+ 2NH, = NH,C1.
If ammonium sulphide be used instead of ammonia the mercurous chloride will be split up into mercuric sulphide and finely divided mercury, and the silver chloride will become converted into a form of silver sulphide. By this means we get a very dense negative, the process being more suitable for intensifying photographs of line drawings for photo-mechanical printing.
If the plate, after being bleached and washed, be treated to a solution of sodium sulphite, the action that will take place will be represented thus 4AgC1 + = Ag2 + Hg + NaCI 3Hg x 2AgNaSO, A darkening action, the blackened image consisting of silver mercury, will commence at once, and when sufficient density is obtained, the plate is removed and thoroughly washed. This is probably the most convenient and easiest method of intensifying, as the washing of the plate after treating with the mercury need not be prolonged. The sodium sulphite solution should be of about ten per cent. strength, and slightly acidulated with a little citric acid.
Another method of darkening the bleached image is with ferrous oxalate. With this the mercurous chloride is reduced to metal, as with sodium sulphite, but the silver chloride also becomes reduced, thus + 2AgC1 + + = + Hg, + + 4KC1 By this method greater opacity is obtained than with the previous one. To prepare the ferrous oxalate solution pour one ounce of a saturated solution of ferrous sulphate into a vessel containing five or six ounces of a saturated solution of potassium oxalate, acidified with a little oxalic acid. Next add three or four ounces of water, and the solution is complete.
Instead of ferrous oxalate, an old hydroquinone developer can be used with very successful results.
The following method of intensifying has also been recommended, one of its advantages over other methods being that it is not necessary to be so particular with regard to the perfect elimination of the hyposulphite :— Bichloride of mercury 6o grains Water 8 ounces To this is added a sufficient quantity of potassium iodide to nearly dissolve the red precipi tate first formed. About iso grains will be required. Next add 12o grains of sodium hyposul
phite in crystals.
In this solution the plate is immersed until sufficient density is obtained. It is then placed for from ten to fifteen seconds in a weak fixing bath. Well wash and dry. The resulting image will consist of a double iodide of silver and mercury of a reddish brown color, with good printing qualities.
The following intensifier was recommended by Farmer :— No 2 is added to No. and the precipitate formed is collected thoroughly washed, and added to Sodium hyposulphite 96o grains Distilled water 6 ounces The mixture is thoroughly well stirred, and then allowed to stand a few hours, when it should be filtered, and distilled water added to it to make up to i6 ounces.
To intensify, the plate is immersed in this solution for about five minutes. It is then drained and a ferrous oxalate developer applied. It is then washed and dried. Instead of the ferrous oxalate developer the following may be used— 4 grains iTled water 2 ounces Silver solution 6o minims with about 3o minims of a io per cent. solution of liquid ammonia .88o added immediately before using.
Uranium recommends the following method of intensifying with uran ium. A solution is made up after the following formula :- Uranium nitrate 15 grains Potassium ferricyanide i 5 grains Water 4 ounces In this the plate is immersed when the details in the shadows are first attacked, then the half-tones, and finally the high lights.
Another method is to first soak the plate in a solution of uranium nitrate (60 grains in io ounces of water), and then immerse it in a solution of potassium ferricyanide. The ferricyanide becomes converted into ferrocyanide, and the resulting image is formed of a compound of ferro cyanide of silver and uranium. With this process of intensification it will be necessary to entirely eliminate the hypo, otherwise a reduction of the uranic salt will take place, producing a chocolate colored stain. This stain can be removed by immersing in a solution of soda or ammonia, but the density which has been given to the negative will disappear.