A smooth or rough paper, according to the size of the prints to be made, is chosen. Papers intended for water-colour drawings should pre ferably be used. Although such papers are already sized, it is a good plan to apply an additional sizing in order to confine the image to the surface of the paper. If this is not done, the image is partially buried in the substance of paper itself.
Put 70 gr. (8 grm.) of white gelatine to swell in about 20 oz. (i litre) of water, melt on a water.bath, and add about i7 gr. (2 grm.) of ordinary alum, filtering, if necessary, through closely-woven cloth. Pour the hot liquid into a dish, and float the paper face downwards on the solution after marking the back of the paper with a pencil. If necessary, the bath should be warmed up from time to time, so as to avoid over-sizing the paper, in which case there would be a risk of the image not adhering to it. After a few minutes in contact with the solution, the paper should be allowed to drain and then put to dry away from dust.
The sensitizing solution should be prepared in weak daylight or in artificial light. For this purpose the following should be dissolved— Ferric oxalate scales . . oz. (25 grm.) Oxalic acid . . 18 gr. (2 grm.) Lead oxalate . 9 gr. (i grm.) Hot distilled water, to make . 2 OZ. (100 c.c.) The solution is then decanted or filtered, to get rid of small quantities of undissolved lead oxalate if present.
The image will adhere to the paper more easily if a very little potassium chloroplatinite is added to the sensitizer, viz., in very much smaller amount than that required to form a platinum image. For example, about 40 minims (r c.c.) of a to per cent solution of potassium chloroplatinite (to gr. dissolved in too minims of water) can be added to 400 minims (to c.c.) of the sensitizing mixture given above. By this method very soft pictures can be obtained from contrasty negatives. The contrasts of a picture can be increased by adding to the same mixture a few drops of a 10 per cent solution of potassium bichromate. When coating rough papers, which tend to absorb a larger quantity of liquid, the sensitizing solution may be slightly diluted.
The sensitizer, contained in a small cup, is spread over the paper with a large brush (this should not have any metal binding) in the same way that a flat wash tint is applied to drawing paper. The paper should be dried fairly quickly it should be allowed to remain in the atmosphere of the room for about 20 minutes, drying then being finished off by hanging the paper near a stove or heater.
The precautions to be observed for the storage of papers and for the filling of printing frames are the same as for commercial platinum papers. When printed, the image is much less visible, and it is only by experience that one able to judge what the appearance of the image should be when correctly printed.
The developer is prepared by adding one part of a to per cent solution of potassium chloro platinite to to parts of a mixture such as the following— Neutral potassium oxalate . 4 oz. (200 grm.)
Di-sodium phosphate . . xoz. (5o grm.) Water, to make , . 20 OZ. (I000 C.C.) Some precautions should be taken to ensure uniformity of development, especially if only a very small quantity of liquid is used. A soft, preferably camel-hair, brush, about i in. to t in. wide, should be used for the smaller sizes, while a 2 in. brush is essential for the large sizes. It should be moved over the picture with a quick, light movement, dipping it in the developer for each stroke so as to ensure an equal and uniform action of the solution.
As soon as the print is uniformly impregnated, it is left until development is complete.
Fixing and washing are carried out as recom mended for the commercial papers.
634. Recovery of Platinum Residues. The extremely high cost of platinum renders it neces sary to recover the platinum remaining in the developing and fixing baths as well as that contained in waste sheets and trimmings. The directions given below are taken from a circular published in 1920 by the Platinotype Co.
Clippings of unused paper and prints which have been discarded during development should be fixed in the old acid baths prints which have already been fixed should be dried and inciner ated, the ashes being placed in the vessel con taining the used developing and fixing baths. (The black deposit which sometimes appears on the dishes used for development consists of platinum and should also be recovered.) The accumulated liquid should first of all be neutralized by adding to it about 1i2oth of its volume of a 15 per cent solution of anhydrous sodium carbonate, in small quantities and with constant stirring. Then a volume equal to that of the carbonate of a 2-5 per cent solution of hydrazine sulphate (prepared with hot but not boiling water) should be slowly added to the mixture, and the whole stirred three or four times for the first half-hour. The liquid soon becomes cloudy, depositing platinum slightly contaminated with iron salts. Allow to stand for at least four days, so that the platinum can settle to the bottom of the vessel. The greater part of the liquid is then siphoned off, taking care to avoid carrying over any of the precious metal. The remaining liquid is stirred up to get the platinum into suspension and is then poured on to a filter resting in a funnel, which should be placed in a bottle, so as to catch the platinum in case the filter breaks. When dry, the filter should be kept for further use.
When an appreciable quantity of platinum has been collected in this way, the filter should be dried and folded so as to enclose the platinum.
Never attempt to sell precious metal residues in very small quantities, as the cost of refining them represents a very appreciable proportion of the total value of the metal.