Kaolinite-Group Minerals in Cultural Heritage Conservation: From Traditional Poultices to Nanocomposite Consolidants (A Critical Integrative Review)

Kaolinite-group minerals and kaolin rock occupy a distinct position in cultural heritage conservation. They occur both as original constituent materials and as components of conservation treatments. This review critically synthesizes the scattered literature on their use in the conservation of cultural heritage objects. It highlights the structure–property–performance relationships, including composition, particle morphology, pore structure, water retention, sorption properties, moisture transport, and residue formation on heritage substrates. Kaolin-based poultices are useful for desalination and cleaning where pore-size distribution, drying conditions, and application parameters can be controlled. Kaolinite and metakaolinite are mainly used in consolidation, the first acting as mineral filler/scaffold and the second as a reactive pozzolanic component, while halloysite is a potential nanocontainer for controlled release. This review highlights that kaolinite-group minerals should not be treated as generic clays, inert fillers, or automatically sustainable additives and particular attention should be given to issues of reversibility, formulation design, performance testing, long-term durability, and residue assessment, as well as to emerging data on toxicity and environmental impact. Future research should prioritize comparative testing, long-term field monitoring, and predictive modelling, alongside clearer assessment of health, environmental and sustainability impacts, to support evidence-based, sustainable use of kaolinite-group minerals in cultural heritage conservation practice.