Bromoil printing and bromoil transfer

by Emil Mayer

To make the swelled grain useful, the secondarily swollen points of the film must permanently retain the difference in swelling which has been imparted to them.

To attain this end, I start from the fact that portions of the gelatine which are treated with alkaline solution will swell much more in a bath of warm water than spots which have not been thus handled. If, therefore, the desired grain can be applied to the film by means of an alkaline solution, all the elements of the grain will swell up more strongly in the water bath than their surroundings, and will therefore protrude above the rest of the film and thus attain and retain a better degree of swelling than the latent tanned image.

The next step was obviously a practical treatment of the film by spraying it as evenly as possible with extremely fine drops of an alkaline solution. It soon appeared that the greatest attention must be paid to the type of apparatus with which the spraying was to be done. Any atomizer whose spray combines fine and coarse drops is useless. Any atomizer which is worked by blowing with the mouth or by intermittent blasts of a pump is unsuitable, for at the instant when the stream of air is interrupted, a certain quantity of liquid remains in the mouthpiece and is thrown out by the next blast of air in the form of coarse drops. Therefore, only continuously functioning atomizers can be used, preferably those types which are actuated by double rubber bellows. Only with such atomizers is it possible to count with a fair degree of certainty on the production of a system of uniformly fine drops. Ammonia, which has previously been generally used in bromoil printing as a swelling agent, cannot be used to produce such a grain, because the ammonia gas volatilizes in great part in its passage through the air. A five per cent solution of potassium carbonate has been found to be most satisfactory.

The next question is at what stage of the process the swelled grain should be produced. Making it on the dry print is not permissible, because the droplets are taken up too greedily by the dry film and diffuse quickly and irregularly. The safest method of working is to place the bleached and dried print in cold water until it becomes limp, then blot it off until quite dry on the surface, and then treat it.

The practical method of producing the swelled grain is as follows: the print, which has been swelled in cold water and thoroughly dried off, is placed on a horizontal support and the atomizer set in action; as soon as it works with complete uniformity, it is passed back and forth across the print as evenly as possible under continuous observation, until the whole print is uniformly covered with a layer of extremely fine drops. The most important precaution is the continuous observation of the print while the spraying is being done, and this is best done by having the light fall on the print at as small an angle as possible. The practical way to do this is as follows: the print is laid on a table near the window. The operator sits in front of the window and gets both his eye and the atomizer very slightly above and in front of the print. Under these conditions there is a reflection of light in every single drop, which makes the observation of the distribution of the drops very easy. At the instant when the whole film seems to be uniformly covered with dew, the atomizer is quickly turned away from the print.

It is necessary to be thoroughly familiar with the action of the atomizer which is being used; with most atomizers the finest drops, on account of their lightness, fall downwards not very far from the mouthpiece, while others project their finest drops to a greater distance. The sprayed print, which naturally cannot be touched on the film side, must be left undisturbed for a certain period, which must be determined by experiment, for it depends, among other things, on the temperature of the room and the peculiarities of the paper which is used. An approximate idea may be had by considering these points: the longer the potassium carbonate solution lies on the print, the more the finest drops evaporate, while somewhat larger drops continue their action, so that the grain becomes coarser through longer action. A coarse grain can also be obtained by the use of coarse drops. If the drops are allowed to dry completely, the diffusion produced during this longer time results in an extra swelling of the whole film, without any grain effect.

After a sufficient time has elapsed, the potassium carbonate solution is removed from the film by rinsing or blotting off, and the print is swollen to the necessary degree. It is obvious that much lower temperatures must be used for this than if the print had not been treated with the graining solution, for the drops of the potassium carbonate solution cover a considerable fraction of the surface of the print. The fact that the greater portion of the surface of the print has been affected by the spray makes it apparent that the alkaline solution cannot be replaced by a tanning solution, for the greater portion of the film would become less capable of swelling if such a solution were used, and therefore the latent tanned image would be destroyed. When the print is blotted off after swelling, it should show a scarcely visible relief when looked at by light falling from the side. The actual effect of the grain cannot be perceived until after the inking has been done.

The prints obtained in this way have, if the treatment has been successful, a very beautiful grained structure which extends over the lights and shadows quite evenly. It is possible to work on such a print quite normally without having to harmonize the degree of relief and the consistency of the ink with great accuracy. Especially is it possible to work up any given part of the print as long as desired with the brush without endangering the structure; on the contrary, it is improved by this treatment. For bromoil transfer, there are the following special advantages: every new transfer has exactly the same coarseness of grain, if this is imparted to the film once for all. In combination transfer, the grain persists in spite of the fact that several impressions are made on the same paper, because the swelled-grain elements are re-impressed in the same spots, if the registration is accurate.

Finally, it might be well to mention a few causes of failure which frequently occur in the first experiments. If the print appears to be covered with many small white spots at a certain distance apart but not in contact, the grain is too heavy and therefore does not take up enough ink. In this case, after rinsing, the print may be sprayed once more, carefully and not too heavily.