All residual fuels contain some inorganic - "non-combustible" - material, which is either naturally present in the fuel or has been introduced from external sources. When the fuel is burnt this inorganic material is converted into solid particles of oxides, sulfates, or more complex compounds, collectively known as ash.

 The ash constituents from the crude oil are concentrated in the residual fuel and this concentration depends upon the refinery processes employed. The following will be, to some degree found in crude oils, vanadium, sodium, calcium, magnesium, zinc, lead, iron, nickel. For distillate fuels the ash level, which is defined as the residue after all the combustible components have been burned, is negligible. The naturally occurring elements in the crude oil could be present in the residue by four times as much as that by the distillation process of the refinery. Aluminum and silicon contamination will contribute to the ash of the fuel. Contamination of the fuel during storage and distribution can increase the levels of ash by the causes such as sodium from seawater, iron from rust in pipelines and storage tanks, dust and general dirt, and even through intended or unintended mixing with other products.