But aren't all soaps metal soaps? A review of applications, physico-chemical properties of metal soaps and their occurrence in cultural heritage studies

Metal soaps, the organic salts resulting from the interaction of fatty acids and metal cations, arouse interest in the scientific field because of their versatility in a great range of chemical applications as well as because of the mechanism of their formation during degradation processes. This article presents a review of the synthetic pathways used to produce metal soaps, their relevant physico-chemical properties, and how these reflect in their applications. Common industrial uses of metal soaps are reported, with a particular focus on those applications, such as cosmetics, paints, and coatings, that have an impact on the cultural heritage field. In addition, the occurrence of metal soaps in cultural heritage studies is presented, ranging from archaeological and ethnographic artefacts to fine art objects, and discussed per class of materials. An overview of the presence or absence of metal soaps in historical artefacts due to the interaction of metal parts or mineral pigments with fatty acids is given herein. This collection shows a variety of situations in which metal soaps-particularly lead, zinc and copper soaps-can form on composite objects made of different materials such as wood, leather and fatty-acid-containing materials (e.g., waxes), in the presence of metal, metal alloys or pigments.