Ambrotype: A Negative Positive

By the summer of 1855 the ambrotype was fast becoming the most popular form of photography among Americans, preferred even over the daguerreotype. The popularity of the medium was owed to the absence of the highly reflective surface that characterized the daguerreotype, which many persons found troublesome. An ambrotype is most simply described as a collodion glass-plate negative that appears to be a positive when viewed against a dark surface. This transformation is caused, with reflected ambient light, by the clear areas of the negative appearing dark, and the opaque silver areas bright.

The process of producing this type of image can be described in eight main steps, as outlined by the American photographer M.H. Ellis in his 1856 manual, The Ambrotype and Photographic Instructor. (1) The glass plate on which the photograph will be made is first cleaned with cotton and a mixture of alcohol and rottenstone, which is essentially a powder made of decomposed limestone. (2) The cleaned glass is then coated with a thin emulsion of iodized collodion that is allowed to flow evenly to the edges of the glass. The collodion solution consists of alcohol, sulfuric ether, gun cotton, chloroform, iodide of potassium, and bromide of potassium. (3) Next, the emulsion-coated plate is rendered light sensitive by plunging it into a silver nitrate solution, which consists of distilled water, nitrate of silver, iodide of silver, and acetic acid. (4) After the plate is rendered light sensitive, it must quickly be placed into the camera for exposure, as the emulsion becomes less sensitive when it dries. Exposure will take between five and sixty seconds depending upon the amount of direct light available. (5) The plate is then ready to be developed using a solution of protosulphate of iron, nitrate of potass, water, and acetic acid. The photographer immerses the entire plate in the developing solution while making a gentle circular motion to ensure even coverage. (6) The emulsion is then desensitized to light with a fixing solution made of hyposulphite of soda and water. The fixing solution is applied to the plate with the same circular motion as the developing bath, and is followed by a thorough water rinse.

(7) The final step in making an ambrotype awards the negative with its magical ability to put on the guise of a positive. This step is called finishing. There are two common methods for finishing an ambrotype. Both use a clear varnish, usually made of amber and a black varnish made of asphaltum or black Japan varnish. The photographer may choose to layer both the amber and the asphaltum on the emulsion side of the plate, or to cover the emulsion side with transparent varnish and coat the back of the plate with the dark varnish. When the black varnish is applied to the emulsion side of the glass, the image is viewed in reverse, and when applied to the back of the glass, the varnish lends a sense of depth to the image. (8) In both cases, a second plate of glass is included over the emulsion side of the original plate, to further protect the image from damage

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