This is somewhat easier to work than the iodide process, as there is less chance of the dyes bleeding; but it is more particularly suitable for the red and yellow constituent pictures, though it can be used for the blue as well; it was first suggested by Traube in 1916.
The process is as follows: the silver image is treated with a solution that converts it into a mixture of silver and cuprous ferrocyanides, both of which are insoluble in water: the copper salt is the mordanting agent, the silver ferrocyanide having but little power in this respect.
It is possible, therefore, to fix out the silver ferrocyanide, leaving the copper salt and the dye. As the former is very transparent, it gives but little trouble in the superposition of the pictures. As stated, the only objection to it is that it has a reddish tinge and this dulls the blues and makes the yellows more of an orange shade, though this last causes but little trouble.
Copper toning processes for bromide prints have been known for a great many years and any of the old formulas may be used, but the following is particularly suitable:
Cupric sulphate 20 g.
Potassium ferricyanide 15 g.
Potassium citrate 120 g Ammonium carbonate 10 g.
Water to 1000 ccm.
In the first place the ferricyanide must be in clear ruby crystals, without any adherent yellow powder, which is the ferrocyanide. The commercial salt is rarely sufficiently clean, and the best thing to do is to weigh out a little more than the correct quantity, say 15.5 g, place this in a small beaker and add about 100 ccm distilled water, stir twice and pour off the water. The clean copper salt and the citrate should be dissolved in half the water, which may be used warm with advantage, the ferricyanide dissolved in 250 ccm, and the ammonium salt in the remainder. Warm water must not be used for the latter. Mix the ferricyanide and the ammonium solutions and add to the copper. The positives should be immersed in the complete solution until there is not the slightest sign of a black image to be seen when examined through the glass. This may take from fifteen to twenty minutes. They should then be washed well and may be immersed in a 20 per cent solution of hypo for ten minutes and well washed. After toning they are of a somewhat opaque reddish brown color when examined by transmitted light; but after fixing they should be a clear transparent red. After being freed from hypo, they should be immersed in the dye solutions, with glacial acetic acid, as recommended for the iodide process, and should be left therein till sufficiently stained up. If at this stage the image appears too opaque, or the blue too dull, as it may, the positive should be immersed in a 10 per cent solution of sodium acetate plus the same quantity of tannin for ten minutes, then rinsed and dipped into a 2 per cent solution of caustic soda, which dissolves the cuprous ferrocyanide, leaving nothing but the dye image.
This is actually an old process, but its use as a mordant was proposed by Namias. We have here the same basic reactions as in the copper process, that is, the precipitation of insoluble vanadium ferrocyanide in situ, with the formation of silver ferrocyanide; the former salt is yellow, while the silver ferrocyanide is white, and again it is the other ferrocyanide that acts as the mordant and not the silver. The usual vanadium salt employed is the chloride, an expensive salt that is so hard to crystallize that it is sold in syrupy form. It has the great disadvantage that it forms silver chloride, which is very opaque. The author has worked out a method of making vanadium oxalate which is comparatively cheap, and much more satisfactory in use, as it gives images so transparent that it is rarely necessary to fix them. The following is the method advised: place in a porcelain evaporating dish 100 g ammonium vanadate and 460 g pure oxalic acid, then add gradually 500 ccm distilled water, and heat. The thick cream, which is white, gradually becomes more fluid and turns first yellow and then green and the vanadium goes into solution; continue the heat till the solution turns blue, then make up the bulk with distilled water to 1477 ccm. This will be a 20 per cent solution of vanadium oxalate with a slight excess of acid. The sulphate may be made in the same way, substituting 197 ccm of pure sulphuric acid for the oxalic; this should form a blue solution, but if not it may be heated, and the bulk in this case also made up to the same as before. Both solutions contain the same ratio of vanadium and they will keep indefinitely in the dark, and it is immaterial which is used. The actual bath is:
Vanadium oxalate solution 50 ccm.
Oxalic acid, saturated solution 50 ccm.
Ammonium alum, saturated solution 50 ccm.
Glycerine 50 ccm.
Potassium ferricyanide, 10 per cent solution 10 ccm Distilled water to 1000 ccm.
The vanadium, oxalic acid and alum should be added to half the water, the glycerine and ferricyanide added to the remainder, and the two solutions mixed. The previous comments as to the ferricyanide apply equally in this case.
The positives should be immersed in the bath until all signs of a black image have disappeared when examined from the glass side, and they can then be washed and dyed up. After treatment with the above bath, the positives will be found to be of a yellowish-orange color and are as a rule sufficiently transparent without fixing.