Copper-containing, binary, Zn-phosphate glasses were melted under reductive conditions. The profile of their optical spectra and their color change drastically when the ZnO content of the host is modified. An increase in the host's ZnO concentration tilts the copper oxidation state's equilibria toward the reduced species Cu+ and Cu0. In hosts containing more than 52 mol% ZnO, the Cu0 aggregates precipitate upon melt cooling to form ruby glasses. The ruby color also appears in glasses including 50 mol% ZnO, after thermal treatments. The effect of ZnO increase, in the composition of binary Zn-phosphate glasses, on the redox equilibria of a copper dopant, is inverse to that caused by increase of Na2O content in the case of Na-phosphate hosts.