The whole operation should not occupy more than about 50 seconds,and the resulting tint on the paper should be level and even; the small ridges left by the hairs of the softener will level down as the paper hardens, which it will do in about half an hour, when it may be desiccated upon the plate rack over a kitchener or in a cool oven, and stored in a pressure frame or calcium tube.
It is, of course, sensitive to light when dry. Paper prepared in this manner will be workable for some weeks if kept in a calcium tube, but will be found to work more freely if used within a few days. The surface should be slightly glossy, and an unexposed piece, if soaked in water, should lose the whole of its coating in about 30 minutes or so; this property will indicate the condition of the paper at this stage.
The brushes must be carefully washed after use, as, if allowed to harden, they will be ruined.
After printing, developing and drying, a second or further coating may be applied in a similar manner, or local coatings may be applied to areas it is desired to strengthen, without adding to other portions of the print.
PRINTING. This may be performed in an ordinary printing frame, or, better still, upon a small drawing board, having two small fillets of wood screwed to the face at right angles to each other; the coated paper is then laid face upwards upon the board, one edge touching the top fillet, and is slid sideways until another edge touches the side fillet ; the negative is then adjusted, film downwards in the same manner, and a piece of heavy glass laid upon the top to ensure good contact ; by this means the negative may at any time be replaced in correct register for a second or third printing.
The negative best suited to the process is one thin, clear and fully exposed, and the paper prepared as described will be somewhat more rapid than P.O.P. It should be fully printed with the same exposure which would make a piece, say, of Paget P.O.P. look as one would desire it.
There is little visible image, and the actinometer must therefore be employed to secure correct exposure, and care should always be taken to avoid over-exposure, as the image then becomes dull and buried.
When properly exposed, the image (unless the coating is very dark) will be clearly visible by transmitted light ; and if the print is not developed at once it must be kept absolutely dry, or insolubility will ensue. A good light is best for printing, but direct sunshine should be avoided.
DEVELOPMENT. Here a variety of methods are available, but that recommended is by a spray, and will require a dish of cold water, a spray diffuser, as sold by artists' colourmen for fixing drawings, a small bottle for water and a piece of glass.
The print to be developed is placed face upwards in the dish of water when, if the gum is in proper condition, the dark parts of the visible image will become light and the light parts dark. As soon as the print is wet, withdraw it from the water and place it upon the sheet of glass, adjusting this at a convenient angle, almost vertically above the dish ; then with the bottle full of water proceed to blow a fine spray over the surface of the print, say at about one foot distance. The operation should be begun on what will be the lightest part of the print, usually the sky, and carried through from this ; the fine spray will loosen the pigment and will run down the face of the print bearing the soluble pigment with it ; care must, of course, be taken not to wash away what it is desired to retain, and it is well to have a print at hand to work by and to know exactly what is required.
If the print has been exposed correctly the surface is extremely delicate, and the spray will remove the pigment easily and readily, without exertion, being advanced to or withdrawn from the print as a strong or gentle abrasion is required. The operation may be carried out with less exertion, especially in the case of large prints, by the use of a " Fletcher's foot bellows " connected to the spray with a length of rubber tubing ; a No. 3 size is suitable and costs about 26 shillings.
A 12 X 10 print should develop under the spray in from 5 to 10 minutes, and, as the surface gradually dries, sharp high-lights may be taken, and other modifications made with a brush or other tool.
The print is best left to dry upon the glass in a nearly vertical position, and if satisfactory may be considered finished ; but, if further modification, more transparency or greater strength be required, a further coating, printing and development may be given as described.
Modifications in the second coating will suggest themselves to the worker, such for instance as a coating weak in pigment with a long exposure for breaking down strong contrasts ; or again, a coating rich in pigment and a short exposure for strengthening a flat print, or perhaps the introduction of another colour.
Other methods of development may be adopted. The image may be washed up under a fine gentle spray from a tap, or pouring the water from a jug on to the margin of the glass and allowing it to run over the print where required ; or automatic development may be adopted, in which case the print may be left face downwards in the dish of cold water to develop itself ; this it should do, if all be well, in from half to one and a quarter hours ; then when sufficient general development has taken place, it may be removed on to a glass plate and any high-lights picked out with a brush, afterwards drying the print in the dark, re-wetting it and working upon it with brushes. The exposure in this method must be delicately adjusted to allow of the automatically developed print drying without running off the paper or becoming flat, and the method is much better adapted for a thicker and stronger single coating, say, 60 grains ivory black to 1 oz. of a 45% to 50% gum solution and 1 oz. of bichromate solution. When the print is dry, if the bichromate stain is objectionable, it may be removed by soaking in a 5% solution of alum or bisulphite of soda, and afterwards rinsing in cold water ; and when dry the print may, if desired, be varnished with gum or celluloid varnish.
In conclusion, it may be noted that the process is in itself extremely simple, and requires only a little patience, delicate handling and the knowledge gained from observation and trial to ensure a satisfactory working. The bare description is necessarily tedious and uninteresting, and differs from the description of most other printing processes, inasmuch as it includes the manufacture of the printing paper ; however, when reasonable facility has been acquired, the personal nature of the operations and results give an interest unequalled in the working of any other process.
J. C. S. MUMMERY