Performance and issues of consolidation polymers for applications in conservation of outdoor cultural heritage: A review

Outdoor stone cultural heritage is constantly subject to coupled temperature and moisture cycling, salt transport and crystallization, solar radiation, atmospheric chemistry, and biological activity. Under such conditions, the field performance of polymer-based consolidants and protective coatings is challenged by the joint effects of polymer chemistry, stone pore structure, and local environment. Protective results cannot be reliably inferred from laboratory testing or small-specimen accelerated protocols. This review examines three major polymer families used for large-scale outdoor stone conservation, namely (meth)acrylics, organosilicon systems (alkoxysilanes and polysiloxanes), and fluorinated polymers as examples to illustrate the key issues. Mechanisms that control durability and side effects are emphasized, including photo-oxidative aging, hydrolysis and acidification, penetration and moisture transport, residual low-molecular-weight components, plasticizer and additive, and thermo-hygro-mechanical incompatibility at treated and untreated interfaces. Evidence from laboratory studies and field observations indicates that short-term gains, such as reduced water uptake or increased contact angle, may be offset by redistribution of evaporation fronts and salt crystallization zones, leading to localized scaling, blistering or cracking risks. Additionally, interactions between consolidation polymers and the local environmental conditions are also discussed, including microbial colonization and attack. This review proposes a workflow that begins with site/climate diagnosis and long-term site-specific trials to calibrate formulation and dosage, followed by in situ evaluation across representative climates and settings. The workflow supports more defensible material selection, retreatment planning, and the future evaluation and development of candidate conservation polymers.