the need for purification techniques. The clay mineralogy of the discrete clay layers consisted predominantly of illite with minor amounts of illite-smectite and kaolinite. They serve to increase the workability of the material while also maintaining the structural integrity of the resultant vessel. Principal Component Analysis (PCA) of raw material and selected Iron Age and Hellenistic potsherds indicated the discrete clay layers could have been used in local manufacturing. There is a paucity of research of clay material (DCL) found as karstic infill. X-ray powder diffraction identified minute traces of rutile in these deposits, a primarily metamorphic mineral not found in other analyzed sediments. This suggested that the discrete clay layers formed from the weathering of nearby metamorphic complexes, likely in the Alps or the Dinarides. The weathered debris underwent aeolian transport to the Adriatic Carbonate Platform (AdCP). Stable isotopic composition of calcite in DCL confirm it was formed in a terrigenous environment created due to the emergence of the platform following the Tertiary global cooling. In further support of a foreign input into the DCL, both authigenic and detrital illite is present. In order to have large amounts of authigenic illite, there must had been a high concentration of K-feldspar. Its dissolution gave rise to K-rich fluids parental to authigenic phyllosilicates. The local geology does not contain large abundances of feldspars which corroborates its foreign provenance. Archaeologically, this study concluded that there were local sources on the island of Hvar and Split basin that could have been used in ceramic production and that were also likely utilized by Iron Age indigenous community as well as the Greek settlers. It is further in line with the idea of ceramic production on Central Dalmatian’s mainland during the Hellenistic period, which has been under debate.

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Clay, Archaeometry, Mineralogy, Clay Mineralogy