Method of forming a glass ceramic material

A process for manufacturing a self-supporting glass-ceramic material comprising, on a weight percent basis, 10-35% MgO, 10-55% BaO and 25-50% SiO2 and possessing a high softening temperature of about 1000° C. or above and a thermal expansion coefficient in excess of 10×10−6 ° C.−1 at 20-950° C., wherein said process comprises:

• a) melting glass raw material at a temperature in excess of 1450° C. and the thereby formed melt is cooled rapidly to a temperature below 900° C. and thereafter to ambient temperature to form a precursor glass,
• b) reducing said precursor glass into a sinterable glass powder with an averaged particle size in the range 1 micron to 100 microns,
• c) consolidating said glass powder into a green body, with or without organic processing aids, and
• d) subjecting said green body to a thermal treatment comprising a heating stage of average heating rate not exceeding 100° C./minute between the temperatures of 750° C. and 900° C. to form the glass-ceramic material.