Iron(III) oxide, also known as rust, is a reddish substance, an iron oxide. It has a chemical formula of Fe2O3. It is made by the reaction of iron with oxygen. It is found naturally in the ground as hematite, the major ore which blast furnaces use to smelt iron. It dissolves in acids. It can be mixed with fine powdered granules of aluminum in a 27:80 ratio to form thermite, a semi-explosive powder.