Solutions of gum arabic and of arrowroot have been found the best vehicular substances for the sensitising liquid, but the former has proved to give better results than the latter. They are mixed in the following manner:
I. - Gum arabic .. .. 50 grm.
Distilled water .. 100 cc.
II. - Arrowroot .. .. 2 grm.
Distilled water .. 130 cc.
The latter is boiled in the usual manner to paste. Before use mix:
Sensitising liquid .. .. 2 parts
Gum solution I. .. .. 1 part
Sensitising liquid .. ..' 1 part
Arrowroot paste II. 1 part
The mixture is well stirred until it forms an entirely homogeneous liquid, and then it is filtered through muslin.
The sensitising solution is prepared as follows: To the normal ferric oxalate solution is added in the dark room, and keeping the dish rocking, as much of neutral ammonium or sodium oxalate as will just dissolve at the ordinary temperature. For this purpose will be required of the above named salts:
To normal ferric oxalate solution........100 cc.
Neutral ammonium oxalate ..........18 to 20 grm.
Or to normal ferric oxalate solution .. .. 100 cc.
Neutral sodium oxalate 15 to 18 grm.
By formation of the corresponding double salt the brownish-grey colour of the ferric oxalate changes to a beautiful emerald green. If the solution begins slightly to darken, this will indicate that the saturation is completed. As soon as this takes place, no more ammonium or sodium oxalate should be added. Keep the dish rocking for some further time, allow to set, and then filter.
The solution may be mixed as follows3
Ammonia ferric oxalate solution, or sodium ferric oxalate solution ..........22 cc.
Gum solution 1...........23 cc.
If ammonium oxalate be added, the prints will be of a more bluish tone; while, by the addition of sodium oxalate, they are rendered more brownish. The former gives somewhat softer prints than the latter. As to the sensitiveness, there has been found no marked difference between the two salts.
Potash chlorate may be added with the effect of increasing contrast, and this may be done in the same manner as with the old platinotype process.
The above given quantity of the sensitising liquid is sufficient for five sheets of the ordinary size. The exposure of the paper no longer offers any difficulty. As the results will be more satisfactory if the paper possesses a certain degree of surface moisture (as in the case of silver printing), it will be well to take the sheets to be worked, a few hours before use, off the tin cylinder, and to hang them up in the dark room. In placing the paper on the negative, it is, with this process, of course, not necessary to place behind it a sheet of vulcanised rubber sheeting or a piece of waxed paper.
1. The printing is carried on until the image has appeared in all its parts. The picture when taken out of the frame should not be darker than it should be when finished.
2. The printing is only continued until the picture is in general visible, but the delicate half tones are still wanting. The picture is taken out of the frame and kept separately. In about 1/2-2 hours the picture will be finished by itself, as the reduction of the platinous chloride, once introduced, is continuous in the dark; or the picture ' is developed out with a cold diluted solution of sodium carbonate.
A good solvent is the following: -
Saturated solution of soda carbonate ............ 5 cc.
Distilled water ........100 cc.
In this solution the picture is Immersed and allowed to remain until all the delicate details are to be seen, which will require only a few seconds.
3. Printing is continued about as far as with the ordinary platinotype paper, i.e. until only those parts of the negative which are clear glass, or nearly so, are distinctly visible. Then development is carried on as with the ordinary platinotype paper.
The pictures printed according to one of the methods 1, 2, or 3, are placed in
Hydrochloric acid........ 1 cc.
Water .. ............80 cc.
For the preparation of the solution of ferric oxalate the following operations are necessary:
1. Manufacturing the ferric hydrate.
2. Dissolving that substance in oxalic acid.
3. Determining the amount of iron and of oxalic acid contained in this solution.
4. Diluting and acidulating the same.
The method of preparing ferric hydrate is generally well known, but for the sake of completeness we will give a brief description of it. Ferric chloride, 500 grm., is dissolved in 5-6 litres of water, and when the solution has been brought to the boiling point, solution of soda is added, until it gives with litmus paper a distinctly alkaline reaction. For this purpose about 250 grm. of caustic soda will be found necessary. The. precipitate is then washed with hot water by decantation, until the wash water is no longer alkaline. It is next placed in a cloth, and by pressure freed from the greater part of the water. With the ferric hydrate thus obtained, which ought to have a sirupy consistency there should be mixed about 200 grm. of finely crystallised oxalic acid, and the mixture be then left to itself for a few days at a temperature of not more than 85° F., and in a place completely protected from the light. Under these circumstances the formation of ferric oxalate will go on steadily.
Some recommend the promotion of this process by digesting the mixture for some time at a high temperature. This we are decidedly opposed to, since by heating for even a very few hours to 110°-140o F., the salt will be partially reduced to a ferrous oxalate. At the commencement the solution has a pure green colour, by continued digestion it turns yellowish-green, and, finally greenish-brown. When this moment has arrived, the remaining ferric hydrate should be filtered off, and the solution be submitted to a quantitative chemical analysis.
Although the determination of the amount of iron and oxalic acid contained in the solution is one of the simplest of analytical operations, we tried at first to dispense with it. Closer investigation, however, proved how necessary it is to confine the relations between rather narrow limits, in order to ensure the success of the platino-ferric process. Now as ferric oxalate cannot be prepared in a stable form, so as to have a constant composition, there remains no other way than that of determining the composition of the solution by analysis, and then diluting it to the required degree. Further, it should be noted that any quantity of the solution may be prepared at once, and then kept in stock.
From the analysis we ascertain the quantity of ferric oxalate contained in 100 cc. of the solution, as well as any slight excess of oxalic acid which happens to be present. The liquid is then diluted with so much distilled water that every 100 cc. of it may contain 20 grm. of ferric oxalate [Fe2(C2O4)2]. Crystallised oxalic acid is then added until, with the free acid already in the mixture, that substance amounts to 6-8 per cent. of the ferric oxalate already in the solution - the normal ferric solution. - (H. C. Gunther.)
Platino- Uranotype & Mercuro- Urano-type. - First, make a saturated solution of neutral potash oxalate, and dissolve dry ferrous oxalate in this to saturation.
The process is very simple, for the operations involved are merely: 1. Coating the paper, 2. Exposing same in printing frame. 3. Development (which takes only 5-10 seconds).
4. Fixing in HC1 (1 1/2 p. cent, solution).
5. Rinsing in water, drying, and mounting.
In recent experiments, uranium chloride has replaced the nitrate in the sensitising solution, and better results' are to be obtained from its use. As in Willis's processes, potash chlorate may be added in minute quantities to the sensitising solution to increase con-trasts.
The following seem good proportions for the sensitising solutions:- -
Saturated solution of uranium chloride.
Potassic platinous chloride .. 60 gr. Water, distilled .. .. .. 1 1/2 oz.
* 90 minims each of A and B are spread over a well-sized sheet of paper, 26 X 20 in.
Now for the most perfect form of platinotype (by any process) to be obtained, it is not difficult to see that the platinum should not form a part of the sensitising solution, but should be deposited on an image already formed by an action similar to that of toning silver prints with gold, and for this reason no platinum in the "paper" or "printing-out" process can ever give the detail of a silver print. The mercuro-urano-type process may turn out to be useful for rapid proofing of negatives, if for nothing else, for the tone is not very objectionable, and if preferred may be modified. Paper is coated with one or the other uranium solution, to which has been added some saturated solution of mercury bichloride (say 1 part mercury solution for every .8 of the uranium solution). A sheet of paper is coated with about 170 m. of this solution, and dried in the dark. It is then printed behind a negative until it attains full strength, which will take only a few minutes; it may then be floated on solutions of gold or platinum to obtain a more pleasing tone, and next placed in water acidulated with HC1, or the toning omitted and the print treated as if it were Pizzighelli paper.
Thus it will be seen that this process is one of the simplest known.
If more attention were directed to the salts of uranium many latent and valuable properties would in all probability be discovered; their reactions are very powerful, and should be more generally known than they are at present. - (A. Reynolds.)