IN the processes now to be described the silver image is converted into some salt which exerts an absorptive action on basic dyes. There is no relief formation and the dyed image is imbedded in the original thickness of the gelatine, so that the whole film has to be transferred to the final support. Consequently it will be found that films give the least trouble.

The first process is the diachrome or iodide process, which was first described by Traube in 1907. The constituent positives are made in the usual way, and after fixing and washing are immersed in the following solution until the image is seen to be completely bleached:

Iodine 15 g.

Potassium iodide 50 g.

Glacial acetic acid 25 ccm.

Distilled water to 1000 ccm.

It may be as well to give the best method of making the above solution. Add the iodide to 100 ccm of water, then stir in the iodine and stir until completely dissolved, then add the remainder of the water and the acid to make the required volume. If the iodine and the iodide are added to the full quantity of water, it takes a very long time for the iodine to dissolve.

As a variant of this bath, the image may first be bleached with cupric chloride, by the action of:

Cupric sulphate 50 g.

Sodium chloride 20 g.

Water 1000 ccm.

Wash after bleaching, immerse in a two per cent solution of potassium iodide for fifteen minutes, and wash again. The first bath converts the metallic silver into a complex silver-copper chloride, which is then converted by the second bath into the corresponding iodide. Possibly an easier method, in which copper iodide is at once deposited on the image, is by the use of the following:

Cupric sulphate 50 g.

Water 500 ccm.

Dissolve and add:

Potassium iodide 33 g.

Water 100 ccm.

A heavy precipitate is at once formed, and strong ammonia should be added till a perfectly clear dark blue solution is obtained, when the volume should be brought up to 1000 ccm by adding more water.

Whichever bath is used, the bleached image should be immersed in a five per cent solution of sodium bisulphite for five minutes. The acid lye can be obtained commercially and can also be made as follows:

Sodium sulphite, dry 500 g.

Water 750 ccm.

Stir well and add, with constant stirring, and slowly:

Sulphuric acid 100 ccm.

This practically forms a 40 per cent solution of bisulphite, which is used as a decolorizing bath, as it instantly attacks the free iodine that is very tenaciously held by the gelatine, and decolorizes it. The bleached positives should be well washed and can then be dyed up.

In all these mordanting processes basic dyes must be used, whereas with the relief processes acid dyes are employed. The terms "acid" and "basic" do not refer to the acidity or alkalinity of the dye solutions. An acid dye is one in which the actual coloring matter is a color-acid, and usually it is combined with an alkaline base such as ammonium, sodium, potassium or calcium. Such dyes are, therefore, salts of color-acids. The color element in a basic dye is a color base combined with an acid, such as hydrochloric, etc.

The fact of having to use basic dyes rather limits one's choice and for the red we have fuchsin, rhodamin G and 3G, which are the best, then Janus red B and pyronin G. For the yellow, auramin, vesuvin, chrysoidin Y, thio-flavin and safranin Y. For the blue, Victoria blue B, night blue, methylene and thionin blues.

The actual dye bath should not be strong, certainly not more than 2.5: 1000, and it is advisable to add about five per cent of glacial acetic acid. The stronger the bath the more rapidly will it act, but the results may be rather flat and wanting in contrast; whereas with a weaker bath, although it takes a longer time, more brilliant prints are obtained. As soon as the pictures are sufficiently stained, they should be rinsed to free them from excess dye on the surface. The image will now consist of silver, plus copper or silver iodide, and the dye, and as a rule, unless rather thin images were obtained in the first place, which is preferable, they will look too opaque and muddy because of the silver iodide. To remove this they have to be fixed, and it is here that one is apt to strike trouble, as, on treatment with an ordinary hypo bath, the dye is apt to diffuse into the surrounding gelatine, or, as it is technically termed, bleed. To prevent this the following bath should be used:

Hypo 150 g.

Sodium acetate 50 g.

Tannin 50 g.

Water to 1000 ccm.

The prints should only be left in this until the images are sufficiently transparent, and then immediately washed. The positives should not be dense for this process, should be well exposed and rather thin, and must be quite free from fog. An acid chrome-alum fixing bath should be used, and after washing they should be immersed for fifteen minutes in a 10 per cent solution of formaldehyde, and dried without washing.