*2.1. Materials*

A 1M DAP solution was prepared by adding the appropriate amount of DAP (Fisher Scientific, Hampton, NH, USA, purity: 99 + %, used as received) to deionized (DI) water. To study the chemical reaction between the DAP solution and the pigments, seven commercial inorganic pigments purchased from Kremer Pigments Inc. were tested, including six pigments commonly used for fresco application, such as cinnabar (Kremer No.10620), lapis lazuli (Kremer No. 10562), white chalk (Kremer No. 58000), French ochre (Kremer No. 40090), burnt umber (Kremer No. 40710), raw sienna (Kremer No. 40400), and one pigment, red lead (Kremer No. 42500), frequently encountered in secco paintings.
