Impurity Elements in a Lead Seals and Stamps of Middle Ages and Phase Composition of Their Patina

Abstract

The paper studied the elemental composition of lead alloys of appendage stamps and seals of Middle Ages, and the influence of impurity elements in the alloy and conditions for long-term storage of monuments on the course of corrosion processes and the formation of patina phase composition. Due to the influence of salts of orthophosphoric, hydrochloric and carbonate acids dissolved in the water environment on the artifacts surface patina coating is produced formed mainly of pyromorphite or pyromorphite and lead carbonate mixture. It was found that the corrosion rate of monuments depends essentially on the total content of Zn, Cd, Sn and Fe in lead alloy, which provide the protect for the main metal due to the electrochemical oxidation. Pink and brown colour of patina of certain attractions is associated with the pink colour of pyromorphite, which it takes as a result of isomorphic substitution of a small number of Pb atoms on Fe (III) ones. A colour of patine coating also depends on the content FeO, FeOOH and PbCO₃∙Pb(OH)₂.Keywords: lead seals and stamps, patina coating, electrochemical corrosion.