This beautiful process is frequently regarded by those who have not tried it as an expensive and difficult one. With care, it may not necessarily be so. A certain amount of judgment is required when printing to know exactly when to stop; this comes with practice, and if the worker will only persevere for a short time the difficulty will be overcome. The paper should always, as far as possible, be examined at about the same position in one room, where there is an even but not strong light. Some form of print-meter may be used if desired.
The sensitive surface of the paper is of a pale yellow colour. Owing to the tendency of the paper to become damp it must be stored in suitable tins. These tins are divided up into two compartments, the one being separated from the other by a piece of perforated zinc. The chamber on one side of the partition is small, and is for storing a small piece of dried chloride of calcium, which absorbs moisture very readily, and so keeps the interior of the tin dry; on the other side of the partition is the chamber for the paper. If the calcium becomes moist and soft, it may be warmed in an oven to drive off the moisture, when it will become hard again. If the tin has been standing empty for some time, before putting a fresh quantity of paper into it it should be held near the fire. The paper will give dull, mealy prints if allowed to become damp. It may be purchased with a rough or smooth surface to yield sepia and white, or black-and-white prints, also for hot or cold development.
We shall here consider the "Cold-bath" variety, as it is the simplest for the beginner to handle. When obtained from the dealer, it is in hermetically-sealed tins. When opened, it should be immediately transferred with the calcium wrapped with it to their respective compartments in the storing tin.
The printing is done in daylight, after the manner of P.O.P., but unlike it in this respect, that when sufficiently printed there is but a very faint image, and the paper will require to be developed to bring it up to its full strength.
The negative should be of a fairly plucky quality, but not too thin in the shadow portion. It is put into the printing-frame in the usual manner, and then the paper; between this and the frame back must be placed a piece of material impervious to moisture, such as a rubber pad, or a piece of oilcloth, smooth surface to paper. As the surface of the paper is rather more sensitive to light than P.O.P., the operation of filling the printing-frame should be performed in a room, but at the corner farthest from the window. Before printing, the worker should ascertain which is the densest portion of the subject-matter of the negative. The frame is now exposed to a well-lighted part of the sky. The paper is examined from time to time, and as soon as there is the faintest suspicion of detail on the paper which comes immediately under the densest part of the negative, the printing may be considered complete.
The solution for developing up the image is made by dissolving neutral oxalate of potash in water. To this may be added phosphate of potash; the solution will, however, work without the addition of the latter. In making up the solution it is best to have it concentrated, so that it can be diluted by the addition of water to suit any requirement. A useful concentrated bath may be made up as follows: -
Neutral oxalate potash ............ 1 oz.
Phosphate of potash or soda ....... 1/4 ,,
Boiling soft water ................ 3 1/2,,
Let this stand to get cold.
For use, take one part of solution and dilute with an equal quantity of water for vigorous prints, and with double the amount for prints of a softer and more delicate grey tint. It is advisable not to be too sparing with the solution; a sufficiency to half fill a deep porcelain dish a size or two larger than the print will be convenient.
The paper is drawn in a sweeping manner face downwards into the solution, or it may be dropped face upwards on the solution, and the dish rocked, so as to quickly cover it. The image will rapidly develop up to its full strength. The print is then placed for several minutes in a clearing bath made of one part of pure white hydrochloric acid to sixty parts of water. The diluted acid is rejected, and a fresh portion poured on. This should be repeated for at least three times. When cleared, the print must be washed in several changes of water for from twenty to thirty minutes. Then allow to dry. By using the developing solution slightly warm it produces a somewhat warmer-toned print.
The "Hot-bath" paper is worked upon similar lines to the "Cold-bath" paper, excepting that the strong solution is used without dilution, and its temperature is maintained at from 150 deg. to 180 deg. Fahr., which necessitates the developing being carried out in well-enamelled iron dishes.
The sepia papers are of the "Hot-bath" variety. The ordinary black-and-white papers may be rendered a warm sepia colour by treatment with perchloride of mercury.