Encaustic is an ancient medium of painting upon a surface with a mixture of heated beeswax and pigments. In Egypt, it was used to paint portraits on mummies, likenesses of the person beneath the mummy wrappings as they appeared in life. It is the most important body of portraiture to survive antiquity. Roman historian Pliny, The Elder, wrote about the process in the first century AD. It was popular with Greek artists as far back as the fifth century BC. After the fall of the Roman Empire, encaustic fell into disuse, but was revived after a long sleep in the mid-18th century. Paul Gauguin experimented with encaustic and Mexican muralist Diego Rivera used the process extensively to intensify the color and enhance the permanence of his images. Paul Klee and Pablo Picasso also tried their hand at the encaustic form.