The Process

Tuscan Centerpiece

In the three-colour gum dichromate process, a sheet of watercolour paper is repeatedly coated with a sensitiser (gum arabic, a potassium dichromate solution to make it light sensitive and a watercolour pigment), exposed through the relevant negative (separation) and processed. The image is built up in three successive printings, one for each of the process colours of yellow, magenta and cyan although this order can vary between printers.

The three parts of the sensitiser are mixed together and painted by brush onto the paper. A thicker mixture containing a higher proportion of pigment and having a short exposure time will place the colour primarily in the shadow areas, whereas a thinner mixture with less pigment and a longer exposure will give colour mainly to the highlights and lighter tones. So some contrast control is possible in the printing.

Immediately the coated paper has dried, it's exposed in contact with the corresponding separation to ultraviolet light. A registration system is crucial to ensure that the negatives are positioned correctly for each printing.