furnace vs kiln

furnace and kiln both are nouns.

Word	Noun	Adjective	Verb	Adverb
furnace	Yes	No	No	No
kiln	Yes	No	No	No

As nouns, kiln is a hyponym of furnace; that is, kiln is a word with a more specific, narrower meaning than furnace:

furnace: an enclosed chamber in which heat is produced to heat buildings, destroy refuse, smelt or refine ores, etc.
kiln: a furnace for firing or burning or drying such things as porcelain or bricks

furnace (noun)	kiln (noun)
an enclosed chamber in which heat is produced to heat buildings, destroy refuse, smelt or refine ores, etc.	a furnace for firing or burning or drying such things as porcelain or bricks