Lead-Glazed Ceramic Fragments: Intentional Glazing or Metallurgical Accident?

The potters kiln is situated in the Lühlingsbach valley 3 km northeast of Brilon-Alme and 3 km west of Bad Wünnenberg-Bleiwäsche (Fig. 1). Geographically, the site belongs to the eastern Sauerland region in North Rhine-Westphalia, Germany. Geologically, the region is part of the Rhenish Massif (Rheinisches Schiefergebirge), which belongs to the Rhenohercynian Zone of the Variscan belt in Central Europe. This fold-and-thrust belt is composed of Paleozoic (Ordovician to Carboniferous) sediments and volcanic rocks which were overprinted by a very low grade metamorphism [1]. The site is situated in an approximately 17-km-long, SW–NE striking anticlinal structure, the Brilon anticline. The core of the anticline is composed of Middle to Upper Devonian biostromal and intertidal limestones with a thickness of > 1250 m [2‐4].

The carbonatic rocks of the Brilon anticline host a great number of mineral veins and metasomatic deposits, predominantly bound to NNW–SSE striking normal faults [5, 6]. In addition to barren calcite, barite, and quartz veins, dozens of Pb-Zn-Fe mineralizations are known. The main ore minerals are galena, calamine, and limonite/goethite. Field observations show that the mineralizations are undeformed and therefore of post-Variscan age [6]. K-Ar dating of illites (< 2 mm) related to the calcite veins of the Eichholz mine near Thülen suggests a mineralization age of ~ 170 Ma (Middle Jurassic) [7], which is in good agreement with other post-Variscan mineralization ages in other parts of the Rhenish Massif [8]. Mining took place predominantly in the oxidation zone [5], so that parts of the primary sulfides (galena, sphalerite/schalenblende, pyrite) are present in weathered form (cerussite, calamine, limonite/goethite). Galena generally is silver-poor; Ag measurements of 27 galena samples from the Brilon mining district reveal low silver contents of 90 ppm at maximum [6, 9]. The silver-poor galena was an important raw material in the production of lead glazes for ceramic products and was therefore named “glaze ore” (“Glasurerz,” “Bleiglasurerz”) [10]. Coarse-grained galena occurs in calcite, barite, and quartz veins, but up to > 100 kg heavy masses also were found in clays (“Letten”) accompanying the veins and in clay fillings of dolines and other karst sediments ([5], own observations). Galena and clays can therefore occur side by side in the former Brilon mining district.

Evidence for galena exploitation occurs already by Roman time (end of the first century CE) and again, in the eleventh century CE [11, 12], which was confirmed by a radiocarbon dating of one charcoal sample [13]. A large number of pinge fields, tapping and forging slags prove extensive ore processing of lead. The produced lead was used for many purposes such as lead anchors, lead pipes, weights, and other daily use. The deposits, the so-called lead glaze veins, were excellently suited for use as a glaze component for ceramic products. The lead in glazes acts as both a network former and a network converter, so lead glazes do not require a complicated recipe to work. The glazing temperatures can also be kept at a very low level.

The fragments are from an archaeological site in the Lühlingsbachtal, which is located approximately 10 km from Brilon. Archaeological site and excavation are described in detail by Essling-Wintzer et al. 2016. The Lühlingsbachtal is surrounded by the mountain range of the Buchholz, a stream runs through the valley, and the slopes are arboreous. There is an extensive field of Pingen [14]. Local amateur historians discovered a vast number of ceramic findings, and likewise, misfires were identified. These suggested that not only ores were smelted at the site (as evidenced by the discovery of slag with adhering furnace walls), but also pottery was produced. An excavation during autumn/winter 2014 was to provide clarification about the condition of the soil monument, but also to localize potential pottery production. During this campaign, remains of a furnace were found. The kiln dome may already have been dismantled after the last fire for opening to remove the content and subsequently weathered. The furnace plate, on the other hand, was torn away by plowing. The lower parts of the furnace, the working pit and the firing chamber, were deeply carved out of the bedrock, which is why they had not been destroyed by plowing and could be excavated. The kiln was a horizontal cross-droughted two-chambered (fireplace and firing chamber) kiln, as supposed from field evidence. The combustion and firing chambers were separated from each other by a clay construction to transfer the hot air more homogeneously into the combustion chamber. The kiln was oriented in the field so that natural wind could support the fire. An important finding in the excavation was a waste dump next to the kiln with lots of clay fragments. These included rim sherds, body sherds, bottom sherds from different kinds of ceramics, but also miniature vessels and some round-bottomed jars and bowls. The sherds are of different thickness and from oxidic firing. A large number, especially the miniature vessels and bowls, have remnants of glazes on the outer and/or the inner side, which was suspected to be lead glazing. The color of the glazes varies from light yellowish green to red–brown. Based on the thin-walled low-fired and unglazed sherds from round-bottomed jars, which remained in the furnace from the latest furnace cycle, it can be determined when the kiln was last in operation. Their typology, especially the protruding rims with grooves and the rounded rim end, indicates the late twelfth/early thirteenth century [13]. This allows the conclusion that as early as the thirteenth century lead-glazed earthenware was produced in the region around Brilon-Alme. Ore mining (lead ore, calamine) was active until the 1880s [5], and calcite deep mining lasted until the end of the twentieth century.

The 13 fragments from the waste dump were provided by the Landschaftsverband Westfalen-Lippe (LWL). Eight of them (Figs. 2 and 3; Table 1) were subject to material investigation. Four unglazed sherds and a miniature vessel were excluded; the latter because it was a distinctly different type and pottery and appeared too precious to be subjected to destructive analysis. The examined fragments consist of coarsely fired clay and glaze attachments. Three samples (A45170048_4; _8b; _8c) were archaeologically identified as crucible rim fragments. One is described as a ceramic wall fragment (A45170048_8a) and two (A45170048_11b; _11d) as furnace wall fragments. Two samples are lumps of undefined shape (A45170048_11e; _11f), which cannot be assigned to a ceramic typology or function. It is obvious that they are all clearly different from the thin-walled, round-bottomed vessels from the kiln, and the archaeological address as crucibles or furnace walls suggests a metallurgical context. The glaze varies greatly in extent and coloration and is found on both exterior and interior surfaces. Some are smoothly coated; others adhere as silvery-gray drips.

For comparison with local ore, a sample of post-Variscan galena (AKZ 4518, 52:007) from a medieval galena mine [15] in the Buchholz Forest 1.5 km southwest of Bad Wünnenberg-Bleiwäsche (UTM 478,244/5701618), less than 2 km away from the archaeological site, was analyzed. The archaeological site was recovered by LWL Archaeology for Westphalia in 2017 from an area that was not remolded by mining operations.