A 10th Century Rotary Mortar Mixing Device - Renovatio...
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A 10th Century Rotary Mortar Mixing Device - Renovatio imperii on the Basler Muensterhuegel? Dr. Sophie Stelzle-Hueglin (Archaeologische Bodenforschung Basel-Stadt, CH) In 2004 the Archaeologische Bodenforschung Basel-Stadt had to undertake a rescue excavation on larger scale in the northern less well known part of the Muensterhuegel in Basel. This northern tip of the plateau is called the Martinskirchsporn after the parish church dedicated to St. Martin and probably dating back to the Early Middle Ages. The southern part of the elevation belonged to the bishop and his church, the Muenster. The archaeological research provides now new facts on the utilisation of the northern part of the Muensterhuegel and gives reason to discuss ownership and building activities of other authorities than the church. The excavation tackled in eight months some 450 m 2 and because the layers comprised in average 3 m thickness this meant about 1300 m 3 of historical strata had to be removed, documented and understood. The time span covers more than 3000 years: in the late bronze age there must have been a fortified settlement on the northern end of the hill as a ditch some 2,50 m deep and 9 m wide shows. Then after a time gap the Muensterhuegel is in the late Celtic period sought again as a dwelling place; at the very south end of the plateau a fortification is erected. In late Roman times a fortification – supposedly a stone wall - surrounds the whole plateau. In Roman times there are several stone buildings on the hill; their layout and function still being subject to discussion. By the end of the Roman Empire layers of so called “dark earth“ start to accumulate. Reasons for it being the surrounding fortification in combination with a blocked drainage system and debris from mostly agricultural usage. In the upper part of this “dark earth“ a round disc of hardened mortar was found. It was about 2.5 m in diametre and had a central posthole. On the surface there were four concentric grooves. The western part of the round plate showed traces of several small sticks; very often two close to each other. They divided this half of the circle into five sectors. One pair of sticks had not left round traces but elongated oval holes. The mortar disc sat in a pit, which was at least 30 cm deep. The structure represents the left overs of a special mortar mixing device as it was common during the Middle Ages on large scale building sites. There are quite a few paralleles mostly in the South of Germany and in Northern Switzerland which have been studied with respect to construction and mecanical function of such a rotary mortar mixing device. First a round pit is dug and a central post is errected, which is hold in place by a large four post construction. The post in the middle serves a spindel for a turnable rake with two or four arms stretching out beyond the pit. They can be turned by man or animal power. What is special in this construction are the sticks at the bottom of the pit. They are arranged in order to let the teeth of the rakes pass between them. This so called "Zwangsmischprinzip" or "forced mixing principle" optimizes the mixing process considerably. There are mounted dents only in one half of the pit to make it easier to add the components and to take out the mortar at the end. This device was probably turned clockwise, which would explain why the first two fixed sticks in this direction have been hit by material and dents of the rake first. The installation of such a complex device has to be seen in connection with a major building project in the vicinity. It was used to produce larger amounts of mortar especially to plaster the walls or to pour a floor from mortar. Micromorphological analysis by Dr. Philippe Rentzel and his team at the IPNA (University Basle) has shown, that there are traces of several mixing processes. The nearest building from stone belonging to the same period is the right angle of a wall which sits on a late Roman foundation, used again in the High Middle Ages. There are two levels in the dark earth, which can be defined as walking surfaces, the upper of the two might be connected to the mortar mixing device and the re-errection of Roman architecture. Because the dating of "dark earth"-strata by finds has proved to be rather difficult, it was tried to get an independent date. Dr. E. Bonani from ETH Zuerich tried to get 14 C-AMS-Dating for the mortar of the mixing plate itself and for charcoal embedded in the mortar. In the end it was not possible to date the mortar, but he succeeded to date several charcoal pieces and could calculate a weighted average which gives a calibrated radiocarbon-age between 936-1018 AD (with a probability of 71.8 %). So the mortar mixing device and with it a major building in stone was most probably errected in the 10th or beginning of the 11th century AD. About the layout of the stone building - be it in Roman or Medieval times - we know very little. It was
Transcript of A 10th Century Rotary Mortar Mixing Device - Renovatio...
A 10th Century Rotary Mortar Mixing Device - Renovatio imperii on the Basler Muensterhuegel?
Dr. Sophie Stelzle-Hueglin (Archaeologische Bodenforschung Basel-Stadt, CH)
In 2004 the Archaeologische Bodenforschung Basel-Stadt had to undertake a rescue excavation on larger scale in the northern less well known part of the Muensterhuegel in Basel. This northern tip of the plateau is called the Martinskirchsporn after the parish church dedicated to St. Martin and probably dating back to the Early Middle Ages. The southern part of the elevation belonged to the bishop and his church, the Muenster. The archaeological research provides now new facts on the utilisation of the northern part of the Muensterhuegel and gives reason to discuss ownership and building activities of other authorities than the church. The excavation tackled in eight months some 450 m
2 and because the layers comprised in average 3 m
thickness this meant about 1300 m3 of historical strata had to be removed, documented and understood. The
time span covers more than 3000 years: in the late bronze age there must have been a fortified settlement on the northern end of the hill as a ditch some 2,50 m deep and 9 m wide shows. Then after a time gap the Muensterhuegel is in the late Celtic period sought again as a dwelling place; at the very south end of the plateau a fortification is erected. In late Roman times a fortification – supposedly a stone wall - surrounds the whole plateau. In Roman times there are several stone buildings on the hill; their layout and function still being subject to discussion. By the end of the Roman Empire layers of so called “dark earth“ start to accumulate. Reasons for it being the surrounding fortification in combination with a blocked drainage system and debris from mostly agricultural usage. In the upper part of this “dark earth“ a round disc of hardened mortar was found. It was about 2.5 m in diametre and had a central posthole. On the surface there were four concentric grooves. The western part of the round plate showed traces of several small sticks; very often two close to each other. They divided this half of the circle into five sectors. One pair of sticks had not left round traces but elongated oval holes. The mortar disc sat in a pit, which was at least 30 cm deep. The structure represents the left overs of a special mortar mixing device as it was common during the Middle Ages on large scale building sites. There are quite a few paralleles mostly in the South of Germany and in Northern Switzerland which have been studied with respect to construction and mecanical function of such a rotary mortar mixing device. First a round pit is dug and a central post is errected, which is hold in place by a large four post construction. The post in the middle serves a spindel for a turnable rake with two or four arms stretching out beyond the pit. They can be turned by man or animal power. What is special in this construction are the sticks at the bottom of the pit. They are arranged in order to let the teeth of the rakes pass between them. This so called "Zwangsmischprinzip" or "forced mixing principle" optimizes the mixing process considerably. There are mounted dents only in one half of the pit to make it easier to add the components and to take out the mortar at the end. This device was probably turned clockwise, which would explain why the first two fixed sticks in this direction have been hit by material and dents of the rake first. The installation of such a complex device has to be seen in connection with a major building project in the vicinity. It was used to produce larger amounts of mortar especially to plaster the walls or to pour a floor from mortar. Micromorphological analysis by Dr. Philippe Rentzel and his team at the IPNA (University Basle) has shown, that there are traces of several mixing processes. The nearest building from stone belonging to the same period is the right angle of a wall which sits on a late Roman foundation, used again in the High Middle Ages. There are two levels in the dark earth, which can be defined as walking surfaces, the upper of the two might be connected to the mortar mixing device and the re-errection of Roman architecture. Because the dating of "dark earth"-strata by finds has proved to be rather difficult, it was tried to get an independent date. Dr. E. Bonani from ETH Zuerich tried to get
14C-AMS-Dating for the mortar of the mixing plate itself and for charcoal
embedded in the mortar. In the end it was not possible to date the mortar, but he succeeded to date several charcoal pieces and could calculate a weighted average which gives a calibrated radiocarbon-age between 936-1018 AD (with a probability of 71.8 %). So the mortar mixing device and with it a major building in stone was most probably errected in the 10th or beginning of the 11th century AD. About the layout of the stone building - be it in Roman or Medieval times - we know very little. It was
demolished in the 12th century, which is proved by fragments of pottery and stove tiles in the debris dating to that time. The situation and orientation of the building make clear that the Martinsgasse does not follow a Roman road, but goes back to a Late Medieval restructuring process. A find from the upper dark earth layers is this beautiful piece of Ottonian metalwork (fig. 3). It is a gilded bronze disc with central opening and pelta-shaped ornament with blue and red enamelling. As it has no traces of a fastening device on the backside, it was not a brooch. What it could have been is part of a chest for reliquies or of a small alterpiece, for example the "Adelhauser Tragaltar", which has survived in the Augustinermuseum in Freiburg im Breisgau (Germany). Who could have stood behind this extraordinary object or the rebuilding of Roman architecture in the 10th century? The one first coming to mind - the bishop of Basle - actually has his cathedral and palace some hundred meters to the south separated from this part of the Muensterhuegel by a ditch. Has the Muensterhuegel of Basle for two centuries or even longer been divided between the bishop and the king? By the way: a similar situation as it used to be on Ile de France. Do the buildings in the Northern part belong to a palatial complex? Was it the king, who ordered in the 10th century to rebuild Roman architecture in a gesture of "renovatio imperii"? The historical sources have to be reread under that light. Bibliography Burnell 1998 - Simon Burnell, Die reformierte Kirche von Sissach BL. Mittelalterliche Kirchenbauten und
merowingerzeitliche „Stiftergräber“ (Liestal 1998) esp. 36-47. Gutscher 1981 - Daniel Gutscher, Mechanische Mörtelmischer. Ein Beitrag zur karolingischen und
ottonischen Bautechnologie. Zeitschrift für Schweizerische Archaeologie und Kunstgeschichte 38, 1981, 178-188.
Hagendorn/Stegmueller/Stelzle-Hueglin 2006 - Andrea Hagendorn, Christian Stegmueller und Sophie Stelzle-Hueglin, Von Befestigungen und Grossbaustellen. Erste Ergebnisse der Ausgrabung Martinsgasse 6+8 (2004/1). In: Jahresbericht der Archaeologischen Bodenforschung Basel Stadt 2004 (Basel 2006) 91-113, esp. 106-108.
Schmidt-Thomé 2007 - Peter Schmidt-Thomé, Ausgrabung im ehemaligen Moenchsfriedhof des Klosters Reichenau-Mittelzell, Kreis Konstanz. In: Ausgrabungen in Baden-Wuerttemberg 2006 (Stuttgart 2007) 227-229.
Fig. 1: Excavation site Martinsgasse 6+8 (2004/1) on the Martinskirchsporn, the northern part of the Muesterhuegel, Basle, CH. Looking North, in the background the church St. Martin and the Rhine (Photo:
Christian Stegmueller).
Fig. 2: The remnants of the 10th century mortar mixing device embedded in layers of "dark earth"; diametre about 2.5 m (Photo: Christian Stegmueller).
Fig. 3: The gilded bronze disc with central opening and pelta-shaped ornament with blue and red enamelling; diametre 24 mm (Photo: Philippe Saurbeck).
Fig. 4: Plan of the Medieval and Modern structures on the excavation site with the Late Roman walls (Graphics: Sophie Stelzle-Hueglin and Christian Stegmueller).
