Mill additions are disclosed which lower the CTE of, and are compatible with, tin-phosphorus oxyfluoride glasses. The sealing materials provide fusion-type seals having a CTE not over about 110.times.10.sup.-7 /.degree.C. and a sealing temperature not over about 350.degree. C.Also disclosed is a family of glass compositions in the tin-phosphorus oxyfluoride system that exhibit very low glass transition temperatures and coefficients of thermal expansion not over about 160.times.10-7/.degree.C. (25.degree.-150.degree. C.). In addition to the four basic elements, the glasses contain up to 11% Nb, and may additionally contain up to 10%, taken individually or in combination, of V, Fe and/or Mo to further decrease the Tg and CTE values.