Potential and limitations of available dual function products to address new stone conservation challenges

The high susceptibility to decay of very porous stones has often justified the application of hydrophobic and/or consolidation treatments to avoid or delay the need for major interventions. This study analyses the effects of two dual-function products, an ethyl silicate with polysiloxane oligomers (CH: consolidation/hydrophobic) and an aqueous emulsion of organosiloxanes (HO: hydrophobic/ oleophobic), applied to highly porous limestone and sandstone. Their potential and limitations in addressing new conservation challenges, such as climate change, graffiti and the demand for more sustainable conservation practices are discussed to support decision-making processes in conservation practice. Changes in water/oil static contact angles, water transport properties, colour, drilling resistance and in-depth distribution were examined. The hydrophobic effects of CH and HO confer immediate protection to both stones against water-related decay processes, which will worsen in the near future. CH efficiently penetrates the pores of both stones, improving their cohesion, and is able to counteract processes of cohesion loss without the formation of superficial hard crusts. No negative side effects were detected for HO treatments, but the CH results suggest a need for careful assessment due to a latent tendency toward changes in colour and water transport properties at depth. Provided they perform adequately, these dual-function products can reduce resource usage and waste compared to single-function products, and they can also contribute to graffiti removal in conservation practices. Although the potential of this additional function warrants further research, the surface effects of CH and HO were shown to make spray paint removal easier and potentially less damaging.