The three letters "P.O.P." are now so widely understood as referring to the Gelatino-Chloride Printing-Out class of Papers that it may be said that P.O.P. is known to many who are not acquainted with the fully-written name of this class of productions.
Also it should be mentioned that when these papers are spoken of as gelatino-chloride papers it is not to be concluded therefrom that chloride of silver is the only silver salt present. What they do actually contain is probably only known to their respective producers. But generally speaking, it is enough to say that so far as the ordinary consumer is concerned, the family resemblance is so strong and chief characteristics so general that the following directions for using them may be held as generally applicable to the various well-known brands now on the market. At the outset, however, it will be convenient to note that for the purposes of manipulation we may roughly group them into two chief classes—viz., the matt (probably from the German word "matt," i.e., dull) and the glazed, glossy or enamelled. The latter comes to us with a highly glazed, i.e., shiny smooth surface, the former being slightly rough, of a surface and texture somewhat like that of very finely ground glass.
Care of the Paper.—The paper is sent out either in the full-sized sheet, measuring about 24 × 17 inches, or in smaller cut sizes, suitable for the usual 1/4, 1/2, 1/1 plate and other popular dimensions of plates. Compared with albumenized print-out paper, P.O.P. is more sensitive to light; therefore, some care must be taken to avoid needlessly exposing it to the influence of daylight or strong artificial light. For example, the printing frames should be filled as far away from any window as possible, and the prints examined from time to time either by gaslight or as feeble daylight as possible.
In handling the paper—cutting it up, etc.—care must be taken to avoid touching the sensitive surface with the fingers in any case. The touch of a moist or hot finger is very likely to produce a mark or stain which is usually irremovable. The paper should be protected from damp, excessive heat and impure air. If kept rolled in a tin tube or flat under pressure in the original packages, it will keep a considerable time—i.e., longer than ordinary albumenized paper.
Printing is done in the usual way. Strong diffused light reflected from the sky or clouds usually gives a better print than direct sunshine. In case, however, of a thin flat negative—i.e., one with insufficient contrast—good results may sometimes be obtained by covering the printing frame with a sheet of green glass and printing in moderately strong sunshine. In hot summer sunshine it is as well to cover the green glass with a sheet of tissue paper or fine-ground glass. Care must be always taken when printing in sunshine or very hot weather to see that the negative itself does not get too warm, or the paper may stick to it. In this case the print is of course lost, and the negative, unless varnished, is also probably seriously damaged by silver stains, which are very difficult to remove. Printing should not be carried quite so far as in the case of ordinary albumenized paper because in the subsequent operations of toning, etc., not so much strength is lost. The same care as regards shielding from light, etc., should be given to the prints after they leave the printing frame. They may be proceeded with at once or kept for some days before being toned, etc., but if this is done the prints should be kept under pressure. Some workers have thought that the light action goes on, "continues" in the print after it is removed from the printing frame. This, however, is not the generally received opinion.
Washing.—It is important that the first washing should be done with some care, or the prints may become stained. The points calling for attention are (1) running water and plenty of it, (2) care to see that the prints do not stick together. What is needed is that the soluble salts should be washed out of the paper as quickly as possible, and that the prints be not allowed to remain in the water containing these soluble salts longer than is necessary. Hence the advantage of running water and plenty of it. The washing water must not be too cold or the salts will not pass out of the paper quickly enough; and again, it must not be too warm or the gelatine will melt. The best temperature is about 65° F., and the limits should not go beyond 60° and 70° F., and preferably are kept within 60° and 65° F. As the paper is usually rather stout, it will need washing in running or constantly changed water for about ten to fifteen minutes, and in any case must be continued until all milky appearance of the water ceases. The print at this stage has a red-brown colour. If it is now passed direct into the fixing bath without toning it becomes somewhat more yellow, and when dry is usually a colour somewhat between yellow ochre and sienna.
Toning is usually our next operation, and for this purpose we have a variety of toning baths recommended by different workers. The sulphocyanide and gold is perhaps the chief favourite.
| 1. | Ammonium sulphocyanide | 10 to 15 | grains. |
| Gold chloride | 1 | grain. | |
| Water | 8 to 10 | ounces. |
The proper way to mix this bath is to add the gold to the solution of sulphocyanide a little at a time. The following method will be found convenient. As ammonium sulphocyanide is somewhat deliquescent, it is convenient to keep it in solution. Therefore, one ounce of the salt dissolved in twenty ounces of (distilled or filtered rain) water gives us roughly a strength of twenty-two grains per ounce. Dissolve the contents of a fifteen-grain chloride of gold in fifteen drams of distilled water. To mix a bath, take of the sulphocyanide solution half-an ounce; to this add eight ounces of water. Now take one dram of the gold solution and dilute with one ounce of water. Then add this dilute gold solution a little at a time to the eight ounces of sulphocyanide solution, and stir well with a glass rod. It will be noticed that as the gold solution drops into the sulphocyanide solution an orange-red precipitate is formed, which is redissolved on stirring. Hence the gold must be added to the sulphocyanide, and not vice versa.
2. Another favourite bath is as follows:—