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Process

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PROCESS Known also as " Cyanotype " (negative) and " Ferro-prussiate " process, and largely used by engineers, architects, etc., for reproducing technical drawings. It is one of the oldest photographic printing processes, having been invented by Sir John Herschel in /84o. Paper is coated with a mixture of ammonio-citrate of iron and potassium ferricyanide dissolved in water, then dried in the dark, and printed by daylight in contact with a negative or drawing on tracing paper, when an image in insoluble Prussian blue (Turnbull's blue) is produced. The print is washed to remove the soluble coating unacted upon by light, leaving a finished print, blue on a white ground.

Another blue-print process is the positive cyanotype, or Pellet's process (which see, under the latter heading), which gives blue lines on a white ground, the opposite process to the above, it being one in which blue is formed where the light does not act. The negative cyanotype or blue-print process proper is the one particularly suitable for negatives, and is that to which attention is here directed.

Blue-print paper, ready sensitised for imme diate use, may be purchased, but as it does not keep well, and is so easy to prepare, it is better to make it as required. A large number of sensitising formula have been published from time to time, considerable latitude being per missible in the quantities of chemicals used as well as in the methods of working. They all, however, resemble one another, and yield prints which are all very much alike. Almost any kind of paper can be coated with the sensitive mix ture ; fairly stout cream-laid notepaper, or a real photographic paper such as Rives, is as good as any ; it should be free from wood-pulp or other impurities usually found in cheap white papers, its surface should be fairly hard and not too absorbent, and it should be tough enough to withstand thorough washing. Common rough papers are better if sized before sensitising, because the size prevents the image from sinking into the paper. For the size use the following arrowroot mixture :—Take f oz. of arrowroot and mix to a smooth stiff paste with a small quantity of cold water. Add warm water to make 22 oz. in all, and boil gently until dear. Thin papers may be immersed bodily in the warm mixture for a minute or two, and then drained and dried. Thick papers should be pinned by the corners to a flat board and the warm size applied first up and down and then across, by means of a soft sponge or a Blanchard brush (see the heading " Brushes "). Then with a clean soft sponge go over the paper again in order to efface all streaks and make the surface smooth ; hang up, and when quite dry it is ready to sensi tise. The two sensitising solutions are made according to the following formulae :— A. Ferric ammonium citrate (brown) . 8o grs. 16o g.

Water . . . I oz. i,000 ccs.

B. Potassium ferri cyanide . . 6o grs. 120 g.

Water . . . t oz. 1,000 ccs.

Unless quite fresh and dear the ferricyanide crystals should be washed before weighing, and dried between blotting-paper, to free the crystals from powder or crust. Mix the solutions, and keep in a stone bottle or in a dark place. The solution is usable at once, but works better when a week or ten days old, but it must be filtered just before using, and if older than this, should be preserved by adding to every 2 oz. of it t gr. of potassium bichromate. The sized common paper or the plain good paper, with blotting-paper underneath it, should be pinned to a flat board, placed (as illustrated) at an angle of about 20° to the horizontal in pre ference to being either flat or upright. Suffi cient of the sensitive solution should be poured into a saucer and then applied to the paper with a sponge, Buckle brush, or large soft camel hair mop. The coating must be done in artificial light or very weak daylight, and the solution should be spread upon the paper by strokes across the sheet, beginning at the top and joining the second stroke to the first. The strokes should then be made vertically in order that the paper may receive a perfectly even coat, without any of the sensitive mixture running in rivulets down the sheet. When evenly coated the paper must be dried as quickly as possible, and in the dark—a warm cupboard is a good place—but no very great heat should be applied to the wet paper to hasten the drying. The coated paper will not keep good for many days ; a heavily coated poor paper will not keep so long as an unsized or lightly sized good one. The colour of the sensitised paper may be 0. dirty greenish yellow tinge, but will vary according to the sensitising formula. The paper is placed in contact with a negative or drawing on tracing paper, and printed by daylight, preferably in strong sunlight. On exposure to light the colour of the paper gradually changes through bluish green and bluish-grey to a kind of dirty olive green, the image having a choked-up appearance when fully printed. The print is washed for about fifteen minutes in water, which should remove the soluble salts and leave a brilliant blue print. The water serves both as a developer and fixer, the print needing no further treat ment. Prolonged washing weakens the image, as will also water containing carbonate of lime. Brighter prints are obtained by adding about 20 grs. of citric acid to the pint of water. A solution of 5 parts of alcohol in 95 parts of water has been advocated for improving the whites, and a 2+ per cent. alum solution has been recommended for brightening the blue colour ; but neither of these aids is necessary if the water is free from lime, the negative or tracing a suitable one, and the paper properly prepared.

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