Adhesives applied to granite cultural heritage: Effectiveness, harmful effects and reversibility

Abstract The use of adhesives to repair stone is a common conservation and restoration practice. Thermoplastic (acrylic) and thermostable (epoxy, polyester, etc) adhesives are often combined to take advantage of the benefits of both types. An intervention layer in the form of an acrylic adhesive is typically previously applied to enhance the reversibility of the structural adhesive used. However, research into the mechanical strength of the bonded stone and into the harmful effects of different adhesives on stone is scarce and is even non-existent for granites. We evaluate the effectiveness, harmful effects and reversibility of the structural adhesives Araldite AY 103-1 (epoxy resin) and Fluoline A (acrylic resin) applied with and without a previous intervention layer of Paraloid B-72, and with and without the addition of micronized silica to the structural adhesives (a common practice among restorers to change the mechanical properties of the adhesives and enhance the reversibility). We used a multi-analytical approach based on assessing shear strength of adhesive bond, water absorption by capillarity, water vapour permeability, colour, stereomicroscopy and scanning electron microscopy. Araldite with a Paraloid intervention layer, both with and without silica, proved to have the best adhesive capacity. However, failure tended to occur in the rock rather that at the join. The Paraloid B-72 intervention layer, while it enhanced reversibility, also acted as a barrier to water transport. The micronized silica enhanced both the hydric properties of the stone and the reversibility of adhesive application. Fluoline had the best reversibility profile. These findings should prove useful in informing granite-built cultural heritage interventions.