Painting with recycled materials: On the morphology of calcite pseudomorphs as evidence of the use of wood ash residues in Baroque paintings

Abstract

The nature of ground preparations is of critical interest to those engaging in the study of historical painting techniques, as certain materials can be identified with specific regions and school of painting. This is the case of a particular ash-based, calcite-rich material obtained as a byproduct of lye production, recently identified for the first time in ground preparations by means of chemical analysis, and which is considered specific to Baroque artists of Spanish school. Because of limitations in the size of the samples that can be removed from works of art, and because of the intrinsic variability of ash composition, chemical analysis alone may not be representative of the whole ash-containing layer, thus limiting the identification of this material. By comparing the morphology, texture and composition of calcite pseudomorphs in laboratory ash to the ground preparations in three Baroque paintings, we provide additional, unequivocal tools to identify calcite particles from ashes in paint cross sections. The results demonstrate that the chemical composition of the ash can vary, but that the morphology and size of the calcite pseudomorph crystals abundantly present in the recycled ash applied to the canvas supports are consistent and extremely characteristic. The unique polygonal shapes and skeletal morphology of the pseudomorphs and their abundance make them ideal markers to recognize ash in paintings' ground layers, even when very limited amounts of sample are available. The study shows also that the practice of using recycled ash in the preparation of ground layers occurred outside Spain, by artists with direct or indirect Spanish lineage.