Abstract: The present invention relates to a method of producing a borosilicate glass exhibiting a low coefficient of thermal expansion, viz., less than about 20 .times. 10.sup..sup.-7 /.degree. C. over the range of room temperature to 300.degree. C., an ability to withstand repeated thermal cycling between room temperature and temperatures up to 1300.degree. C. and higher without devitrification, and the ability to flow, when present in particulate form, at temperatures between about 1000.degree.-1250.degree. C. The method contemplates impregnating a porous, essentially alkali metal-free, high silica material with a solution or a molten mass of a B.sub.2 O.sub.3 -containing compound, the B.sub.2 O.sub.3 -containing compound also being essentially free from alkali metals. The impregnated glass is thereafter dried to eliminate any solvent. High emissivity can be imparted by incorporating minor amounts of such materials as CoO, NiO, MoO.sub.3, Fe.sub.2 O.sub.3, and V.sub.2 O.sub.5 as dopants during the impregnation step.