If the negative be in the state best described as nearly dense enough, careful washing in the first solution will give just the requisite density, and then a thorough washing and immersion in Ammonia...... 1 dr. Water........ 20 oz. will yield a result as perfect as possible. After the ammonia solution has done its work, the negative does not gather any more density, no matter how long it' may be left in. One precaution is, however, necessary during both stages, and that is, the dish must be kept gently rocked, or streaks are likely to form. (Brit. Jour. Photog.)
(6) The mercury intensifier for gelatine plates, now largely used by photographers, has been somewhat improved by H. J. Newton quite recently. The advantages claimed for it are simplicity, speed, and giving to the negative a good colour. The intensifier, combining mercury, potassium iodide, and sodium hyposulphite, sometimes gives to a negative a yellow colour, which makes it a slow printer. The solution will not keep well, but soon precipitates.
Newton's formula overcomes these objections. He first takes 10 gr. mercury bichloride, pulverises it in a mortar, and dissolves in 10 oz. water. He next dissolves 190 gr. potassium iodide in 3 oz. water, and gradually pours the same into the mercury solution. A red precipitate occurs, but will be redissolved when the whole amount of potassium iodide has been added. The 13-oz. concentrated solution thus formed is now diluted by the addition of 24 oz. water. The intensifier will keep clear for a long time, and so retain its strength.
To intensify, Newton pours a sufficient quantity of the intensifier into a tray, and immerses in the same the dry or dried negative. The action of the intensifier takes place in a few seconds, and the intensification is completed in 2-3 minutes.
The plate is then washed and immersed for a few seconds in a very dilute solution of sodium hyposulphite, again washed, and dried. Negatives in which there was very little detail in the shadows have been very easily brought up to good printing density with this intensifier. It is essential that the soda hyposulphite shall be eliminated from the plate before intensification. To avoid an extended washing for that purpose, Newton quickly dissolves out the hypo from the film by pouring over the latter, after fixing, a solution of 5-10 gr. lead nitrate to the oz. of water. Its action is easily observed by the formation on the film of a milky precipitate, which may be easily washed off.
(7) Dr. Nicol remarks that several of the published methods of mercurial intensification may be relied upon as both safe and practical; but the following, which has been repeatedly described, he believes to be the best: -
(a) Mercury bichloride .. 1 oz. Ammonium chloride .. 1 oz. Potassium iodide quant. suff.
Dissolve the mercury and ammonium salts in 10 oz. water, putting them both in together, and add sufficient of a strong solution of potassium iodide to dissolve the red mercury iodide formed by the first additions. Then make up the bulk with water to 20 oz.
(b) Silver nitrate .. 1 1/2 oz.
Potassium cyanide quant. suff.
Dissolve the silver in 5 oz. water and add sufficient of a strong solution of the cyanide to dissolve the precipitate formed by the first additions, and make up the bulk with water to 20 oz. The solutions will keep indefinitely, and, where very much intensification is required, should be used at the full strength; but when only a slight action is desired, (a) may be diluted to 1/2 or 1/3
The fixed and well-washed negative should be placed in a dish with sufficient of (a) to cover it, and keep in motion for a few seconds. Let the action proceed, examining the plate from time to time, till apparently sufficient - or, rather, a little more - density is produced. At this stage the negative will have the appearance of a rather dense but thoroughly good printing collodion image, and the operator may feel inclined to "let well alone." On well washing the plate, however, he will find the whole deposit has assumed a yellow colour, and the washing must be continued till that colour is uniform all over. When that change has been accomplished, the plate must be placed in another dish, covered with (b), and kept in motion for a few seconds as before. Gradually, beginning with the higher lights, the yellow will give place to a fine olive brown, and the action must be allowed to continue till the whole negative has assumed that colour. A final wash completes the operation.
Regarding the practical permanence of the image thus intensified, Dr. Nicol has no doubt whatever. He has practised the matter pretty constantly during the last 2 years, and there lay before him while he wrote a negative from which some hundreds of prints had been taken, and it was, so far as it is possible to judge, absolutely unchanged. (Brit. Jour, Photog.)
(8) Scolik, of Vienna, has recently experimented extensively with a soda sulphite intensifier, and recommends the following formula: -
(a) Mercury bichloride .. 1 oz. Potassium bromide .. 1 oz. Water...... .. 50 oz.
The above may be diluted 4 times its volume if desired, in order that the action may be gradual and less energetic. The fixed and well-washed negative is allowed to remain in (a) until the film becomes well whitened. If a small degree of intensification is desired, it should be left in but a short time.
The plate is next slightly rinsed off (a thorough washing not being required at this point), and immersed in.
(b) Saturated solution soda sulphite......5 oz.
The darkening action will be observed to take place gradually, as in the case when ammonia is used, and will impart a rich brown-black colour to the negative, which should be well washed; negatives thus intensified are believed to be permanent. Dr. Eder describes the following as the chemical reaction which takes place. The whitened negative contains mercurous chloride (calomel), and this is reduced to the metallic state by the sodium sulphite, just as appears to be the case when potassium cyanide is used; thus the method now described may be regarded as analogous with Monckhoven's potassium argento-cyanide method. Mercuric chloride is not reduced in the cold by alkaline sulphites, because stable double salts are formed; still, at a boiling temperature, reduction sets in, the mercurous chloride being first formed, and then the metallic mercury. The above fact explains why it is unnecessary to wash away all traces of mercuric chloride before treating with sodium sulphite.