The following are the details of the principal methods of intensifying with mercury :— First method A. Mercuric chloride 120 grs. 25 g.
Hydrochloric acid 24 mins. 5 ccs.
Water . . . io OZ. i,000 „ B. Liquor ammonia: . oz. 25 ccs.
Water . .
• 5 500 „ Immerse in A until the image is thoroughly bleached, and then, after treating with acid and washing as above described, immerse in B until thoroughly blackened. Wash, and dry. This method is not recommended.
Second method— The negative is bleached in solution A (above given), treated with acid, washed, and then thoroughly blackened in— Sodium sulphite . I oz. 25 g.
Water . . . io „ 25o ccs. This solution must be used at once, as it will not keep. The intensified negative is a good black colour, free from veiling, and the result is permanent ; but the increase of density is very slight.
Third method H. Chapman Jones advocates this method. The plate is bleached in solution A, and then, after the acid baths and washing, is re-developed with ferrous oxalate. From the developer the negative passes to a weak acid bath for two min utes, and is then washed. This approximately doubles the printing value, and may be repeated as many times as desired, a corresponding increase of strength resulting each time. Care must be taken that the ferrous oxalate developer is freshly prepared and in good condition, or the negative will be badly stained.
Fourth method— Bleach in solution A, and then, after the acid baths and washing, re-develop with amidol or any clean-working alkaline developer. Pyro is not suitable. The result is similar to that pro duced by the third method. The image is a good neutral black, there is no tendency to staining or loss of quality, and the result is permanent.
The increase of density is rather less than that given by the third method, but the operation may be repeated with a similar increase of strength.
Fifth method— Bleach in C. Mercuric chloride . 120 grs. 27 g.
Potassium bromide 120„ 27 „ Water . . . io oz. i,000 ccs. Treat with three acid baths, wash, and then immerse in a freshly prepared solution of D. Sodium sulphite . f oz. 55 g.
Water . . . 5 „ soo ccs. until thoroughly blackened. The resulting nega tive is a good neutral black without the slightest staining or loss of quality, and it is quite per manent. The printing value is approximately doubled, and repeating the process will not pro duce any increase of strength.
Sixth method— The working is as in the fifth method, but the bleached image is blackened in E. Sodium sulphite . . oz. 55 g.
Sodium carbonate. f „ 55 >, Water . . . 5 „ 50o ccs.
This solution will keep indefinitely. The result is exactly the same as that produced by the fifth method.
Seventh method— Bleach in solution A, and then, after the acid baths and washing, re-develop in amidol or other clean-working alkaline developer. The gain in printing value is slightly less than that produced by methods 5 and 6, but the strength may be increased by repeating the operation as many times as desired.
Eighth method. Monckhoven's process— Bleach in solution A, and then, after the usual washing, etc., blacken in F. Silver nitrate . . 5o grs. 23 g.
Potass. cyanide (about) 50 „ 23 „ Water . . . 5 oz. i,000 ccs.
The silver nitrate is dissolved in the water, and potassium cyanide added, d. little at a time, until the precipitate that forms is just dis solved. The plate must be taken from the cyanide solution directly it is blackened through, as a reducing action on the shadow details begins as soon as the blackening is accom plished. If the first washing is not rapid and effective, there will be a similar reduction. It is
impossible to prevent a slight reduction in the extreme shadows. The original printing value is rather more than doubled, excepting in the shadows, where the strength is actually reduced. The operation may be repeated if the density is not sufficient, and a proportionate increase of strength will result each time.
Ninth method. Mercuric iodide, Lunuere's method G. Sodium sulphite 20 grs. 46 g.
Mercuric iodide . I „ 2.3 „ Water . . . 1 oz. i,000 ccs. The sodium sulphite must be dissolved first, and then the mercuric iodide, which is insoluble in water, added ; and the solution well shaken until the iodide is dissolved. The solution must be prepared when required, as it will not keep. Immerse the negative in this solution, rocking the dish to ensure even action, and a steady gain in strength will result ; the maximum intensity is attained in about seven or eight minutes, but the negative may be withdrawn at any time. Afterwards the plate is washed, immersed in the acid baths as already described, washed, and then re-developed in any alkaline developer. Pyro or any other developer may be used, the printing value not being affected by the choice of developer, but when gyro is used the plate is not quite so clean as with other developers. If the plate is not re-developed, the image will gradually bleach. If the action is carried out fully, repetition will not produce any increase of strength ; but if a smaller degree of intensifica tion only is attained at the first operation, repe tition at any future time will complete the work and produce the same strength that a full action would have secured in the first instance. The intensity is greater than that given by any other form of mercurial intensification at one oper ation, the printing value being multiplied by 3.
In all these methods of mercurial intensifica tion, a moderate washing after the final operation of re-developing or blackening completes the work.
In all excepting the ninth method (Lumiere's mercuric iodide process), the treatment with the mercuric solution must be thorough and the operation must be taken as far as it will go, the completeness of the work being judged by the thorough whitening of the image. If this bleach ing is not thorough, the intensification will be irregular, some portions being fully strengthened and others only partially. It follows, neces sarily, that the increase of strength is a fixed quantity ; one degree of intensification only can be obtained from any negative, neither more nor less, excepting in those modifications which can be repeated, in which cases the result of repetition is a series of regular steps.
In process work, the negatives have always to be intensified in order to get the necessary intensity for photo-mechanical printing. In wet collodion negatives for line work, the lead intensifier is generally used, followed by ammo nium or sodium sulphide for blackening. For half-tone negatives, the copper bromide intensifier is more generally used, both with wet collodion and collodion emulsion, though the lead intensifier can also be employed. With both intensifiers the method known as ” cutting " is adopted. This consists in treating the plate with a solution of potassium iodide and iodine, and afterwards applying a solution of potassium cyanide ; this cuts off the fringe of the dots or lines, and so sharpens up the image, at the same time clearing the transparent spaces. For dry-plate process negatives the silver cyanide intensifier is found best, and " cutting " is done with potassium ferricyanide and " hypo " solution.