Gruppo Rodolfo Namias

   I will describe here the various steps for executing a gum dichromate with colour separation.

   By this term I mean a polichrome gum, whose different colours are added in subsequent layers.

   This method is different from the trichromic or quadrichromic separation: in the trichromic process one prepares three negatives corresponding to the fundamental subtractive colours (Yellow, Magenta and Cyan, YMC). Printing them, from their superimposition all the intermediate colours are formed, black included. In the quadrichromic process the formation of the black is "helped" by means of a fourth negative; in fact the three fundamental colours seldom are so pure and well balanced to form a deep and neutral black.

   In the colour separation one prepares the negatives for dome well defined and not-fundamental colours (i.e. brown, ochre, green, etc.). In the phases of selection and separation of the colours one must take in account the main hues, enhancing them. In this sense this printing technique looks like the lithographic process: it is made by means of a number of negatives bigger than for the quadrichromic process; sometimes some colours of the original image are absent.
   In addition, in the colour separation it is possible to change more or less dramatically the colours of the selection, substituting them with other chosen by ourselves.

Colour separation
   We will use a photo-retouching programme, like Photoshop ®.
   At first, the principal colours of the image must be determined. This is a completely subjective process, and it will determine the final aspect of the print.
   The colour masses, their relative weight, the hues of the shadows, etc. will be evaluated. It is like when one puts itself in front of a canvas and begins to paint with the subject in front of him, but in addition it also means to remake the same reasonings, more or less conscious, that were done at the moment of the shot.

   The picture has been shot on a slide a January Sunday morning about at 8:30, both to take advantage of the particular characteristics of the light and to avoid the water traffic which would have too much stirred the water of the canal.
   Owing to that hour, the image has an evident cold dominant needing some correction.

   The Sun light, still low above the horizon but enough intense because the clear sky, hits the facades but does not reach the water, enhancing the plasters' colours and firing their reflections over the water.

   The main subject is the parallelepiped-shaped structure of two houses side by side, red and pink, and the dark mass of a wooden structure of the nearby "squero" which emphasizes the perspective. The reflection of the houses looks like their downward prolongation.
   The water has a fine, intense blue hue in the foreground and shades in a pale blue in the background, where the same colour continues into the sky.

   On the basis of the observations above, by means of the tool "colour selection" of Photoshop we look at the chosen colour on the image and, moving the mouse in this area, we continue till we will see the desired result in the window showing the selection. To modify the amplitude of the selection, change the value of "tolerance". It is convenient to note the parameters of the selection (colour and tolerance) for remaking or other modifications.
   Once the desired colour has been selected, fill the corresponding area with black and its inverse with white. Change also the colour mode to "grayscale".
   By means of the tool "levels", or with "automatic levels", expand the grayscale so that it goes from deep black to pure white.

   These are the selections taken from the original in colours:

red *

pink

yellow

blue 1

blue 2

black

* Note: the red selection is the combination of two selections, one for the sunny part of the facade, the other for the shadow part. The printing of the black will contribute to differentiate the two parts.

It is possible, again with Photoshop, to determine the general effect of the separation and verify that there are no "forgotten" hues. To do this, it is neded:

to modify in each image the colour mode from "grayscale" to "RGB"

to copy each selection in a new file, with an image per level

each selection will be coloured (image> adjustments> hue/saturation, then choose "colorize") with its proper colour

in the levels window, set the method of each level from "normal" to "multiply"

You will see an image showing the effect of the superimposition of the separation negatives. Naturally, when you will make a gum many things will change, for example the colours could not be perfectly transparent, nor they will be identiocal to those set up in Photoshop, the contrast will not be the same, etc.

The selections have to been inverted and tilted horizontally in order to produce the negatives. Mine have been printed on Epson Photo Quality Glossy Paper (S041126).
Some previous proofs indicated that the grayscale would be modified in order to have a 130 black output instead of 0, and a 240 white instead of 255. With such a negative I expose for 20' a paper coated with gum and ammonium dichromate 28% (gum/dicr 1:1). The development needs 30' in water at 25°C, with three water changes.

Registered printing:

Paper is Fabriano artistico rough, 180 g/m², previously shrunk in hot water and sized two times with gelatine 2.5%.

Gum arabic solution is 30% (30 g of gum in 100 ml of total solution).

All the colours are in powder form: natural earths, synthetic oxides and ceramic pigments (fire resistant).