Abstract: A substantially homogeneous ceramic mass comprised of about 48 to 88 wt. % of alumina; about 4 to 12 wt. % of polyvinyl butyral; about 0.5 to 5 wt. % of polypropylene oxide - ethylene oxide block polymer; about 3 to 10 wt. % of a polymer solution comprised of acrylic acid esters, methacrylic acid esters and mixed polymers thereof; about 3 to 10 wt. % of a polyvalent alcohol and mixtures of such alcohols; about 1 to 5 wt. % of triethylene glycol and about 0.5 to 10 wt. % of water is produced and vacuum-extruded into an endless relatively flat strip, dried to a green state, shaped into a desired substrate shape and, if to be used with thin layer circuits, subjected to a plane-parallel pressure of about 9,000 to 12,000 N/cm.sup.2 and then fired or sintered into rigid ceramic substrates.