From the storage tank 1 we draw 1,061 lbs. sodium silicate solution having a content of 9.0% of NaaO and 29.0% SiOz, into a mixing tank 2 located-on scales 3 and having a stirring device. We heat the silicate solution in any convenient manner, preferably with steam coils and add 292 lbs. caustic soda in solid or flake form. Stirring is continued until all the caustic is dissolved 5 I and heating continued until 243 lbs. water have been evaporated, as determined by balancing the mixing tank 2 on the scale 3, or until the resulting solution contains the equivalent of NaaSiOs.6HaO. The solution of sodium meta sili- 10 cate may be pre-cooled by passing cooling water through the coils previously used for steam; it should not, however, be' cooled below C. to avoid solidification in the mixing tank.