The ambrotype process is based on the light sensitivity of silver halides suspended in a collodion binder on a glass support. They are  direct positives; they are one-of-a-kind positive images made directly in the camera. Collodion wet-plate negatives were first introduced in the late 1840s in England by Frederick Scott Archer, when he found that collodion could produce a thin binder when poured on glass. Collodion is made from a mixture of ether, alcohol and gun cotton (purified cotton mixed with nitric or sulfuric acid, today known as cellulose nitrate). In 1850, Louis Désiré Blanquart-Evrard noticed that a slightly underexposed negative image on glass could appear positive when placed against a dark background. This phenomenon was due to highlights made up of heavy deposits of silver particles with little to no silver in the shadows, thus allowing the dark backing to show through.

Blanquart-Evrard’s discovery led to the development of the ambrotype process, which used essentially the same processing technique as wet collodion negatives, though sometimes with an additional bleaching step after development to produce a fainter image. To make an ambrotype, a glass plate first had to be well-cleaned and polished. It was then coated with salted collodion (collodion containing a halide, such as potassium bromide) and dipped in a sensitizing bath of silver nitrate and exposed in a camera while still wet. After exposure, it was placed in a developer bath where length of development was determined by eye until the image had reached the correct density. The plate was placed under running water to stop development and then fixed in a solution of potassium cyanide or sodium thiosulfate. After drying, the finished plate could be hand-colored with dry pigment before being coated in a protective varnish consisting of gum sandarac (resin), alcohol and lavender oil. The plate was placed against a dark background, typically along with a sheet of cover glass, identical in size, which acted as another layer of protection. It was also common for a mat, which could range from plain to extremely ornate, to be added as part of the finishing process. The layers were then folded into a thin preserver frame and placed into a protective case. Though the order of layering could vary a little, the dark backing used to make the ambrotype appear as a positive image was always behind the image plate.

The dark backing used to make ambrotypes appear positive could be made from a variety of materials such as paint, fabric, paper, or even a tintype plate. If paint (lacquer) was used as the dark backing, it could have been applied on either side of the glass support. If applied on the image side, the resulting image would be oriented correctly. If applied on the non-image side, the image would appear laterally reversed like a daguerreotype. Dark varieties of glass, such as red glass, could be used for the actual plate on which the image was developed, thereby removing the need for a dark backing material.

In 1854, American photographer and inventor James Ambrose Cutting developed a method for adhering the two pieces of glass together using Canada balsam. Though meant as a way to hermetically seal the ambrotypes as a preservation method, the process was ultimately unnecessary as the varnish layer itself worked extremely well as a protectant. In fact, ambrotypes that utilized Cutting’s patent are known to exhibit deterioration caused by the technique. Interestingly, his lasting contribution to the ambrotype was his naming of the process, which was taken from the Greek word meaning ‘imperishable’ and suggested to him by fellow photographer Marcus A. Root. Although the name took hold in the United States, ambrotypes continue to be called collodion positives in England. Cutting changed his middle name to Ambrose in honor of the process later in his life.

Most ambrotypes are portraits that were produced by studio or itinerant photographers. Since the process cost considerably less to create than daguerreotypes, they were less expensive to buy and far greater numbers of people could afford to have their photograph taken. Because of this, ambrotypes came to be known as the “poor man’s daguerreotype”. It is important to note that a large number of ambrotypes were produced in a relatively short amount of time as a direct result of their affordability. Ambrotypes remained popular until the end of the American Civil War, which coincided with the rise of yet another similar and even less expensive process: the tintype.