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| dichromated gum | |
| The dichromated gum process or simply 'gum' (socalled on account of the use of the vegetable rosin 'arabic gum' colloid instead of the photographic animal 'gelatine'), was born about 150 years ago and has been largely successful for at least 40 years. Often it had a revival and to day is probably the most popular technique for experimentation, among the obsolete photographic processes.
The success is due to the simple way of procedure in comparison to others and to the endless possibilities to operate permitted both in addition or in subtraction on the copy.
How to proceed begins preparing a sort of "mush" made of an arabic gum' solution, toned with powder pigment or watercolour hue and "salted" with a sesitizer, to reach a proper concentration and 'thickness'. The best drawing paper - better if suitably prepared - is spread with the aforesaid mixture and rapidly dried by a mild warm. The dry sheet of paper is then exposed under a negative continuous tone film, to UV light in a vacuum exposure frame or UV lamp or directly to the sun, to create the positive image through the tanning (hardening) of the rosin proportionally to the amount of light it received. The sheet is then soacked in the 'stripping bath' or 'clearing' - not exactly a develop - that means floated face down in a bath of fresh water. The unhardened (or partially hardened) stuff, will dissolve into the bath, carring away the surplus pigment while leaving the image grasped to the paper. The tonal range of the gum process is rather narrow and so often it requires multiple layers of colored gum on the same printed copy. For this reason it is important to have sized paper (by a pre-shrinking) and pushpin holes at the corners of the negative mask, to permit a correct 'register' among the layers. The process, permits infinite operations either in the humid or in the dry fase, bat planned results need great deftness and good exercise. |
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