SALTED PAPER AND ALBUMEN PAPER 523. Salted Paper. Salted papers are sized with starch 1 paste containing a suitable quan tity of chlorides and other soluble salts, which are sensitized after drying by floating on a solution of silver nitrate and again drying (de Brebisson, 1854). The keeping qualities of this paper are very limited, and it is suitable only for very vigorous negatives ; it is no longer a general article of commerce. The absence of any coating, other than the normal sizing of a good quality drawing paper, makes it possible to draw on the paper with pen or pencil ; the photograph can thus be used as a guide and then caused to disappear when the drawing is nearly finished. Photographs on salted paper can also be worked up with water-colours ; the image is faintly printed and toned to a neutral colour, which gives the outlines and gradation in the shadows.
The preparation of salted paper is one of the easiest of operations, but owing to its bad keeping qualities it is sensitized only in small quantities at a time.
524. Preparation of Salted Paper. Paper of good quality, suitable for water-colour painting, is first of all marked with pencil on the back to avoid subsequent confusion, and then pinned to a small clean board or stretched out on a drawing board. The salted size is then applied with a thin flat brush with cross-brushing. If the paper is of pronounced texture it is necessary to brush the size well into the cavities. Finally the coating is treated with a soft badger brush until it appears to be uniformly matt.
The size is best made with arrowroot (a starch chiefly used as a food-stuff). The amount required can be calculated on the basis of about 2 oz. (fl.) per io sq. ft., but allowance must be made for loss in working.
Grind about I oz. (35 grm.) of arrowroot in a mortar with a small quantity of water, until a thick paste free from lumps is obtained. Dis solve separately 25 gr. (3 grm.) of citric acid and oz. (35 grm.) of sodium chloride (table salt) in 20 oz. (950 c.c.) of water. This solution is
brought to the boil in an earthenware vessel, and the arrowroot cream is then added in small quantities with constant stirring with a stirrer or wooden spoon. Boiling is continued for a few minutes, and the mixture is left to cool ; after cooling, the surface skin is removed.
As paper thus coated has good keeping qualities, a large quantity may be prepared for sensitizing when required.
525. Sensitizing must be done in the absence of white light ; it is convenient to work at night by artificial light. The drying must be done in darkness. On the commercial scale a room lighted by yellow glass windows, or the dark corner of a feebly lighted room is used.
The sensitizing bath is prepared by mixing'— Silver nitrate (cryst.) 22 oz. (too grm.) Citric acid. . i oz. (75 grm.) Alcohol (9o) . .i oz, (fl.) (75 c.c.) Water, distilled, to make . 20 oz. (r,000 c.c.) The solution is poured into a perfectly clean glass dish and should be about in. deep. The paper is caused to float on the surface of the bath face downwards by lowering it gradually ; it is then removed to make sure that it is uniformly wetted and re-floated with the same precautions. After about five minutes the sheet is lifted by one corner and withdrawn from the dish ; by drawing over a glass rod, most of the adhering solution is removed. The paper is allowed to drain for a few seconds and is then put to dry in the dark. The clips used for hanging the paper from the drying line should have been previously soaked in melted paraffin to render them impermeable. Drippings from the papers should be caught by any waste paper available, the ashes being afterwards added to the silver residues. The dry paper should be packed and kept in a dry place.
The silver bath may be used repeatedly ; in proportion to its use, it becomes poor in silver and rich in nitrate of soda ; the presence of this latter salt makes the readings of a Ramie hydrometer, which is sometimes used for esti mating the concentration, entirely misleading.