The Collaborative International Dictionary of English v.0.48:

Spiegel iron \Spie"gel i`ron\ [G. spiegel mirror + E. iron.]
   (Metal.)
   A fusible white cast iron containing a large amount of carbon
   (from three and a half to six per cent) and some manganese.
   When the manganese reaches twenty-five per cent and upwards
   it has a granular structure, and constitutes the alloy ferro
   manganese, largely used in the manufacture of Bessemer steel.
   Called also specular pig iron, spiegel, and
   spiegeleisen.
   [1913 Webster]