<human>:  What is the oldest known use of gold leaf?
<bot>: The oldest known use of gold leaf is in the Egyptian Book of the Dead. The Book of the Dead is a collection of spells and prayers used to help the deceased in the afterlife. The spells and prayers are written on papyrus scrolls and then covered with gold leaf. The gold leaf is believed to protect the scrolls from decay and to help the deceased in the afterlife.