Thermodynamics of the System Cu‐O and Ruby Formation in Borate Glass

The CuO‐Cu 2 O and Cu 2 O‐Cu equilibria in glass solution were studied in 30Na 2 O 70B 2 O, and 25Na 2 O · 10Al 2 O 3 · 65B 2 O 3 glasses at varied partial pressures of O 2 and temperatures. The CuO‐Cu 2 O equilibrium was studied in a Pt crucible at p O 2 = 1 to 0.05 atm with ∼0.5 wt% Cu; the Cu 2 O‐Cu equilibrium was studied in Cu crucibles at much lower p O 2 's (10 −14 to 10 −17 atm), where no detectable amount of CuO was present. Plots of log [CuO]/[Cu 2 O] 1/2 ) and log ([Cu 2 O] 1/2 /[Cu]) (where the brackets designate concentrations in glass solution) vs log p O 2 gave best‐fit lines having slopes of ∼ 1/4. From the variation of the equilibrium constant with temperature, the enthalpies of the reactions °Cu 2 O( glass soln ) + 1/4 O 2 ( g )→ CuO( glass soln ) and Cu( glass soln )1/4 O 2 ( g )→ Cu 2 O( glass soln ) in the glasses were estimated to be − 14.57 and − 54.34 kcal/mol, respectively; these values are near the data for isolated Cu‐O systems. The enthalpy for the solution of Cu in glass was estimated to be + 23.7 kcal/mol. On heat treatment, the best ruby was formed in glasses melted at p O 2 = 10 −14 atm; at higher p O 2 's no ruby was formed, and at p O 2 ≤10 −15 the glass produced progressively poorer rubies. Using the thermodynamic data estimated for the system Cu‐O in glass, possibilities for the precipitation of pure solid Cu and Cu 2 O are discussed. It was shown that Cu 2 O is primarily responsible for the bright red ruby color in glass.