Exploration of the rules for the use of organic additives in the mortar of the Forbidden city

The Forbidden City exemplifies the official architecture of the Ming (C.E. 1368–1644) and Qing (C.E. 1644–1911) dynasties in China. The buildings within the Forbidden City adhere to a specific construction system and hierarchy. This study examined the organic additives in eighty-three lime mortar samples collected from the Forbidden City. These mortar samples were categorised into two main groups: garden mortar and construction mortar. The construction mortar was further subdivided into masonry, rendering, tile roof, and pointing mortars. Classical chemical analysis was performed to identify organic residues in the samples, including glutinous rice, sugar, oil, blood, and protein. Additionally, ovalbumin, casein, collagen, and peach gum were detected using enzyme-linked immunosorbent assay (ELISA). The results confirmed historical records regarding to the use of organic additives in ancient mortars. Statistical analysis revealed that the organic additives added to different types of mortars were selected according to the location and function of the mortar. This evidence supports the validity of traditional Chinese composite mortar scientifically. Ancient Chinese artisans adhered to rules aimed at practicality and functional realisation when selecting organic additives.