Abstract: From the characteristic curve of gelation of silicic acid of N-Shape, it is possible to control the time of gelation by varying the pH of the solution. When a suitable amount of hydrochloric acid or calcium chloride is added to the water glass solution to lower its pH to a definite value, the moulding sand with which the solution is mixed, Will harden at a definite time corresponding to that particular pH value as demanded by actual production operation. Results of preliminary experiments indicate that both of the two above named hardeners may be tried in actual practice when their use is adopted; it not only saves large amounts of carbon dioxide gas, but also makes possible the mechanization and automatic operaion of the casting process.