Verreotype was an alternative name for an ambrotype, this example is heavily over painted and in a passe partout frame.

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

Ambrotype

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

Relievo Ambrotype

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

Alabastrine

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