Process for making cordierite glass-ceramic having nucleating agent and increased percent cordierite crystallinity

In a process for producing a cordierite glassceramic by fusing a mixture of ingredients to form a melt consisting essentially of MgO, Al.sub.2 O.sub.3, and SiO.sub.2, quenching the melt to a solid glass, and then heating the solid glass to crystallize such oxides to a cordierite crystalline structure in the presence of a glassy phase;

• the improvement of increasing the amount of cordierite crystalline structure formed and reducing the amount of glassy phase obtained, comprising:
• admixing with said mixture or glass at any time prior to such heating step a nucleating agent in a relatively small amount of about 0.5% to about 3%, based on the weight of said solid glass, said agent consisting essentially of a metal compound, the metal of said compound being selected from a class consisting of Mo, Ta, Zr, Nb, Ti, Li, As and mixtures thereof, and said compound of the metal being selected from the class consisting of the metal itself, an oxide of the metal, a compound convertible to the oxide by heat, and lithium fluoride, comminuting said solid glass to an average particle size no greater than about minus 325 U.S. Standard sieve to provide substantially increased surface area, heating said particles and nucleating agent to a nucleating temperature and forming a plurality of sites for crystallization of said comminuted glass to a cordierite glass-ceramic by the combination of said relatively small amount of nucleating agent and said increased surface area, and then heating the particles and agent at a higher temperature to promote crystallization and form a cordierite glass-ceramic containing at least 80% by weight of a cordierite crystalline structure corresponding to 2MgO.2Al.sub.2 O.sub.3.nSiO.sub.2, n being a whole number within the range of 5 to 8, said glass-ceramic having a coefficient of thermal expansion less than 1.60 .times. 10.sup.-.sup.6 inch/inch/.degree. C from room temperature to 600.degree.C as determined by the Orton dilatometer.