The word 'porcelain' is a noun, a word for a white, translucent, ceramic; a word for things made from this ceramic; a word for a substance; a word for a thing.
Porcelain is a type of ceramic; it is not part of a steel tub. If the tub were made of porcelain it would not be made of steel. Enamel is a kind of tough paint that can be used on steel, or on porcelain.
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.
The part of speech for "indefinite" is an adjective.