The ambrotype is a direct positive monochrome photographic process. It is a wet collodion glass plate negative which when viewed against a dark background looks like a positive photograph. The choice of the term "ambrotype", from the Greek "ambrotos" meaning "imperishable" or "immortal", is probably linked to the durability of the glass base.
Although the first portraits made using this process were presented by Frederick Scott Archer in 1851, then by Adolphe Martin in 1852 under the name of "amphitype", the process was only patented under this name in 1854 by James Anson [Ambrose] Cutting. The patent states that a collodion direct positive is made on a glass plate hermetically sealed using Canadian balsam resin, a resin from Canadian fir trees.
Less expensive to produce than daguerreotypes and requiring shorter exposure times, ambrotypes were regularly used from 1854 up to the 1870s. It especially found favour in the United States, particularly with portrait photographers.
To produce the negative part of this unique object, one side of a clean glass plate is covered with a thin layer of collodion containing ammonium or potassium halides (iodide or iodide and bromide). This collodion emulsion is richer in ether than the one used for producing conventional collodion negatives so as to produce a whitish image, making it show up better against the dark background used for viewing it. Dipping in a silver nitrate solution ensures that it will be uniformly sensitive to light prior to exposing the plate when still wet. After development in a nitric acid and iron sulphate developer, sometimes with added silver nitrate, the image is fixed in a bath of sodium thiosulphate or potassium cyanide. Using iron sulphate gives the ambrotype its characteristic creamy tint. After drying, a transparent varnish is applied, and sometimes, additional colour highlights.
The dark background against which the negative is placed may be of various kinds: paper, velvet or a varnish made from a base of bitumen of Judea and turpentine. When the background is placed against the glass and not against the emulsion, an impression of depth is formed by the space between the light and dark areas.
Then the complete assembly is placed in a frame or case, just like the daguerreotypes that ambrotypes are often confused with. This is because the aesthetic aspects of the two processes are similar, although ambrotypes show less detail.
Ambrotypes, comprising a glass base, are fragile objects and may crack, break or even become opaque over time. The collodion emulsion, protective varnish and the dark varnish may also suffer from various alterations due to their nature and how they are preserved. Most common are flaking of the dark varnish, lifting or reticulation of the collodion layer and silver tarnishing. Once the dark background alters, the picture losses its legibility and seems to disappear.
Less fragile varieties using different kinds of supports as a thin iron sheet (tintype) or a wax-canvas (pannotype) were developed.