To mitigate possible environmental pollutions, the direct to blister (DTB) route is an important process for copper making because a single, SO2-concentrated stream can be produced. In this study, the ferrous-calcium silicate (FCS) slag system for the DTB process with a CaO content of 15 wt pct was studied, as well as its equilibria with spinel, blister, and gas phases. The investigation was targeted in the spinel primary phase field with accurately controlled P(SO2) and P(O-2) to be 0.4 and 1 x 10(-5) atm, respectively. The equilibrium chemical compositions of various phases in the samples after high-temperature experiments were quantified using an electron probe X-ray microanalyzer (EPMA). Accordingly, the relationship between the liquidus temperature of slags and Fe/SiO2 was established, as well as that between the equilibrium "Cu2O" content and temperature. The solid solubilities in the spinel phase were also identified, including the concentrations of S, CaO, SiO2, and "Cu2O." Last, the present results on FCS slags were compared with those on iron silicate (IRS) slags and the FactSage predictions, which provided more clues for further slag development and thermodynamic modeling.