The photograph was produced using the wet plate collodion process, which gave birth to three separate photographic techniques – the salt print, the tintype and finally the ambrotype (Ancient Greek ‘immortal impression’), of which this is an example.

The different techniques are distinguished by the material on which the print is made. The wet plate collodion process is messy, difficult, toxic and requires the use of a naked flame in the presence of flammable liquids in darkened and confined spaces, so it is not a surprise that other, safer techniques were soon developed and the process became less and less popular.

To take an ambrotype image the first thing the photographer has to do is go into a darkroom and coat one surface of a glass plate in collodion, a sticky liquid used in medical dressings, made up of nitrocellulose, alcohol and ether. Nitrocellulose, also known as gun cotton, was also used very successfully in artillery ordnance, and spectacularly unsuccessfully in billiard balls.

Coating the glass plate is a critical process that ultimately dictates the quality of the image – an uneven coating will result in an uneven exposure.

The coated plate is then placed carefully in a light-proof flat box that is attached to the camera at the focal plane. The subjects are posed, and often their heads would have been held in place with a discreet head clamp for the duration of the exposure.

In a similar manner to the traditional film printing that superseded it, the developing process was carried out in a dark room on the exposed glass plate. Once developed, the glass plate would be dried using a naked flame, probably the most dangerous part of the process.

When dry, the plate would have a negative image of the subject on it, but the photographer would then paint the uncoated back of the plate with black paint and the negative would magically become a positive.

Each ambrotype is unique, and copies cannot be made, as the negative becomes the final positive image. An interesting quirk of the ambrotype and its siblings is that the print is a mirror image of the scene. Using digital editing software, we can see the scene as it actually appeared.

The image was taken at a time when the new process was becoming a practical technique that allowed photographers to think of making a living from it, and its popularity was spreading rapidly. It had become fairly common to have a portrait taken, although it would still be some time before the technology was effective enough to allow the more candid images that we have become familiar with. Photography was still very much a formal process, and this image is a classic example of that style.

Today the wet plate collodion technique is becoming very popular with photographers unsatisfied with the almost clinical precision that has become associated with digital photography. Photographers such as Sally Mann, Rob Gibson and Kurt Grüng, among others, have popularised and advanced the technique by applying some of the modern lighting techniques unavailable to the unknown photographer who made this image.