This may be the 1965 movie adaption of H. Rider Haggard's She, starring Ursula Andress.
from the IMDB page:
After doing military service in the Middle East, British archaeologist
Holly, his valet and his handsome young friend Leo are spotted in
Palestine and approached by a mysterious oriental who identified Leo
by his uncanny likeness to the portrait on an ancient coin. An
adventurous search for the lost Pharaonic city of Kuma proceeds
relying on a recently-acquired map. Their hasty mini-expedition
reaches a hidden city where the Hellenistic age is still alive. It is
ruled by the supernatural Ayesha, 'She who must be obeyed', who
believes Leo is is the reincarnation she waited for all those
centuries of Callicrates, a companion of Alexander the Great. She
offers him immortality and to share the throne as her eternal love,
but it goes tragically wrong...
It's a largish image, but here's some thumbnails of She aging in the fire: