(37) T. C. Roche gives the following method of making fine blue prints on paper, wood, canvas, etc, and only requires washing to fix properly. First solution: red prussiate of potash, 120 gr.; water, 2 oz. Second solution: ammonio citrate of iron, 2 oz.; water, 140 gr. The solutions should be made separately, and, when dissolved, mixed and filtered; then pour it into a dish, and float plain photographic paper on it for 3 or 4 minutes. When the paper is dyed, it will keep for months. Print in the sun for 8 to 10 minutes; then simply wash the paper under the tap with running water. The result will be a strong blue picture on a white ground. The addition of a little gum arabic water to the above solution, when made, will render the colour of the picture richer and the whites purer.
(38) Charming Whittaker describes an ingenious apparatus of his own contrivance for obtaining prints which shall be free from the defects produced by the printing-frames in common use. In the latter the pressure required to keep the paper in contact with the glass is applied at the periphery of the glass and of the back-board. This causes the centre of the glass to spring, and the contact of the paper and the negative being consequently imperfect, only an imperfect print can be expected.
The ordinary printing-frame used in photography is an excellent one for small negatives, when the back-board is well cushioned with cotton-flannel or woollen blanket, but in large sizes the plate glass is expensive, hard to handle, and liable to be broken by any uneven pressure. The improvement described is simply the adaptation of an air-cushion in place of a solid pad to ensure the perfect contact of the sensitive paper with the negative. The backboard is in one piece, being clamped to the frame that holds the glass, and is covered by a piece of manila paper coated with shellac varnish. Over this a sheet of the thinnest rubber is laid, and then a single thickness of cotton cloth, the whole being secured at the edges by strap-iron, fastened by bolts to the wooden frame.
The air-cushion is charged by blowing from the lungs. A rubber tube, provided with a glass mouthpiece, leads to a T, one end of which is connected with a nipple introduced through the back-board and the other end of which is connected by a rubber tube with a pressure gauge.
Prints made in this apparatus are entirely free from blue lines, or any blotches or blemishes due to imperfect contact between the negative and the paper. An important addition to the apparatus is that by which it is so adjusted as to have the surface of the glass always at right angles to the direction of the sun's rays. This is secured by providing two adjustments, one by which the glass is rotated in a direction opposite to that of the rotation of the earth, and another in which a secondary axis is mounted on the primary one and at right angles to it, so that it can be rotated to the required position when the sun is either north or south of the equatorial plane.
In addition to this the author gives some notes of experiments on the sensitising liquid, and the proportions for one yielding the best results. When the process was first introduced into America from France, the formula in use was as follows:
Red prussiate of potash, 8 parts; citrate of iron and ammonia, 8 parts; gum arabic, 1 part; water, 80 parts.
Beginning with the proportions - Red prussiate of potash, 10 parts; citrate of iron and ammonia, 1 part; water, 50 parts - different solutions were made up to the proportions: Red prussiate of potash, 1 part; citrate of iron and ammonia, 10 parts; water, 50 parts.
The plan followed was to coat a sheet with a given solution, and after cutting it into strips to expose them all to direct sunlight and withdraw them one after another at stated intervals, thus giving a different time of exposure to each one.
The conclusion drawn from these experiments was that each mixture would give a deep blue after each exposure, that this would turn to a gray if over-exposed, but that 2-3 minutes' deviation from the proper time of exposure does not materially alter the result. The best formula, he finds, would be: Red prussiate of potash,
2 parts; citrate of iron and ammonia,
3 parts; water, 20 parts.
An excess of the prussiate lengthens the time of exposure, while that of the citrate shortens it.
(39) Below is a formula which Dr. L. H. Laudy of the School of Mines has prepared and long used with excellent results: -
Solution No. 2.-53 grammes (816 grs.) of citrate of iron and ammonia dissolved in 230 cubic centimetres (8 oz.) of distilled water. These solutions must be kept separate.
When ready to prepare the paper, mix equal parts of Nos. 1 and 2 and apply to the paper either with sponge or soft cloth, and hang up to dry. These operations must be conducted in a dark room. As soon as the paper is dry, place under negative or tracing, and expose to direct sunlight. After printing, place in water and wash thoroughly.
(40) A black process, which will compete for favour with the above blue process, is given in the Photocopie of A. Fisch. The process is technically known as heliography, is simple, and inexpensive, while the prints are ink-black, and are made from drawings or positives and negatives. We owe this process to Poitevin, but it has been slightly improved.
Dissolve separately: -
1. Gum arabic . 0 13 dr. Water ... 17 oz. 0
2. Tartaric acid. 0 13 dr. Water ... 6 oz. 6 dr.
3. Persulphite of iron . 0 8 dr. Water ... 6 oz. 6 dr.
The third solution is poured into the second, well agitated, and then these two solutions united are added to the first, continually stirring. When the mixture is complete, add slowly, still stirring, 100cc. (3 fl. oz. 3 dr.) of liquid acid perchloride of iron at 45° Baume. Filter into a bottle and keep away from the light. It keeps well for a very long time.
Here especially it becomes necessary to select a paper that is very strong, well sized, and as little porous as possible. By means of a large brush or sponge apply the sensitising liquid very equally in very thin and smooth coats; then dry as rapidly as possible with heat without exceeding, however, a temperature of 55° C. (131° F.). The. paper should dry in obscurity, and be kept away from light and dampness ; notwithstanding all these precautions it does not keep well long, and if it is desired to act with some certainty it is better to have a stock to last only a fortnight. Freshly prepared it is better than a few days afterwards. It should be of a yellow colour.