Diammonium hydrogen phosphate (DAP) for consolidation of wall paintings: A pilot study on original fragments from eight churches (IX to XV century)

Consolidation of wall paintings is a challenging task, as the available products often exhibit limitations. The use of aqueous solutions of diammonium hydrogen phosphate (DAP) has provided very encouraging results on natural stones and lime-based mortars, but its effects on pigmented plasters have not been reported yet in the literature. Therefore, the present study reports for the first time the performance of DAP on fragments of real mural paintings coming from 8 churches in northern Italy, differing in terms of creation time (IX to XV century), pigments and painting technique. Overall, more than 50 samples were treated with an aqueous solution containing 1 M DAP+ 1 mM CaCl2 and then characterized to assess the DAP-treatment effectiveness (by indentation, scotch tape test and ultrasonic test), compatibility (in terms of color change, alterations in water transport properties and composition of the new consolidating phases) and durability (by freeze-thaw cycles). The results of the study show that carbonate hydroxyapatite was formed when no dolomite was present in the substrate, while also struvite was formed in the presence of dolomite. In all cases, significant increases in mechanical parameters indicate strengthening of both the pigmented layer and the underlying plaster. Only in 7 cases out of 30 a color change exceeding the common acceptability limit was found after consolidation, which seems to have no obvious correlation with struvite formation, the nature of the pigment or the painting technique. In all cases, modest alterations in water transport properties were registered. Moreover, after 10 freeze-thaw cycles all the consolidated samples showed lower material loss than unconsolidated ones, thanks to the increase in mechanical properties experienced after consolidation. Overall, the DAP-based treatment appears as a promising method for consolidating wall paintings. To reduce chromatic alterations, the use of more diluted DAP solutions or the adoption of different applications methods could be explored in the future.