Complex sentence: The kilns at Arita formed the heart of the Japanese porcelain industry, which developed in the course of the 17th century, after the white kaolin clay was discovered, and came to rival the output of the Chinese kilns at Ching-te-Chen.
Split into simpler sentences: The kilns at Arita formed the heart of the Japanese porcelain industry, which developed in the 17th century, after the white kaolin clay was discovered. Arita soon came to rival the output of the Chinese kilns at Ching-te-Chen.