Dual Functionalities of Few-Layered Boron Nitrides in the Design and Implementation of Ca(OH)2 Nanomaterials toward an Efficient Wall Painting Fireproofing and Consolidation

Preserving ancient wall paintings from damage has become a challenge over the years. Nanosized calcium hydroxide (Ca­(OH)2) has been identified as a promising material to preserve wall paintings. However, the synthesis of nanosized Ca­(OH)2 is extremely difficult. Here, we demonstrate a breakthrough in wall painting protection enabled by boron nitride nanosheets (BNNSs) through strategic synthesis Ca­(OH)2-BNNS nanohybrids using an aqueous method. The BNNS have two significant functionalities in the design and implementation of the Ca­(OH)2 nanomaterials. First, the introduction of BNNS results in the successful synthesis of uniform and nanosized Ca­(OH)2 (∼80 nm) in the nanohybrids, which can be attributed to the supersaturation-induced “etching–stripping” mechanism. More interestingly and importantly, a unique gradient penetration structure is strategically formed when applying Ca­(OH)2-BNNS hybrids on the wall paintings, i.e., the BNNS-rich layer will be at the surface of wall painting, whereas Ca­(OH)2 nanomaterials prefer to penetrate deep in to the wall paintings. This gradient structure will allow the BNNS-rich layer to protect the wall paintings from fire, which is the first report to date among the protection materials for wall paintings; at the same time, nanosized Ca­(OH)2 shows superior wall painting consolidation strength compared to commercial Ca­(OH)2 material. These results endow new applications of the newly emerging two-dimensional nanomaterials for protecting cultural heritage.