Borosilicate, opal glass article

Claim 1

An integral, colored spontaneous opal glass article composed of an internal, opaque portion encased within a continuous, unbroken surface layer of transparent glass, the vitreous article having a uniform elemental composition throughout its mass, consisting essentially, expressed in weight percent on the oxide basis as calculated from the batch, of 55-80% SiO 2 , 5-18% TiO 2 , 4-15% B 2 O 3 , 3-20% of at least one mono- or divalent metal oxide selected from the group of Na 2 O, K 2 O, BaO, CaO, MgO, and SrO, the total monovalent metal oxide content not exceeding 6%, and a TiO 2 coupler selected from the group in the indicated propotions of 0.1-5% of an oxide capable of coupling with TiO 2 selected from the group of As 2 O 5 , CdO, CeO 2 , CoO, Cr 2 O 3 , CuO, Fe 2 O 3 , MnO, MoO 3 , NiO, Nb 2 O 5 , Nd 2 O 3 , P 2 O 5 , Sb 2 O 3 , SeO 2 , SnO 2 , SO.sub. 2, TeO 2 , and V 2 O 5 , and 0.001-1% of a noble metal capable of coupling with TiO 2 selected from the group of Ag, Au, Pt, Rh, and Ru, the internal portion being composed of a borate-rich phase separated from and dispersed throughout a silicate-rich phase, the two glass phases being of such different refractive index as to render said internal portion opaque, and the surface layer being a transparent, single-phase borosilicate glass.