The negative is immersed in it until the image becomes yellow throughout its thickness. After thorough washing it is blackened in a solution containing 0-25 per cent of silver nitrate to which about i per cent of sodium acetate has been added (so preventing the copper nitrate formed during the reaction from attacking the silver).
The silver salts, other than silver iodide (chloride precipitated in the gelatine due to the use of ordinary water and silver nitrate combined with the gelatine), are removed by immersion for about two minutes in a bath containing about I per cent of ammonia, which has no action on silver iodide.
The process is completed by reducing the silver iodide to the metallic state by means of a solution of hydrosulphite of soda containing a little bisulphite of soda, or by means of an amidol developer made alkaline with carbonate of soda.
456. Other Methods of Intensification. The methods of intensification described in the preceding paragraphs are amply sufficient for all practical requirements, so that we shall be content to mention only briefly some others which are sometimes used, or which are in themselves curious because of the means employed.
Intensification by precipitation of silver ac cording to the method already described for physical development before or after fixing (§§ 395 and 396) is especially suitable for fine grained images. It is often an advantage to precede this process by immersion in a very dilute solution of permanganate acidified with sulphuric acid (the reversal bath described in § 439 diluted twenty or fifty times with water), and to follow it by treatment with hyposulphite of soda. (This method of intensification may
be carried out before fixation.) Various methods which will later be described under methods of toning may be employed for intensification (uranium toning, quinone toning, arid dye-toning with a mordant).
A method giving great intensification but requiring rather delicate manipulation has been described by K. Hickman and W. Weyerts (1933) : a sulphide toned image (§§ 588 and 589) is placed in a solution of silver sulphite and, under the action of a strong artificial light, it progressively becomes intensified by deposition of silver. After intensification to the desired degree the image is rinsed, placed in a fixing bath and washed.
It has been suggested that a negative may be intensified by " doubling " the image in " car bon" on the gelatine of the negative by the Carbro process, or by an image obtained by dusting-on, the sensitive layer (hichroniated glue) being coated on the gelatine face of the negative, which has been previously varnished.
Lastly, it has been suggested that an image may be intensified by contraction of the film after detaching it from its support.