Disclosed are water soluble core wherein a particulate refractory material is combined with a binding agent comprising Na.sub.2 CO.sub.3, Na.sub.2 O . NSiO.sub.2 where n is 0.5 to 4, and SiO.sub.2 as ingredients and a process for producing the water soluble core. The process comprises mixing the particulate refractory material with a water glass, casting the mixture of the particulate refractory material and the water glass, hardening the casting by CO.sub.2 gas, and calcining the casting at between 100.degree. C. and a temperature less than that of an endothermic peak of the water glass in differential thermal analysis. Also, a process for die casting a metal is disclosed using the water soluble core which process comprises pre-heating the water soluble core from a temperature of 300.degree. C. below a melting point of the metal to a temperature of the endothermic peak.