The Autotype company was the leading supplier of carbon and carbro materials. The Trichrome process is the adaptation of the Carbro process for three-colour work producing prints or transparencies.

The process uses bromide enlargements from red, green and blue separation negatives. Each bromide print is placed in contact with a pigmented gelatine sheet, coloured cyan, magenta and yellow, (referred to as pigment paper or carbon tissue). During the process the gelatine sheet is sensitised with potassium bichromate making the gelatine insoluble after contact with silver.

Outline of the process

• Bromide prints are produced from three-colour negatives (red, green and blue). It was important to ensure that the bromide print did not have an additional gelatine coating.
• Pigment paper, which was pigmented gelatine attached to a paper support, was sensitised by soaking, separately, in the A and B solutions.
• The pigment paper was placed in contact with the appropriate bromide print and left for 15 minutes. The 'red' bromide print was matched with the cyan pigment paper, the 'green' print with the magenta paper and the 'blue' print with the yellow paper. On separating the two, the image on the bromide print will be found to have been bleached and no longer visible, it could be recovered by re-developing the print. The sensitised gelatine becomes insoluble in proportion to the silver in the bromide print, i.e. shadow areas in the print will produce a thick layer of insoluble gelatine.
• The pigment paper was then placed on a waxed celluloid sheet (with the surface previously in contact with bromide print next to the celluloid).
• The celluloid and pigment paper were placed in warm water to 'develop' the image, the paper support could be removed and the soluble gelatine would dissolve leaving an image consisting of pigmented gelatine of varying thickness.
• The above was carried for each of the three prints.
• The three images were then mounted on a single temporary support (gelatine coated paper). The order of mounting on the temporary support was cyan, magenta and yellow. The use of a temporary support was to give the correct left-right orientation in the final print. After attaching each image to the paper support the celluloid support (to which the pigmented gelatine was still attached) was removed.
• The last operation was to mount the images on a final sheet of paper (single transfer paper), which leaves the cyan image on top.