Abstract: A process for the hydrothermal fusion of aluminum silicate and alkali aluminum silicates with aqueous sodium hydroxide solution under pressure and at elevated temperatures in the presence of soluble silicates, in which an aqueous solution containing from about 20 to about 50% by weight of NaOH is mixed with aluminum silicates and/or alkali aluminum silicates, and with a hydrothermally fusible silicate component, wherein the ratio by weight of SiO.sub.2 to Al.sub.2 O.sub.3 in the mixture amounts to at least 30:1 and the ratio by weight of SiO.sub.2 to Na.sub.2 O amounts to at least 1.5:1, and the mixtures are reacted at temperatures of from about 180.degree. to about 250.degree. C. and under the saturated steam pressures corresponding to those temperatures. The resulting sodium silicate solution, which contains the dissolved aluminum, is then separated off.