A chemical reactor and a method for arranging a ceramic barrier in a gasifying reactor, which reactor is arranged to convert high energy organic waste of black liquor type with a large amount of organic or inorganic alkali metal compounds, by high temperature oxidation with air or oxygen, whereby the organic waste is converted to a hot reducing gas containing a considerable amount of water vapour and the inorganic compounds form an alkali-containing salt melt at a temperature of 750-1150° C., and which reactor is arranged to comprise an outer shell (14) with associated inlet and outlet devices for reactants and products, the method comprising the arranging of a lining (16, 18) comprising one or more layers of ceramic barriers (16, 18) on the inside of the shell (14), the innermost ceramic barrier (16) of the reactor being composed of a lining material that primarily contains compounds of aluminium oxide (Al2O3), and at least one of alkali metal oxides (Me2(I)O) and alkaline earth metal oxides (Me(I)0), forming compounds of the type Me2(I)O Al2O3 and Me(II)0-Al2O3.