The lignite deposit of the Southern Field Mine in the Ptolemais Basin is characterized by the multiple interchanges between lignite and thin intercalated sterile layers of marly limestones, marls, clays, and sands. This form of deposit imposes selective excavation where, unavoidably, thin intercalated sterile layers are coexcavated with lignite ones, thus increasing the quantity of ash. Apart from quantity, the quality of ash is of great importance since it affects the overall power plant performance. In this study the role of the thin intercalated sterile layers coexcavated with lignite is investigated, producing laboratory mixtures of lignite-marly limestone, lignite-carbonaceous marl, and lignite-clay (in proportions of 10 and 20% by weight in sterile layer). These mixtures were analyzed and their ash quality parameters were compared with the pure lignite and the run-off mine lignite. The addition of carbonates increases the CaO and CaO(free) content and reduces the SiO2 and Al2O3 content of ash, respectively. On the other hand, the addition of clays increases the SiO2 and Al2O3 and reduces the CaO content.