suitable glass - ceramic base compositions , including some silicates , aluminosilicates , lithium aluminosilicates , and boroaluminates , are modified by the addition of suitable quantities of transition metal oxides prior to melting and forming into thermally - crystallizable glass articles . appropriate crystallization and post - crystallization heat treatments are thereafter employed to cause crystallization of the articles and the formation and growth of films thereon , which films are characterized by the presence of transition metal compounds , typically of spinel structure , therein . such films demonstrate useful electrical , magnetic , catalytic and / or light - absorptive properties .