As a rule, a solution of about 5 per cent of anhydrous sulphite of soda (or io per cent of crystallized sulphite) is employed in a slightly acid condition ; the acidification is effected by the addition of bisulphite of soda or of an acid. Solutions which have been used must be thrown away after treatment of each negative.
The intensification becomes a little more energetic if, instead of converting the image into the mercurous-silver chloride, it is converted into the corresponding bromide. This may be done by bleaching in a solution containing equal weights of mercuric chloride and potassium bromide (about 260 gr. of each salt dissolved separately in io oz. of water and then mixed, i.e. 30 grm. of each in Soo c.c. of water). In addition, some of the irregularities of intensi fication are diminished, due to the fact that silver bromide is much less soluble than the chloride in solutions of sulphite of soda. 1 In either case the intensified image is quite stable.
450. Darkening with Ferrous Oxalate. By means of a ferrous oxalate developer (§ 346), an image which has been bleached in mercuric chloride is reduced completely to the metallic condition without exposure to light being neces sary. This process is so exact that satisfactory photometric measurements may be made of the lower densities after intensification, the propor tional factor for the increase of density being applied. If the gelatine is hardened and other suitable precautions are taken, including inter mediate washing, the whole set of operations (bleaching and blackening) may be repeated as many times as desired, the densities and contrast increasing each time with exact proportionality. 2 The developer may be used several times over.
This method of intensification is only to be recommended for certain scientific applications of photography.
451. Darkening with an Organic Developer. Common developing solutions are capable of darkening images which have been bleached in mercuric chloride or bromide. Due to the chem ical reducing action of the developer, the greater part of the metals which would be dissolved in solution of pure sulphite is reduced to the metallic condition, though the reduction is never complete. Amidol, whilst it is far from
giving the greatest intensification, gives very satisfactory proportionality between the den sities before and after intensification.' Whatever the developing agent, each portion of the solution should be used once only.
452. Various Other Methods of Darkening. A great number of reagents have been suggested for darkening images bleached in mercuric chloride ; hyposulphite gives only a negligible intensification, except for the accidental forma tion of sulphide which generally occurs as stains in the lighter parts of the image. 2 Sodium monosulphide is very largely employed for blackening intensified images on wet collodion plates, and in such circumstances gives excellent results ; its application to gelatino-bromide plates is, however, dangerous, because of the intense fog which appears if a little of the mercuric chloride has been retained by the gelatine. Other solutions which may be men tioned as capable of giving strong intensification are as follows : a very dilute solution of caustic soda, to which formaline has been added (Blake Smith, 1901), a solution of stannous tartrate prepared when required for use by dissolving a little stannous chloride in a dilute solution of tartaric acid (Main, 1901), and many other substances which are reducing agents in the chemical sense.
Special mention should be made of the process of darkening by means of silver cyanide (D. van Monckhoven, 1879), which is frequently used commercially in the copying of originals, since it reduces the lower densities (fog, etc.) at the same time that it increases the higher densities considerably. The darkening solution may he prepared by dissolving oz. of silver nitrate in about 10 oz. of water (25 grm. in 500 c.c.), and adding in small quantities at a time a solution of 175 gr. of pure cyanide of sodium' or of potassium in about 4 oz. of water (20 grm. in 200 c.c.) until the precipitate first formed is almost entirely redissolved. The volume is then made up to 20 oz. (I,000 c.c.) with water. The solution may be used several times over.