spiegel iron vs pig iron

spiegel iron and pig iron both are nouns.

Word	Noun	Adjective	Verb	Adverb
spiegel iron	Yes	No	No	No
pig iron	Yes	No	No	No

As nouns, pig iron is a hypernym of spiegel iron; that is, pig iron is a word with a broader meaning than spiegel iron:

spiegel iron: pig iron containing manganese; used as a deoxidizing agent and to raise the manganese content in making steel
pig iron: crude iron tapped from a blast furnace

spiegel iron (noun)	pig iron (noun)
pig iron containing manganese; used as a deoxidizing agent and to raise the manganese content in making steel	crude iron tapped from a blast furnace