Complex sentence: Gold leaf is often thinner than standard paper today, and when held to the light is semi-transparent; in ancient times it was typically about ten times thicker than today, and perhaps half that in the Middle Ages.
Split into simpler sentences: Gold leaf is often thinner than standard paper today, and when held to the light is semi-transparent. In ancient times it was typically about ten times thicker than today, and perhaps half that in the Middle Ages.