Ceramic fabrics and lead glazes of late medieval redware pots in the Helsinki, Turku and Tallinn regions (ED-XRF, SEM-EDS)

Abstract

We report on ceramic and glaze chemical and technological data on lead-glazed redware pottery, sampled from late medieval, ca. 14th–17th century CE contexts at five sites across the Gulf of Finland, in modern Finland and Estonia. Redware pots first arrived in this region as Scandinavian and Continental imports, and their local manufacture was established in the 15th–16th centuries. We aimed to chemically characterize ceramic and glaze recipes, distinguish between local and imported redware pots at the sites, and to examine glaze preparation and application practices used. Altogether 34 sherds of ceramic artefacts from three hamlet sites in the Helsinki region (Gubbacka, Mankby and Mårtensby), and the towns of Turku and Tallinn were geochemically and microstructurally analysed by energy-dispersive X-ray fluorescence spectrometry (ED-XRF) and scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS). The majority of the analysed pots were made of clay originating from the Gulf of Finland region, produced at least in Turku and highly likely in Tallinn. Redware was also exchanged between these trading areas. High-lead-content (PbO < 67 wt%) glazes, sometimes opacified with tin, were applied as lead-oxide itself or as a lead-oxide-plus-sand mixture on unfired non-calcareous, iron-rich ceramic bodies. This glazing technology to produce impermeable pots was achievable at relatively low temperatures and cost, hence it was commonly adopted by ceramic producers in the North, and stayed in vogue for centuries. Redware pots from different sources appear visually and morphologically related, yet microstructural inspection reveals varied glazing technologies.