Hydrophobic protection and consolidation of porous stone: Between multifunction and multilayer treatments

This study compares multifunction and multilayer conservation treatments designed to couple both hydrophobic and consolidation effects in porous stones, having in common ethyl-silicate based products. The practical difference lies in how the products are applied and where their effects concentrate. Multifunction treatments result from the application of products that co-deposit a consolidant and a hydrophobic agent in a single deep-penetrating step, whereas in multilayer treatments, a consolidant is first applied and then a hydrophobic product that acts mainly near the surface. Both strategies were investigated in two stones (limestone and sandstone) characterized by contrasting pore structures and mineralogy, using complementary assessments. The treatments met the primary objective of limiting liquid-water ingress while restoring cohesion, delivering comparable in-depth consolidation, more evident in silicate stone, and broadly similar changes in water-vapour permeability, which were governed by stones pore size. The colour change was associated with ethyl silicate, a component common to both systems. Multifunction treatments offered simplicity and stronger protection against capillary uptake, but the superficial hydrophobic effect diminished within months of outdoor exposure, and salts were retained in depth with potential for interfacial damage and stone collapse. Multilayer treatments allowed the choice of more stable hydrophobic agents, providing more durable superficial hydrophobic effect, and tended to favour salt crystallization nearer the surface, typically more visible yet easier to monitor and manage. Operationally, multilayer systems were more complex and included an interval between steps for consolidant curing; however, the lower need for re-treatments to restore the superficial hydrophobic effect may offset their apparent operational disadvantage. Overall, the results provide clear, actionable criteria for evidence-based, substrate-specific selection, allowing practitioners to balance operational constraints with technical performance.