Abstract: A core material for making cores to be used in molds for the casting of metals and comprising a mixture of hollow ceramic particles uniformly distributed throughout a resin binder of organic material. The hollow ceramic particles are individually in intimate surface contact with adjacent individual hollow ceramic particles throughout the core materials to allow the heat of the casting to be conducted efficiently through the core material. The amount of resin binder being maintained at a minimum to reduce the amount of gas generated by the resin binder as it is exposed to the heat of the metal being cast. The hollow ceramic particles comprising about 99.0 to about 96.5% by weight of the core material and the resin binder comprising about 1.0 to 3.5% by weight, respectively, of the core material prior to the core material being cured, and the hollow ceramic particles preferably ranging in diameter from about 200 microns to about 450 microns, with a preferred mean diameter of about 325 microns.