
These days, falling ill will hopefully land you in hospital with a doctor wearing a white coat and stethoscope by your bedside.

But not so for Navajo Indians at the start of the 20th century, who could expect to find themselves in a sweat lodge being attended to by these masked figures.

This fascinating collection of images, taken between 1904 and 1905 by Edward S Curtis, documents a Navajo Night Chat - a healing ceremony that lasts for nine days and involves the patient being routinely sweated while rituals are performed by medicine men dressed as deities from tribal mythology.

Edward S Curtis spent more than 20 years photographing and recording North American Indian culture in the early 20th century. In this set of images he documented a Night Chant, a Navajo healing ritual involving men dressed as spirits from the tribe's mythology. Here two men are dressed as the Black God (left) and House God (right), two of the four deities who presided over the Third World and created the first man and woman, according to the Navajo creation myth

This image shows the Sacred Twins, known in English as Monster Slayer (right) and Born-of-Water (centre). According to Navajo mythology they were the children of White Shell Woman, one of the first beings, and roamed the earth slaying monsters that threatened humanity and scalping them. They are accompanied here by a deity named only as The Beggar (left)

The ceremony itself it sacred, so Curtis is unlikely to have photographed it in progress, but rather documented a recreation put on by the Navajo for his benefit.

Included in the photographs are the Sacred Twins, known in English as Monster Slayer and Born-of-Water, who roamed the Third World slaying monsters and scalping them, according to the Navajo creation myth and documented by Eastern New Mexico University.

Other figures from the Navajo creation story featured in the pictures is the House God, one of four deities who presided over the Third World and turned the first man and woman from spirits into humans, and the Black God, a fire deity.

Curtis was paid $75,000 in the early 1900s by financier J P Morgan in order to document Native American Indian culture, and spent the next 20 years dedicating himself to the project.

In total he produced 10,000 wax cylinder recordings of Native American language and music and 40,000 images of more than 80 different tribes.

These images were colourised by bank technician Frédéric Duriez, 53, from Angres, in northern France.

'These portraits with these masks are fascinating,' he said. 'These people are mysterious behind their masks and seem to come out of a strange and mysterious story; from another world.

A Navajo man stands holding his ceremonial mask (left) and then appears dressed in it (right). It represents a god known as Fringe Mouth, a water-dwelling deity who participates in the Night Chant. Navajo boys are typically required to undergo a coming-of-age ritual before they are allowed to see which members of their tribe wear the masks

Left is Haschebaad, a representation of several benevolent female deities used specifically for the Night Chant. Right is Gaaskidi, deity of harvests, plenty and rain, who is usually depicted with a hump filled with seeds and water that is so heavy he has to lean backwards using a stick to support it

'The colorised portraits were without relief and without souls. So I applied a light effect so that it would appear out of the shadows. This makes them exceptional I think.'

Originating in the Southwestern United States, the Navajo are the second-largest Native American tribe recognised by the U.S. government, after the Cherokee tribe.

As of 2015 over 300,000 people were enrolled tribal members, with over two thirds of the tribe's population residing in Arizona and New Mexico.

Striking images like these are featured in British author Michael D. Carroll's new book, Retrographic on the colourisation of historical images.