In this study, the aim was to reduce copper losses to slag in copper production by using a boron compound, called colemanite (2CaO center dot 3B(2)O(3)center dot 5H(2)O), as flux, besides silica. In order to achieve this purpose, two different matte-slag couples were used; the flash furnace matte-slag obtained from Eti Copper Inc. (EBI.) and a master matte-slag without copper produced synthetically under laboratory conditions. Chemical analyses of flash furnace slag and synthetic master slag were 0.88%Cu-36.1%SiO2-43.9%FeO-3.7%Fe2O3 and 37.6%SiO2-60.0%FeO-2.4%Fe2O3 respectively. In each experiment, the same amounts of slag and matte were mixed with the addition of predefined amounts of colemanite and melted together at 1250 degrees C for different durations under nitrogen atmosphere in a silica crucible. Results showed that it was possible to decrease the copper losses to about 0.3-0.4% by accelerating the settling rate of matte particles by the addition of colemanite, which was attained by the expected increase in flash furnace slag fluidity and lowering of its liquidus temperature.