[Unveiling medieval Parisian craftsmanship]: Investigation into the fabrication process of brass mechanical pencil through microstructure and sulfide inclusions analysis

Details

Supervisors

Pascal Jacques ; Lise Saussus

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

In 2003, a brass mechanical pencil was discovered beneath the hôtel de Mongelas in Paris. This unique artifact, dated to the late 15th to early 16th century, is the oldest known mechanical pencil according to current knowledge. It is composed of five separate parts that fit together with intricate micro-mechanics. This thesis focuses on a specific piece of the pencil: a tube measuring 30 mm in length and 0.3 mm in wall thickness. The tube features a helical opening that begins at one extremity and does not reach the other. The primary research question is to understand how this object was formed using the techniques available at the time. To address this, the thesis examines the sulfur inclusions present in the brass. Unaffected by annealing, these inclusions serve as indicators of the deformation the object has undergone. By studying their morphology, it is possible to trace the history of the plastic deformation of the brass component and estimate its initial state. The inclusions were analyzed using SEM imaging of perpendicular cross-sections and high-resolution X-ray microtomography. The study of these inclusions demonstrated that the tube had been elongated, leading to a reduction in its wall thickness. The aspect ratio analysis of the inclusions provided an estimate of the initial wall thickness of the tube after casting, which was between 0.5 and 1 mm. X-ray tomography analysis also revealed grooves on the internal and external surfaces of the tube, indicating final surface polishing and the use of a mandrel during shaping. The microstructural analysis revealed equiaxed grains, indicating that one of the final steps in the deformation process was annealing. Subsequently, this research focused on producing brass with a similar composition to that of the mechanical pencil. Brass was melted by combining brass of known composition and a pure Cu ingot at 1100°C for 20 minutes. A mixture of Cu2S/Pb powders was then added to the molten alloy and cast. This process yielded brass with sulfur inclusions in the matrix, similar to the original mechanical pencil. The behavior of these sulfur inclusions during cold rolling was then observed, showing a progressive increase in aspect ratio with increasing plastic deformation. From the measured aspect ratio, an estimate of the initial thickness was calculated and compared to the actual initial thickness using the formula applied to estimate the initial thickness of the tube. Preliminary results indicate that the formula provides conclusive results only at low levels of deformation.