Decorated Corams of Palazzo Chigi: Characterisation and Preservation Treatment

Collagen based materials derived from animal skin like goat, sheep and others are frequently employed in the manufacturing of cultural heritage artefact. Leather, parchment or alum-tawed skin have been used through the centuries to produce artefacts for the everyday use, such as clothes and shoes, but also to produce artistic items, writing supports and books. The peculiar physical properties of collagen-based materials are mainly due to the fibrillary structure of the molecule itself, its orientation in fibrils and bundles of fibrils to compose the fibre, resulting in high strength and durability. For alum-tawed skin and leather, the tanning step corresponds to the process, during the manufacturing route, that converts the collagen of the raw skin into a stable and undecomposed material. Unlike parchment, in fact, the alumtawed skin undergoes an alum tanning process, while for leather two main types of tanning can be performed, namely vegetable and mineral (mainly chrome based) tanning. Through the tanning, the raw skin undergoes a series of preparatory steps in order to eliminate the outer layer of the epidermis, containing the animal hair, and the bottom one, i.e. the flesh, leaving behind only the middle layer of the whole skin, called corium that, accordingly, can be tanned.