583. There is, unfortunately, no change in the appearance of prints to show when they are completely fixed. It is desirable, therefore, to ascertain by experiment the time necessary for fixing the various papers in use.
The following method may he adopted. A strip of the paper to be fixed should be taken and immersed, for about half its length, in a solution of potassium iodide, of about x per cent strength. The chloride or bromide of silver will be thus converted into light yellow iodide of silver, strongly visible in contrast with the whiteness of the silver salts not so converted, and also much less soluble than either the bromide or the chloride of silver. The time necessary for the disappearance in the fixing bath of the line of demarcation, between the iodized portion and the part left unchanged, is certainly longer than the time necessary for fixation. It must be borne in mind, however, that the fixation of an isolated strip is much more rapid than the fixing of a number of prints one on top of the other ; they protect each other against the action of the solution. If fixing is done for twice the time ascertained by this test, it may be taken as complete.
It is well to ascertain from time to time, either by repeating the test just described or by one of the methods previously indicated (§ 498), that the useful life of the bath has not been overstepped. This is a matter of more vital importance than in the fixation of negatives or lantern-plates.
The recovery of the silver from used fixing baths can be done by the methods already given for fixing solutions for negatives (§§ 412 to 414).
584. Intensification. The intensification of a transparency, or a print on paper, allows of increasing the strength of a print when develop ment has yielded insufficient contrast, or of remedying an unpleasant tone.
Most of the toning processes described in the succeeding paragraphs strengthen the image, and can be used for intensifying when a black tone is not essential.
Among the various methods of intensification described for negatives, the only one which is suitable for strengthening prints on paper or transparencies is the chromium (§ 454). The use of mercury intensifiers should. be especially avoided ; they spoil the trans parency of the shadows, increase the graininess to a disagreeable degree, and only yield per manent results when the whole of the mercury salts formed in the image have been reduced to the state of metallic mercury.
585. Reduction. The various reducers given for negatives (§§ 457 to 464) can be used equally for transparencies, under the same conditions and by observing the same A very complete study of the reduction of prints made on the different types of develop ment papers has been carried out by L. A. Jones and C. E. Fawkes (1921). It contains a number of photometric measurements of prints before and after reduction, and shows that all reducers do not act in the same manner on the com paratively coarse-grain images of bromide prints and on the fine-grain images of gaslight and chloro-bromide prints.
An acidified solution of permanganate, which acts as a surface reducer on negatives (§ 460), reduces proportionately all the densities on almost all papers, and therefore best compen sates for over-development. It should be used of much lower strength than for negatives, e.g. the following, made up at the time of use— Potassium permanganate, .1°,4, solution if dr. (so c.c.) Sulphuric acid, 1% solution . f dr.(35 c.c.) Water, to make • . . 20 oz. (1 .000 C.C.) If the whites of the print become slightly stained to a brown tint, which is not very likely on account of the great dilution of the reducer, the print, after a rapid rinsing, should be immersed for a few moments in a very weak solution of sodium bisulphite.
For reducing the light tones without pro ducing any appreciable effect on the heavy deposits of the image, Farmer's reducer may be used very dilute (§ 459), or, alternatively, a mixture of iodine and cyanide. The latter is preferable, as it leaves no trace after reduction, while ferricyanide frequently leaves a yellow coloration which, however, will usually dis appear in a few minutes in a new acid fixing bath.
Formulae for these two reducers are here given. They are practically equivalent, and may be used with advantage for prints taken from negatives with insufficient contrast for the paper employed when the exposure has been chosen for obtaining the maximum, black. Each of these reducers should be pre pared in a small quantity only, and at the time of using ; their activity disappears in a few minutes.
Farmer's Reducer— Hype., so% solution 5 oz. (250 c.c.) Potassium ferricyanide, 1% solution 5 oz. (250 c.c.) Water, to make . . 20 oz. (1000) Iodine and Cyanide' Reducer— Iodine, x % solution 2. 9i dr. (bo c.c.) Sodium cyanide, 1% solution . if dr. (so c.c.) Water, to make . . 20 oz. (1,000 C.C.) One or other of these solutions is generally used for clearing the margins of prints which are slightly veiled or defaced by stress or abrasion marks (§ 548) ; also for local reduction with a For reducing the deep shadows of a print without appreciably weakening the details in the light tones, the ammonium persulphate reducer should be employed (§ 463). For avoid ing the deepest shadows being reduced to such an extent that they possess less strength than the intermediate tones, it is necessary, with some very fine-grain papers, to add a small quantity of a chloride to this reducer. Only the quantity required for immediate use should be prepared.
Persulphate Reducer for Prints on Development Papers Ammonium persulphate oz. (2 grin.) Sulphuric acid, s% solution 4f dr. (3o c.c.) Sodium chloride (common salt), 1% solution . . 4 dr. (25 c.c.) Water, to make . . 20 oz. (.1,000 C.C.) This reducer is used specially for reducing the exaggerated contrasts of a print from a nYgative too strong in contrast for the paper employed, when the exposure has been timed to obtain full details in the light tones, and thus causing the shadow details to be lost in the black deposit.