hard lead vs antimonial lead

hard lead and antimonial lead both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard lead	Yes	No	No	No
antimonial lead	Yes	No	No	No

As nouns, hard lead and antimonial lead are synonyms defined as:

hard lead and antimonial lead: a lead alloy that contains about 5% antimony

hard lead (noun)	antimonial lead (noun)
a lead alloy that contains about 5% antimony	a lead alloy that contains about 5% antimony
unrefined lead that is hard because of the impurities it contains