Question

How did the knowledge of silk spread further? Why was it expensive?

Answer 1

Some Chinese people went to distant lands on foot, horseback and on camels, carried silk with them.
Moreover the knowledge of silk spread in the west as Chinese rulers sent gifts of silk to rulers in Iran and west Asia. Silk became fashion for rulers and rich people in Rome about 2000 years ago.
It was very expensive as it had to be brought from China passing through mountains, deserts and dangerous roads, People who lived along the route asked for payments from traders to pass through.