Although ash and water free, the structure of activated charcoal does not consist of pure carbon. Depending on the mode of manufacture and starting material, not only carbon but also oxygen and hydrogen, and sometimes nitrogen, are present in bound form. The carbon content mostly amounts to 85 - 98%. Oxygen forms so-called surface oxides, which can endow the charcoal surface with alkaline or acid characteristics. A reaction of the surface compounds can be observed through pH-value changes of a charcoal suspension (slurry). In many cases, surface oxides are the cause of catalytic properties of activated charcoal, particularly in the case of reactive processes. As with graphite, pure carbon surfaces are water repellent. However, through the presence of surface oxides, hygroscopic areas are formed, so that carbon with a high content of surface oxides is easily wetted by water. Activated charcoal, a carbonaceous substance, is naturally combustible. However, carefully prepared activates, particularly water vapour activates, are not auto-igniting. By the point of ignition, one means the temperature at which the first exothermal reaction takes place in the course of being heated. The ignition temperature of activated charcoal lies in the range of 250 - 400°C.
