Tuesday, September 9, 2008

Lithographic Transfers From Bromide Prints.

         The Bromoil process has for several years had an important application in the lithographic trades as a means of readily making enlarged or reduced productions of line or "stipple" copies. The method also lends itself to the production of coarse-grained halftone lithographs. In this process a negative is made from the original line or tone drawing, or from an existing reproduction.
From this a bromide print is made of the size required, which is subsequently treated by a modified Bromoil process so as to become transformed into a lithographic transfer.
         Details are given below of a method which has been produced after numerous experiments. These were undertaken by the writer with a view to obtain general reliability and ease in results.

Character of the Negative.

         The negative may be made either on a dry plate or by the wet collodion process. It must be quite sharp. Line negatives should be made with a fairly large stop, or there will be a slight diffusion of detail in the finer lines. This is due to the fact that the anastigmats lenses generally in use are designed primarily to work at large apertures. F /16 to //22 are about the correct stop to use.
         Half-tone negatives must have the dot formation well joined in the high-lights. The particular screen to use for half-tone work must be calculated. For example, if the print from the negative is to be enlarged two diameters and 75 lines per inch grain is required; the negative must be made with a 150-line screen.

Making the Print.

         The ordinary copying camera may be used for making the print, the negative being rigged up a foot or so in front of the copy board, which is covered with white paper, so as to reflect light through the negative. A better way, when work is to be done in quantities, is to use an enlarging lantern. Whichever method is adopted, care must be taken to focus quite sharp, and again to use a fairly large stop.
         The most suitable developer is the regular smidol or diamido-phenol formula, using plenty of bromide. The fixing bath must consist of plain hypo and water, and nothing more. Exposure should be just long enough to produce a full strength deposit in the finest lines. Development should be full. After fixing the print it should be washed for not less than ten minutes, and then dried.

Making the Transfer.

The print, when dry, is ready for bleaching. This should be done by means of the following bath :

A. Copper bichloride .............. 60 grs. 5 gms

Ammonium chloride ......... 240 grs. 20 gms

Hydrochloric acid, about ... 20 drops 2c.c.s

Water………………………l0 ozs 400 c.c.s.

B. Sodium bichromate…….12 grs. 1 gms

Water………………………2½ ozs. 100c.c.s

         For use take 2 ozs.(50 c.c.s.) A., ¼ ozs. (6c.c.s) B., and 4 ozs. (100 c.c.s.) water.
         The print should be fully bleached in about two minutes. Occasionally a strong print will fail to beach right out. The partly bleached portions will, however, take the ink quite well. After bleaching the print is washed for not less than 4 minutes in running water.
         While the print is washing the inking slab should be get ready. Take a little re-transfer ink on the end of a palette knife and rub it cut on an old litho stone, or other suitable slab, thinning it down with xylel or benzole. Turpentine is unsuitable for this process.
          The washed print is now blotted off, and laid on a sheet of zinc or glass. Take a fairly tough letterpress roller, or better a rubber-covered roller, and distribute the ink all over the inking slab, diluting with xylol until the roller has a tendency to skid over the surface of the slab. Now roll up the print with the roller in this condition. At first the print assumes a uniform grey tinge, and then as the xylol evaporates the stiffening ink leaves the whites and adheres more and more to the bleached parts. In a few seconds the maximum effect is reached and the rolling stopped.
         The print should at this stage appear full of detail and of a grayish-black colour. There may be a very thin film of ink left upon the whites. In order to remove this, take a piece of thoroughly wet cotton wool and rub lightly over the print until clean. The transfer is then ready for the lithographer.

Weak Prints.

         Sometimes a print is too weak in character for the bleaching solution to act with full effect. In this case it will be found that fine details do not ink up. Such a print may be saved by a re-development operation, as follows: Clean all ink from the surface with a piece of cotton wool moistened with xylol, and then put it in an ordinary smidol developer, such as was used to make the print originally. It quickly blackens, and should be washed for four or five minutes, when it may be re-bleached in the Bromoil bleacher. No fixing is necessary before re-bleaching. The print will be found to have received an extra dose of hardening action, and will usually ink up well.
          Inking up of the transfer by means of the Bromoil brush is favored by some workers. It is useful at times for the purpose of bringing out portions of a print which may lack detail. In order to use a Bromoil brush some re-transfer ink must be mixed with a mere trace of boiled linseed oil and the tip of the brush charged with this, no xylol being used. The charged brush is dabbed upon the required parts of the print until sufficient ink has been taken up, and the inevitable dirtiness of the whites removed with wet cotton wool. The print can 'be persuaded to take up more and more ink by adding a greater proportion of boiled oil. As a rule, however, attempts at faking of print* are not to be recommended.'

General Considerations.

         Almost any grade of bromide paper can be used for bromoil transfers. The most suitable is a matt smooth paper, which is coated on a substantial base. It is well to be sure that the emulsion has a fine grain. Glossy paper gives bright-looking prints, which, however, the lithographer finds difficulty in transferring to stone or plate, owing to the extremely high relief.
          Transfers may be re-inked and re-used a number of times, the limit being governed by the toughness of the paper base.
          Some grades of paper have a tendency for the gelatine coating to strip off during inking. This tendency may be minimized by using the bleacher given above. Lack of strength hitherto has been apparently, due to the softening of the baryts base on which the emulsion has been coated. By substituting ammonium chloride for the more usual sodium salt this defect is overcome. The object of hydrochloric acid in this formula is to enable ordinary tap water to be used. The acid neutralizes any hardness in the water. Sodium bichromate was found to be the most reliable chromic salt to use.

Stretching of Transfers.

         Sometimes it is important that the impression must be of exact size. In such cases the bromoil transfer process hitherto has been hardly feasible, owing to the tendency for the paper base to stretch unevenly. A bromide paper, known as Kerotype, has recently been placed on the market, which to a large extent overcomes this defect. It is a stripping paper i.e., the prints are first made on a bromide emulsion which has been coated on an impermeable base. These prints are then soaked in a mixture of spirit and water, and the emulsion is transferred by means of a gelatine solution to a suitable support, such as celluloid.


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