The Zarnegar House, located in Evaz County, Fars Province, is a Qajar-era building that exhibits a remarkable variety in the application of mortars. Various types of mortars have been used in different parts of the structure, including mortars for stone masonry walls, wall plastering, floor finishing, arch construction, false arches, filling wooden ceiling beams, and lattice window frames. This study aims to technically investigate and identify the compositions of historical plasters and mortars using instrumental analysis methods (XRD, XRF) and petrographic studies of polished sections under a polarizing microscope. Additionally, the research seeks to answer the following question: What are the primary (binding materials) and secondary (fillers and additives) components used in the historical mortar and plaster samples of the studied building, and what similarities and differences exist in their compositions and constituent materials? Laboratory results indicate that the studied mortars can be classified into three groups: the first group consists of high-purity gypsum mortar used for wall plastering, flooring, arch construction, and lattice windows; the second group comprises gypsum-soil mortar with silicate additives used for filling wooden beams and false arches; and the third group includes lime-silicate mortar used for stone masonry walls. Furthermore, the results reveal that the primary structure of the gypsum used consists of a combination of anhydrite, gypsum, and dolomite phases, which are common across all gypsum and gypsum-soil samples. These findings align well with geological data and indicate a specific calcination method at varying temperature ranges and the use of a specific source for the production and application of gypsum mortar in this region.