If intensification is required, a solution of mercuric bichloride, followed by ammonia, can be used as in dry plate operations. If the image is very flat, or when black-and-white work is re quired, intensification with lead nitrate can be adopted, using : Lead nitrate . . . x drm.
Potassium ferricyanide . . r Water . . . . . io oz.
The plate is immersed in the above solution till sufficient density is reached, then washed under the tap till all yellowness has disappeared. The picture, ua most cases, need not be blackened when intensified by lead, as sufficient density is obtained without the use of an alkali. When absolute opacity is required, ammonium hydro sulphuret can be used, after all the yellow stain has been washed from the film. It must be noted that the lead intensifier has a drastic action and must be used only for flat pictures or for the reproduction of black-and-white draw ings.
Another method of intensification, known as re-development, is perhaps the best for beginners. When the image, after fixing, seems to be lack ing in contrast, the plate is rinsed and fresh developer, mixed with a few drops of a to per cent, solution of silver nitrate, is flowed over the film. The addition of the silver to the developer gives vigour to the image. After fixing the plate, mercuric intensification will give further con trast, if necessary. When dry, the plate should be varnished, as the collodion film is easily torn.
For ferrotype plates, a thinner film is neces sary, and the iodised collodion should be further diluted with sulphuric ether. A developer giving a white deposit (see " Collodion Positive ") should be used. The general procedure of sen sitising, developing, and fixing ferrotypes is the same as for wet-plate negatives.
In process work, a very clean working collodion is required, and at the same time one that gives great density. Further, the film must be tough to withstand intensification and reduction. Celloidin collodion of not more than 2 per cent. strength is usually employed. Pure solvents have to be used in order to avoid fog and scum. Ammonium and cadmium iodide and cadmium bromide with cadmium chloride are the general ingredients of the iodiser. A typical formula for collodion suitable for process work is Celloidin . . . I oz. 3.r g.
Alcohol ( 8o5) . . 40 1,136 ccs.
Ether (.72o) . . 6o 1,704 ,, The following is the iodiser : Alcohol (820) . . ro oz. 284 ccs.
Cadmium iodide . f 15.5 g.
Ammonium iodide . 18o grs. r r6 Cadmium bromide . 15 Cadmium chloride . 30 2 Iodine . . . to 6 Take one part iodiser to nine parts collodion, and allow to stand for ten to fourteen days. The silver bath is usually 35 to 4o grs. per oz. Development is with the iron developer ; fixing with potassium cyanide ; intensification with either lead nitrate (for line negatives), or with copper bromide (for half-tone), followed with ammonium or sodium sulphide. " Cutting " or reducing to sharpen up the dots or lines is gener ally resorted to, the solutions employed being iodine and cyanide. For stripping, the negatives are coated with rubber solution and then with collodion to which a small proportion of castor oil has been added to make it flexible. The glass plates are, as a rule, edged with rubber solution before coating with collodion, to make the latter hold.