Treatment of very fragile and porous stone kept in a highly dry condition with potassium silicate was experimentally evaluated on its consolidating effect and durability for the conservation of grottoes in China. Results obtained are as follows: Impregnation with potassium silicate increased the splitting tensile strength of stone 1.5 to 2.0 times, the consolidation effect being almost the same as that with acrylic resin and silane. The durability was as high as that of the one treated with silane. Stone treated with potassium silicate whose molar ratio of SiO₂ to K₂O is 4.0 showed quite high water resistance, while the one with the molar ratio of 3. 0 showed very low resistance. Efflorescence (crystallization of potassium carbonate) on the stone treated with potassium silicate having molar ratio of 4.0 occurred by far less than the case in which the molar ratio is 3.0. The above results show that potassium silicate whose molar ratio of SiO₂ to K₂O is high (4.0) is quite a possible consolidant for very porous and fragile stone. -- AATA