We have dealt so far with the various processes as totally distinct, but it will frequently be found that a combination of one or more of these processes will give excellent results. For instance, suppose the blue constituent picture is made by blue toning of a bromide print, then the red and yellow images may be made by transfer by the imbibition method.
There is only one disadvantage in using bromide paper for one of the prints, and that is, its unequal expansion, which may cause some trouble in registration. This may be overcome to a great extent by soaking the paper in water for half an hour before exposure, which gives it opportunity to fully expand. If a piece of thin celluloid be placed between the negative and paper, the latter may be squeegeed down to the celluloid and the loss of sharpness is so slight as to be negligible. Or, the negative may be given two coats of celluloid varnish and the paper squeegeed direct to it, and if the paper be stripped immediately after exposure and the negative wiped dry no harm will ensue. The paper may be treated before exposure with celluloid varnish, the back being painted rather thickly, which can be easily done if the paper is placed in a printing frame with a glass behind it to hold it flat, the sensitive side being inside, of course. Old celluloid films, freed from their gelatine, and dissolved in methyl alcohol with acetone and a little amyl acetate, make an excellent varnish.
If the constituent pictures have been formed by one of the mordanting processes, and are thus carried by the original thickness of gelatine, the best plan to adopt is to strip the films, and while this may at first sight seem a ticklish matter, it really becomes very easy with a little practice. The great danger is, of course, expansion of the gelatine and consequent lack of registration; but, if the following directions are carefully followed, there is little chance of this.
Place the plates to be stripped on a flat surface, and, with a straight edge and a sharp penknife, cut through the gelatine film right down to the glass, about an eighth of an inch from the edge all round; this considerably facilitates later operations. The actual stripping solution is:
Glycerine 37.5 ccm.
Water 37.5 ccm.
Hydrofluoric acid 37.5 ccm.
Denatured alcohol to 1000 ccm.
Care must be exercised in the use of the acid, as it readily attacks the finger nails and skin even in this dilute strength, and when strong it causes the most painful and persistent burns. As the solution attacks glass and enamel, it must be used in papier-mache or vulcanite dishes; on this property of attacking glass is based its stripping action, as it practically dissolves the surface of the glass and thus loosens the adhesion of the gelatine to the latter. It will be found advantageous to make up a stock solution of the above without the acid, and only add this just before use. Still more desirable is the abandonment of the acid altogether and its replacement by an alkaline fluoride. The commercial hydrofluoric acid is a 40 per cent solution, therefore, in the above quantity there will be actually 15 parts of acid. As we can set free the acid from an alkaline fluoride by the action of an acid, such as sulphuric or nitric, we may use either 30 g of sodium fluoride or 44 g potassium fluoride and 75 ccm pure nitric acid to obtain the same result. Therefore, the procedure would be to make a stock solution of the glycerine and alcohol and add the fluoride and acid just before use.
The positive should be placed on a level plate and a little of the stripping fluid poured on its surface and spread with a flat brush; this is more convenient than using a dish. In about five minutes the film will begin to loosen, and this can be easily determined by trying to lift one of the narrow margins of the film with the penknife.
If this comes away freely then one may proceed to strip the film; if not, a little longer time should be allowed. In no case must any force be used to lift the film; it is better to allow double or treble the time for the solution to act naturally, as any force will distort it and cause trouble in subsequent registration. When the film seems loose, gently lift the glass and allow the mixture to drain off slowly so as not to give the film a chance to slip; then pour on a little of the stock solution without the fluoride, leave for two or three minutes, and then drain this off in turn.
One may now use either waxed paper or celluloid as the transfer medium. If the latter is used, it should be thin and preferably rubbed over with a little vaseline; it requires very little of the latter, the merest dab just rubbed all over the surface and then well polished with a clean rag or two. Waxed paper will be found the easier of the two, but it must be smooth and free from folds, and larger than the film. It should be laid down on the surface of the plate and very lightly squeegeed, and then the paper may be lifted and it will bring the film with it away from the glass.
An alternative method, which obviates the use of the fluorides, is to use an alkaline formaldehyde solution, such as:
Potassium carbonate 100 g.
Formaldehyde 100 ccm.
Glycerine 100 ccm.
Denatured alcohol 300 ccm.
Water to 1000 ccm.
The carbonate should be dissolved in the glycerine and water and then the other ingredients added. The film should be cut round as before, immersed in this solution in a dish, and left therein for from twenty to thirty minutes. The progress of the stripping action is tested by the edges, as previously suggested. When the film proves to be loosened, lift the plates from the solution, drain, and immerse in a mixture of 400 ccm alcohol and 600 ccm water for five or ten minutes, then lift out, drain, and lightly squeegee down on the waxed paper.