A glass composition comprises a low glass transition temperature glass and a higher glass transition temperature glass, which glasses are uniquely capable of forming together a continuous vitreous phase over their entire compositional range. During thermal cycling, the low glass transition temperature glass flows out and solubilizes the higher glass transition temperature glass to thereby synthesize in situ a new glass. The low glass transition temperature glass is composed of 14.1% SiO.sub.2, 72.8% PbO, 12.5% B.sub.2 O.sub.3, 0.2% Al.sub.2 O.sub.3, 0.1% CaO, 0.2% Na.sub.2 O, and 0.1% MgO, and the higher glass transition temperature glass is composed of 71.11% SiO.sub.2, 2.38% Al.sub.2 O.sub.3, 7.13% CaO, 14.45% Na.sub.2 O, 3.76% MgO, 0.30% of K.sub.2 O, 0.13% B.sub.2 O.sub.3, and 0.74% PbO.