An aqueous solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel.[5] When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2CoCl4 or cobalt chloride CoCl2 is added.[5] This will cause the gel to be blue when dry and pink when hydrated.[5] An alternative indicator is methyl violet which is orange when dry and green when hydrated. Due to the connection between cancer and cobalt chloride, it has been forbidden in Europe on silica gel