Complex sentence: In the era of its productions, viewers would also admire the sculpture by candlelight, whose lustre was not only due to the fine quality of the marble but also to the waxed surface, which has been recently restored.
Split into simpler sentences: In the era of its production, viewers would also admire the sculpture by candlelight. The sculpture's lustre was not only due to the fine quality of the marble but also to the waxed surface, which has been recently restored.