Process for preparing a borosilicate glass

Claim 1

A process for preparing a surface-darkened glass, comprising the steps of sequentially: (a) providing an alkali alumino-borosilicate silicate glass comprised of from about 50 to about 85 weight percent of silica, from about 4 to about 26 weight percent of boron, from about 1 to about 26 weight percent of alumina, from about 0 to about 20 parts per million of silver ion, from about 0 to about 20 parts per million of halide ion, from about 0.2 to about 10.0 weight percent of antimony ion, and alkali, wherein said alkali is selected from the group consisting of from about 2 to about 8 weight percent of lithium oxide, from about 4 to about 15 percent of sodium oxide, from about 6 to about 20 weight percent of potassium oxide, from about 8 to about 25 weight percent of rubidium oxide, from about 10 to about 30 weight percent of cesium oxide, and mixtures thereof; and (b) subjecting said glass to a temperature of from about 350 to about 1,000 degrees Celsius for from at least about 30 minutes while contacting said glass with hydrogen-containing gas.