Commercial production of formaldehyde began in Germany just over a hundred years ago, but it was the development of a process for the synthesis of methyl alcohol in the 1920s which provided the spur for its large-scale manufacture. The formaldehyde produced from methyl alcohol is usually marketed as an aqueous solution. Formaldehyde can be produced from natural gas or methane, but the mixture of products obtained presents problems in subsequent separation stages. Controlled volumes of water are fed into the top and a 40–50% aqueous formaldehyde solution is obtained from the base of the condenser. Formaldehyde solution from the bottom of the distillation column is passed through an ion-exchange bed to remove any acid before further concentration by distillation as in the complete oxidation process. Gaseous methyl alcohol is mixed with air and preheated to 340°C before entering the reactor which contains tubes filled with a catalyst based on iron, molybdenum or vanadium oxide.