Ambrotype

About 

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. 

Manufacturing

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, whose function is to invert the polarity of the image, may be of various kinds: paper, textile or a varnish made from bitumen of Judea and turpentine. When the background is placed against the glass and not against the emulsion, an impression of depth is formed, due to the separation between the whites and black of the image. The resulting set is then placed in a frame or case, just like the daguerreotypes which ambrotypes are often confused with. The aesthetic aspects of the two processes are similar, although ambrotypes show less detail. 

Identification

An ambrotype can be identified by the milky, whitish appearance of the image, which is visible to the naked eye, the thickness of the glass plate, often evident in the blacks, and the absence of the amphitype (positive-negative) characteristic of the daguerreotype, which helps to distinguish it from the other, as these two processes can be easily confused when they are presented framed or mounted in cases.

Deterioration

Ambrotypes are fragile objects made of glass and may crack, break or even become opaque over time. The collodion image layer, the protective varnish and the black background painting may also suffer from various deteriorations 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 black background alters, the picture loses its legibility. Over time, the image may darken and/or a coloured ring may appear as the silver tarnishes. Less fragile variants using different kinds of supports such as a thin iron sheet (see tintype) or a waxed canvas (see panotype) were developed.