PIGMENT PROCESSES 635. Advantages of the Carbon Process. The carbon process is unquestionably the most beautiful printing process that a photographer can employ when he is not aiming at a personal modification of the negative. It is one of the most flexible in regard to choice of colour and of contrast. It is also one of the few photo graphic processes which yield practically per manent images, and on any type of support at the choice of the photographer. In addition, it is one of the easiest processes to work. There exists a fixed opinion in the minds of those who have never worked this process that it presents innumerable difficulties and so is very little used. The absence of competition amongst the few manufacturers of materials, and the very limited markets, result in papers for carbon printing being sold at relatively high prices. But the cost ought not to exceed materially that of bromide papers. This naturally acts as a deterrent to the more general use of carbon printing.
636. Action of Light on Bichromated Gelatine. When gelatine is impregnated with a bicliroinate, dried in a current of air, and exposed to light, 1 a brown image is formed on a yellow ground. It can then be ascertained that the parts which have darkened to a brown tint have lost their solubility in warm water ; or, at least, can only be dissolved in water much hotter than that which would dissolve those parts which have been protected from the action of light. The property of swelling in cold water is also very much reduced in the exposed parts. 2 Under normal conditions the proportion of chromium oxide in combination with the gelatine is much greater than that which combines with gelatine tanned by immersion in a solution of chrome alum (Lumii_`!re and Seyewetz, 1905).
While in a liquid condition, or even when semi-liquid in the form of a jelly, bichromated gelatine will only darken after an exposure to light lasting several days.
The sensitiveness is so slight that it can be regarded as non-existent.
Bichromated gelatine which has been dried by being kept for several days in a box in which the air has been dried by lumps of fused calcium chloride, although it may still retain as much as 5 per cent of water, has largely lost its sensitive ness, and has, in addition, become very brittle.
Bichromated gelatine gradually becomes in soluble if kept in the dark. This insolubilization is much more rapid and much more complete when the gelatine contains a large proportion of bichromate, and when it has been kept in a warm and moist atmosphere. This spontaneous insolubilization (by the chromic acid present as an impurity in the bichromate or liberated by hydrolysis of this salt) is considerably retarded by the addition to the bichromated gelatine of small quantities of alkaline citrates and oxalates (R. Namias, 1903), or, better still, of neutral chromate which has not the disadvantage of decreasing the speed as do all the other adju vants suggested for the same purpose (E. Elod and H. Berczeli, 1936). Alkaline tartrates and lactates produce the opposite effect. A solution of gelatine to which bichromate has been added and which has then kept liquid for some time has its solubility considerably reduced.
Gelatine treated with chromic acid becomes completely insoluble instantly, and even loses its property of swelling in water.' Similar reactions occur with albumin, casein, gum arabic, various natural resins (gum lac dissolved in an alkaline medium), or artificial resins, and a large number of cellulose deriva tives, which thus lose by exposure to light the property of swelling or of dissolving in their usual solvents.
637. Chromates and Bichromates. The sensi tizing of gelatine for the pigment processes is done by using the bichromate 2 of potassium or ammonium. The deliquescent bichromate of sodium is very difficult to purify, and cannot be used with Potassium bichromate occurs in large orange crystals, unaffected by exposure to air, soluble in water to the extent of about 8 per cent at 50° F., and more than 50 per cent at 212', boiling point. It is insoluble in alcohol, and is precipitated from aqueous solutions when an appreciable proportion of alcohol is added.