partially devitrified silica cores with exceptional high temperature strength are disclosed for foundry use in sand casting of ferrous alloys . precision cored holes of small size and long length , which heretofore could not be cast accurately or had to be machined because of limitations in existing core technology , are formed using accurately machined extruded porous silica cores fired to eliminate combustibles and partially devitrified to develop a strong crystalline phase bond that resists viscous flow at a temperature of 1500 ° c . and to increase the refractoriness of the vitreous silica grains , whereby the permeability and thermal shock resistance are such that the core can be heated very rapidly by molten steel to above 1500 ° c . without spalling , cracking , sagging , breaking , or loss of integrity and without gas holes , scabs , fissures or other serious casting defects . the cores are made from an extrudable composition containing vitreous silica , a mineralizer , an organic binder , and a plasticizer and / or tempering fluid . high - purity vitreous silica grains are mixed with a mineralizer and large amounts of finer vitreous silica particles which bond the silica grains together to provide extremely high strength after firing . the extruded cores are fired to form cristobalite in major amounts in the bond region between grains and in minor amounts in surface portions of the silica grains . the sand casting process of the invention can employ hollow porous thin - wall cores having small diameters , such as 0 . 4 to 2 centimeters or less , and length - to - diameter ratios , such as 20 : 1 or more . the problem of casting defects due to gassing is solved by causing the expanding gases to move radially inwardly to the central passage of the hollow core .