Spiegeleisen (literally "mirror-iron", German: Spiegel—mirror or specular; Eisen—iron) is a ferromanganese alloy containing approximately 15% manganese and small quantities of carbon and silicon. Historically, this was the standard form in which manganese was traded and used in steel making. Being remarkably free from impurities, as phosphorus, sulphur, silica, it was much used in the Bessemer process for the purpose of reintroducing carbon. It reduces the iron oxide and is itself subject to oxidation. It mostly originated in Germany and was made from hematite.
Today, manganese is usually traded and used in more concentrated form, 80% manganese content being typical. Spiegeleisen is sometimes also referred to as specular pig iron, Spiegel iron, just Spiegel, or Bisalloy.