Collodion Positives on Glass (Ambrotype)
A collodion negative was to some extent also a positive in that the reduced silver forming the highlights reflected light. If the image could be whitened an underexposed collodion negative placed against a black background would have the appearance of a positive when viewed by reflected light. This describes a process that was popular in Britain from the time of its introduction around 1852. Being inexpensive, quick and easy to use it soon replaced the Daguerreotype in commercial portrait studios. The process was far less common after the early 1860s, following the introduction of the carte de visite.
A collodion negative was given a short exposure, after development, usually in ferrous sulphate, it was placed against a dark background and mounted in a case or framed. The dark background could be paper or cloth, or a varnish applied to either side of the glass plate. Some images were produced on purple glass and did not need a backing. If the dark varnish was applied to the plain side of the glass the image was given a clear varnish and mounted under a cover glass for protection, the image would be laterally reversed. The dark varnish could be applied to the image, which was viewed though the glass, in which case the image was not reversed. Collodion positives were often hand coloured.
The use of ferrous sulphate developer gave a silver-grey image, pyrogallic acid gave a brown image which would have required whitening with a mercury bichloride solution. To hold back development and allow the mid-tones to appear the collodion used to produce the negative was weaker than that used for a negative intended to be printed from. As well as iodide, bromide was sometimes used in sensitising the collodion.1
The collodion positive process was patented in America and in Britain by James A. Cutting, the only distinguishing point of the patent being that the image was cemented to a cover glass using Canada balsam. The patent calls the process 'Ambrotype', in America the name came to be used more generally for all similar processes.2
These were produced in the same way as ordinary collodion positives, the black varnish applied to the plain side of the plate was removed from all but the subject area, the plate was then placed against a white background which gave a relief effect to the subject. A painted background (such as a clouded sky) might be used in place of the white background. The process was patented by John Urie.3
In his manual, Frederick Scott Archer proposed the use of mercury bichloride in hydrochloric (muriatic) acid to whiten an image for use as a positive.4 Mercury bichloride was sometimes used in the ordinary collodion positive process to whiten the image.
The term Alabastrine dates to around 1858 and describes a process producing more vigorous whites together with a method of colouring the image. Alabastrine solution, probably, sulphate of the protoxide of iron, mercury bichloride and sodium chloride was sold as a commercial product without its constituents being disclosed, its main advantages over the simple use of mercury bichloride being the improved white tones it gave and that it did not produce a blue tint as was often the case with mercury bichloride.5
A full toned negative was treated with alabastrine solution to give a whitened image. Powder colours were then, optionally, applied and the image varnished. The action of the alabastrine solution enabled the varnish to penetrate the collodion so that the colours lie within the film. The image was viewed through the glass against a black velvet background.6
References & NotesThornthwaite, Guide to Photography. (1860), p. 24. Monckhoven, Popular Treatise on Photography. (1863).
Information on the Wet Collodion Process.
 Hardwich, Phot. Chemistry, p. 141. Hardwich writing in the Journal of the Photographic Society in 1854 gives a full account of his method of working. Phot. Journal, May 1854, pp. 202, 215.
 BP 1638/1854.
 BP 407/1854.
 The Collodion Process on Glass, 1854. The whitened image could be turned into a full-toned negative with potassium cyanide.
 Article by G.W. Simpson in Phot Notes 1/1/1860, p. 17.
 Brothers, History, p. 63. Phot. News 15/3/1860, p. 73. Phot. News 17/12/1858, p. 180.
Early ferrotypes were produced by the wet collodion process on a support of thin iron sheet having a japanned or lacquered finish. They were produced directly in the camera and are in many respects the equivalent of an ambrotype but on metal, that is, they are thin negative images placed against a dark background so as to appear as a positive. The metal support was at first coloured black later brown was more common. The image is laterally reversed.
Ferrotypes became popular in the United States when introduced in the mid 1850s, there, the process was used in studios as a replacement for the ambrotype. In Britain the process had little support until the 1880s and 1890s when it became popular with photographers working at fairgrounds and seaside resorts.1
The ferrotype process was capable of yielding good quality images though a bit flat in tone. Because the process was easy and very quick to use it became popular with itinerant photographers and associated with the lower end of the portrait trade. In their cheaper forms they required little or no mounting, they had the advantage over glass of being unbreakable and were much lighter; ideal for the souvenir trade.
Most images are small, better quality images are found in cases or mounted in frames, they are also found mounted in albums and in jewellery. Cheaper images were simply placed behind a card with an oval aperture, the tintype was held in place by paper stuck to the back of the card. Another method, used on cheaper images, was to mount it between a sheet of folded card of a similar size to a carte de visite,
The name ferrotype is unfortunate as it had already been used by Robert Hunt for an un-related process. In Britain the name was also used to describe the metal plates themselves that were used for glazing prints as well as for use in the ferrotype process.
Dry FerrotypeIn the late 1880s dry plates coated with gelatine silver bromide emulsion were introduced.
References & NotesCyclopedia of Photography, p. 240. Eder, History, p. 369. Information on the Wet Collodion Process.
 The process was patented in America by Hamilton L. Smith in 1856 and assigned to William and Peter Neff. US Pat. 14300/1856.
Smith's process was described in Phot. News 1/1/1856, p. 11.
An excellent description of the early history of the tintype in the USA, including the relationship between Smith, Neff and Victor M. Griswold is in Robert Taft's book, Photography and the American Scene.
Collodion images were also produced on leather, black glazed paper and other materials. Thomas Bullock of Macclesfield developed a process of producing direct positive images in the camera on black card. The card was coated with collodion and dipped in a silver nitrate bath in the normal way.1
'Ferrotype cards' were a later introduced commercially consisting of gelatine silver bromide emulsion on a card base.
References & Notes
 Bullock - Phot. Notes 15/1/1858, p, 22; 15/9/1858, p. 221. A process for producing collodion images on papier mâché, wood or metal was patented by William Kloen and Daniel Jones, BP 2887/1856.