Simplify the following text for a Grade 10 native English-speaking person.
Original: Scarlino remained part of the Principality of Piombino until the early 19th century , when it became part of the Grand Duchy of Tuscany .
Simplified: Until early 19th century , Scarlino was still a part of Principality of Piombino few time later when it became part of the Grand Duchy of Tuscany .