So admired were these pieces that they encouraged the development of
earthenware made in imitation of porcelain and instigated research
into the method of their manufacture.
"So" is a degree adverb (not unlike "very") modifying "admired".
The phrase "so admired" has been preposed (fronted), no doubt to serve as a link to the preceding discourse. Preposing expressions with so + noun/adjective is fairly uncommon and found mainly in formal discourse/textbooks/literary works etc.
Preposing certain elements like this triggers subject-auxiliary inversion: the basic order would be These pieces were so admired that they encouraged ... .
Note also that the complement clause that they encouraged the development of earthenware ... . is licensed (specifically permitted) by the so that modifies admired. Complements like this are called 'indirect complements' because although they follow the head word, it is not the head word that licenses them.