A mold for metal casting contains a bonded refractory composition comprising hollow alumina-containing microspheres in which the alumina content is at least 40% by weight. The mold may be an ingot mold and the bonded refractory composition may be in the form of a sleeve or boards located in the top of the mold or in the head box thereto. The mold may be a sand mold and the bonded refractory composition may be in the form of a sleeve or boards located in a feeder cavity or in the form of a board or pad located so as to constitute a metal casting surface where it is desired to promote directional solidification in cast metal. The bonded refractory composition may also be in the form of a breaker core. In a preferred composition the microspheres contain alumina and silica and the composition may also contain one or more other particulate refractory materials, a readily oxidizable metal, an oxidizing metal, an oxidizing agent for the metal and a fluoride salt.