697. Theory of the Carbro Process. The potassium ferrocyanide formed during the action of the mixture of potassimn ferricyanide and potassium bromide (§ 588, footnote) on the silver constituting the original image reacts slowly with the bichromate, reducing the latter to chromic compounds which insolubilize the gelatine ( 636), whilst the ferrocyanide reverts to A small fraction of the silver is attacked directly by the bichromate after immersion in the acid bath, but the chromic compounds arising from this secondary reaction insolubilize the gelatine of the bromide paper in situ, and cannot contribute to the formation of the final image ; the fact that this reaction removes a certain fraction of the silver from participation in the fundamental reaction ex plains the reduction of the contrast of the final image when immersion in the acid bath is prolonged.
The favourable effect of the acid in the matter of the preservation of the gradation of the high-lights appears to be as follows (F. J. Tritton, 1926) : The very hard gelatine of the bromide print has swelled to the maximum possible extent during its long immersion in water before being placed in contact with the carbon tissue. The soft gelatine of the carbon tissue, on the other hand, immersed for a short time only in a neutral bath, and then, at the last moment, in an acid bath which favours swelling, does not have time to swell much ; its swelling therefore continues after it has been applied to the silver image, at the expense of the water contained by the latter, which is thus attracted, by a species of suction, towards the layer of coloured gelatine. In this way, the diffusion of the potassium ferrocyanide formed in contact with the silver is directed towards the carbon tissue, in a direction normal to the surface of contact, and this is the reason why there is no appreciable loss of sharpness, as would necessarily be the case if the diffusion were not directed. It has also been found that the images are much less vigorous and sharp if the bromide paper is slightly swelled and the carbon tissue much swelled at the moment of placing them in contact.
By producing a very slight superficial tanning, the formaldehyde facilitates the insolubilization of the gelatine by the very small quantities of chromic tanning agents produced in the details in the high-lights of the image. Finally, the acetic acid provided for in some formulae allows the gelatine to swell without promoting ex cessively the direct action of chromic acid on the silver, as hydrochloric acid does."
(b) THE BROMOIL PROCESS 698. The Bromide Print. The conversion of a bromide print into an image in greasy inks was described by E. J. Wall and C. Welborne Piper in 1907.
The bromide papers prepared specially for this process have a thick layer of emulsion, made with a very soft gelatine ; failing ordinary bromide papers can be used, provided choice is made of those with a gelatine which gives good relief on swelling. It is well to choose matt papers on a stout, smooth base, with a coating able to stand transfers. For one thing, matt papers, as a rule, do not have the over-coating of gelatine used with the emulsion of glossy papers to protect it from abrasion (§ 548) ; for another, matt emulsions contain starch grains, which, on wetting, do not swell so much as the gelatine, and thus produce a granulation of the surface of the gelatine which greatly facilitates inking.
The lightest tones of an ordinary image on bromide paper do not usually come out in the final inked image, whereas the heavy tones are distinctly accentuated. Hence the working con ditions must be adjusted, so that the tones of the silver image range from a clear grey to a deep grey, and not from white to black. To obtain such an image, of sufficient depth and with only slight_ contrasts, ample exposure must be given with short development, but all fog must be avoided, as this would reduce the swelling.
For the same reason, old developer should not be used, since the oxidation products of development may cause a slight general tanning of the gelatine (§ 350). If a tanning developer (e.g. pyrogallol) is used, the quantity of sulphite needs to be increased in order to confine the hardening action to the gelatine of the image.
In order to allow of the maximum swelling of the gelatine after wetting and before inking, the silver forming the image should extend, at any rate in the blacks, the whole depth of the emulsion. An aid to this is the use of relatively short development, in a slow-acting developer, of a fully-exposed image, or, better still, an acid diaminophenol developer (§ 386), much appre ciated by some workers.
Fixing can be done in a bath acidified with sodium bisulphite ; the hardening of the gelatine thus produced lasts only a very short time. The fixing bath must on no account contain alum, which would reduce the permeability of the gelatine, and, consequently, its swelling.