The advantage of this paper over the ordinary silver paper is that it is 5-6 times more rapid, is more brilliant, and very readily toned to any desirable colour. Any suitable paper of a firm texture can be used, but it is preferable to employ special Saxe or line plain photographic paper, or, instead, chalk paper used for Lichtdruck purposes. The paper is first coated with a weak solution of gelatine and water, 5 gr. of gelatine to each oz. of water, and then dried. It is next coated with a sensitised collodion emulsion, made after the formula devised by F. W. Gold-macher, as follows: - "The preparation of this silver chloride emulsion demands, in the first place, chemicals of the utmost purity, and a strict observance of the order in which the different compounds are made. A 2 per cent, collodion will answer if the coaling of the plate is proceeded with slowly. With 3 per cent, of cotton and the addition of 1/2 per cent. of castor oil, the film is stronger and more tenacious.
Ordinary plain collodion sold by dealers in photographic materials will answer, but should be diluted with ether to the proportions above stated.
Three separate solutions are now prepared as follows: -
Silver nitrate.. .. 20 grm. (308 gr.).
Water......20 cc. (5 dr.).
Alcohol .. .. .. 50 cc. (12 1/2 dr.).
Citric acid .. .. 5 grm. (77 gr.). Alcohol......70 cc. (2 1/4 oz.).
Strontium chloride.. 5 grm. (77 gr.). Alcohol......70 cc. (2 1/4 oz.)
Besides the strontium chloride, other chlorides may answer as well. Calcium chloride causes higher sensitiveness, but collodion prepared with it prints violet, while strontium proofs are more of a reddish tone, which allows a more correct observation in the subsequent toning process. A reddish tone allows us to see more distinctly the action of the gold bath. The citric acid solution (No. 2) is mixed with chloride solution (No. 3) and the whole added to 800 cc. (25 fl. oz.) of plain collodion. This chlorated collodion can now be sensitised by adding the silver solution (No. 1) very gradually and by frequent agitation, by a subdued light (gas or lamp light). The emulsion should be filtered through a thin layer of cotton, after standing a few hours. But as the silver chloride is held in suspension, the bottle should be vigorously shaken just before filtering, in order that the particles of silver may pass through. Shaking the bottle prior to coating is also necessary.
The paper is stretched on a board, and clamped by a light frame around the edges. The collodion is then poured on near one corner, and by giving the board a series of movements with the hand, the collodion gradually flows over the entire surface (similar to the coating of a glass plate with collodion), while the surplus is drained off at the opposite corner into a separate bottle. A special hole or niche should be provided in the lower corner of the frame to allow the collodion to easily flow off.
It is also better to use a thin collodion, and recoat the paper two or three times, running off the collodion of subsequent coatings at the opposite corner from that of the previous coating, which tends to produce a very even film. Paper coated very heavily is apt to roll up in the water in a very objectionable manner, and will crack when drying. By bending the edges of a sheet of paper over a glass plate, the paper can be very readily coated. It is also advisable to add additional alcohol and ether occasionally to the collodion, to replace that evaporated by frequent lifting of the stopper.
The paper must be dried in a well ventilated room. Pampness is very injurious. Quickly dried paper is also more sensitive, and prints with more brilliancy. It is preferable to cut the paper with a dull pair of scissors than with a knife, as there is less danger of injuring the film.
The prints should be washed before toning sufficiently to take off the surplus silver.
An ammonia sulpbocyanide gold bath without the addition of soda hyposulphite answers best for toning, and the latter operation proceeds very smoothly, provided the paper is not too old. If kept longer than a week, it is liable to tone very irregularly.
A great variety of tones' can be obtained, from a pronounced red to a distinct blue, according to the length of time the print is kept in the bath;
Goldmacher advises the following proportions for a toning bath: -
Gold chloride, 1 gr. water, 200 cc, (6 oz.).