Lustering, a technique which involved the application of coloured translucent glazes over gilding and silvering was widely used to enrich and decorate altarpieces in the Baroque period. The decorations consist of a micrometric multilayered structure including several color glazes, metallic leaves and bole applied over a plaster ground. The collection of Baroque altarpieces, the oldest dating 1671 and the newest 1775 from the cathedral of Tortosa (Catalonia), is a perfect case of study of the materials used and the techniques employed throughout the period. Further information is obtained from the analysis of the reaction and aging compounds resulting from the interaction among the compounds present in the layers and between the different layers. A combination of sensitive analytical techniques, Synchrotron based µ-XRD and µ-IR and SEM-EDS which enabled the luster samples to undergo analysis without altering their original layered microstructure were selected. The nature of the compounds used to produce the yellow, green, red and blue glazes (pigments, pigment lakes and resins), metallic leaves and boles is presented. Relevant information from historical and conservation viewpoints about the origin and nature of the materials used, the making of the lusters and their reactivity and aging are also discussed.

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