Deciphering the Iron Provenance on a Medieval Building Yard: The Case of Bourges Cathedral

Abstract

This paper presents the provenance study of the iron reinforcements of Bourges Cathedral (13th c.): the links of a 100 m long iron chain, surrounding the eastern parts of the cathedral at the triforium level and 4.5 to 5 m long tie-rods consolidating the arches of the inner aisle at the same level. The analytical methodology is based on the determination of trace rare earth elements analyses by LA-ICP-MS in the slag inclusions of the artefacts and in the slag found on candidate production sites combined with statistical approaches. This chemical approach is crossed with archaeological and historical studies on the monument itself and on the production sites. Ninety-nine iron samples were analyzed on the bars and chains and 238 iron slags from 3 presumed areas of supply. For the first time, iron circulation and trade around a single building yard over a time of 30 to 40 years is studied with a precision never obtained before with historical sources. It shows that mainly four different metallurgical districts, local and more distant, supplied the building yard, mostly depending on the construction phases and also on the types of iron armatures needed.

Vice-versa: The iron trade in the western Roman Empire between Gaul and the Mediterranean

“…ore, furnace lining etc) is preserved from the slag discovered on the smelting sites to the fragment of these slag that remain entrapped under the form of inclusions in the metallic matrix of the produced metal. Coustures et al [ 9 ] were the precusors of this approach but it was significantly refined particularly by introducing statistical data treatments of the results to examine the provenance hypothses [ 9 – 13 ]. The second approach is based on the principle that the isotopic signature of some elements initially present in the ore is preserved in the metal.…”

Notre-Dame de Paris: The first iron lady? Archaeometallurgical study and dating of the Parisian cathedral iron reinforcements

“…This feature is once again very specific to the case of Notre-Dame. Most of the building yards studied so far such as Brussels, Bourges, Metz or Rouen, showed some consistency in the provenance groups [22,29,33,85]. The relatively small number of staples studied and their scattering over the whole building could be a reason for this great diversity (Fig 17 ).…”

“…Iron reinforcements therefore seem to have been used extensively in Notre-Dame's construction in its entire height. Moreover, the sampling possibilities, which gave access to entire staples that were removed and will be replaced, is a particular feature of this study compared to most previous ones made on other monuments [11,22,23].…”

“…Table 2 describes the main analytical parameters. Comparison of trace element ratios between different iron armatures as proposed by Leroy et al, Disser et al or L'He ´ritier et al [22,33,34] can also indicate their common or different area of production. Building site supply, construction phases and ancient metal trade routes can then be explored.The concentrations of 37 elements are routinely determined in SI.…”

“…So far, these series of staples are the earliest known example of iron armatures used in the initial design of a Gothic monument. They were designed probably 15 to 20 years earlier than the tie-rods installed in Soissons cathedral [48] southern transept and about 40 years before the iron reinforcements of Chartres or Bourges cathedral, which until now were considered as the first examples of a systematic use of iron in such masonries [10,15,16,22]. These staples are placed on the floor of the tribunes, on the line corresponding to the extrados of the arches between the outer and inner ambulatory, as well as on the transverse arches of the outer ambulatory.…”