If, on the other hand, the carbon-content is to be raised, then carbon and manganese are usually added together in the form of a manganiferous molten pig iron, called spiegeleisen, i.e.
Part of the carbon of this spiegeleisen unites with the oxygen occluded in the molten iron to form carbonic oxide, and again a bright flame, greenish with manganese, escapes from the converter.
Mushet had no such exclusive knowledge of the effects of manganese that he alone could have helped Bessemer; and even if nobody had then proposed the use of spiegeleisen, the development of the Swedish Bessemer practice would have gone on, and, the process thus established and its value and great economy thus shown in Sweden, it would have been only a question of time how soon somebody would have proposed the addition of manganese.
It thus resembles magnetite in external characters, but is readily distinguished from this by the fact that it is only slightly magnetic. It is found in considerable amount, associated with zinc minerals (zincite and willemite) in crystalline limestone, at Franklin Furnace, New Jersey, where it is mined as an ore of zinc (containing 5 to 20% of the metal); after the extraction of the zinc, the residue is used in the manufacture of spiegeleisen (the mineral containing 15 to 20% of manganese oxides).
A small amount is used for spiegeleisen, and the rest as a flux.
His results prompted Bessemer to try the purer iron obtained from Cumberland haematite, but even with this he did not meet with much success, until Robert Mushet showed that the addition of a certain quantity of spiegeleisen had the effect of removing the difficulties.