Abstract: The present invention is directed to a method of manufacturing low thermal expansion modified cordierite compositions which are especially useful in the manufacture of mirrors for high temperature applications, said method comprising the steps of: providing, on a molecular basis, a mixture of about 2 moles magnesium oxide, 2 moles aluminum oxide, and about 5 moles divided between silicon dioxide and germanium oxide; blending said materials; milling the mixture until finely divided and thoroughly mixed; forming said mixture into a desired shape; gradually heating the shape to a temperature at which reactive sintering occurs and a cordierite type structure is formed; gradually cooling the structure; and hot compacting the structure to consolidate the sintered body to very nearly 100 percent of theoretical density.