When the thick liquor is burned under limited air intake, a melt of sodium carbonate (Na2CO3) and sodium sulphide (Na2S) is formed. Dissolving this in water gives "green liquor". The sodium carbonate is causticized with calcium hydroxide (slaked lime, Ca(OH)2) to sodium hydroxide solution. Filtration produces "white liquor" which is re-used in the process. The filtered calcium carbonate is burned in a rotary drum furnace to calcium oxide (burnt lime, CaO).