Dislocations in milled galena (PbS)
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9/27/04
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Dislocations in Milled Galena (PbS) C. DEEB, J. CASTAING, P. WALTER, P. PENHOUD, P. VEYSSIÈRE, and P. MARTINETTO Transmission electron microscopy (TEM) has been used to examine milled galena (PbS). The microstructure of coarse particles consists of dislocation networks created by plastic deformation under loads applied along various directions during the milling process. Processing conditions (intensity of milling, temperature) have a strong influence on dislocation arrangements. Structures introduced in PbS by processing are compared in archaeological and modern specimens to deduce aspects of powder manufacturing in ancient times. The specific nature of dislocation reactions and observed dislocation structures are treated in detail.
I. INTRODUCTION
POWDERS based on lead compounds have been used for thousands of years as eye make-up with supposed beneficial effects for various types of eye maladies. In a recent study, cosmetics from ancient Egypt were investigated by X-ray diffraction to determine their composition and to deduce the processing of the constituent materials.[1–4] Most dark cosmetics are based on a mixture of black galena (PbS) and white cerussite (PbCO3), including two other white constituents: laurionite (PbOHCl) and phosgenite (Pb2Cl2CO3).[1] Because of the small quantities of material available, synchrotron radiation was essential to allow quantitative interpretations of the X-ray diffraction patterns.[2,4] In particular, diffraction profile analysis was performed for galena to determine both the size of the coherent diffracting domains and the average lattice distortions (or dislocation densities). Galena powders have been prepared in the laboratory by hand milling with a mortar and pestle, ball milling, sieving, and annealing. Comparison of the data for laboratory specimens and for archaeological specimens sheds light on the processing of galena in ancient times.[2,4] Dislocations can be studied in detail using transmission electron microscopy (TEM). This has motivated TEM investigations into PbS particles and the dislocation arrangements induced by crushing, milling, and heating. Transmission electron microscopy provides a direct view of particles and grains as well as of dislocation structures in the volume of the foils transparent to electrons. Since TEM and X-ray diffraction profile analysis use different size scales and different experimental approaches, the agreement between their respective results for milled PbS is, not surprisingly, poor.[3] For example, dislocation densities from TEM are about 10 times smaller than those determined by X-ray diffraction.[3] The crushing of materials into small particles is obtained by the brief, random application of a load along various crysC. DEEB, J. CASTAING, and P. WALTER are with the Centre de Recherche et de Restauration des Musées de France, CNRS UMR 171, Palais du Louvre, 75001 Paris, France. Contact e-mail: [email protected] or [email protected] P. PENHOUD and P. VEYSSIÈRE are with the Lab
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