THE FERRO-PRITSSIATE PROCESS.
The method of making and using ferro, prussiate paper (for obtaining blue prints) is described below. Prepare two solu tions :—(a) Citrate of iron and ammonia 1 oz., water 4 oz. ; (b) Potassium ferri cyanide 1 oz., water 4 oz. Coat any toler ably pure paper of fine texture with solu tion (a), using for the purpose either a broad flat brush or a tuft of wool, the paper being pinned on a clean board. The wool may be fixed in the end of a piece of glass tubing and held so by passing round the wool a wire and bringing the two ends now mixed by pouring the chloride into the silver, which ensures the silver nitrate being always in excess. This emulsion out through the opposite end of the tube. • The brush should be drawn from side to side, and there should be a slight over lapping at the edges. Keep the coating as even as possible, although considerable unevenness seems to have no bad effect. The paper prints very slowly. The details . should be brought out fully, and the dark parts should have a bronzed appearance. When a fairly dark brown image appears, (b) solution is applied by flowing it over the print. The result will be a picture in Prussian blue. The yrint is then passed through a weak solution of citric acid, washed for a few minutes in water, and hung up to dry in gentle heat. This is the cheapest of photographic printing pro cesses, costing very little beyond the plain paper, but is, of course, very unsuitable for general work. The ferro-prussiate process is chiefly used for copying plans and draw ings or for moonlight scenes. In making up the solutions before dissolving the ferri cyanide it should be rinsed in warm water to get rid of any yellowish powder adher ing to the crystals, which after rinsing should be of a bright ruby colour. After
dissolving the ferricyanide, take a small portion of the solution, dilute it to say one tenth of its strength, and to this diluted solution add a small quantity of a solution of ferric salt, such as ferric chloride, If a dense dark blue precipitate forms in the ferricyanide solution, the presence of ferro cyanide (yellow prussiate) of potassium is indicated, or it may be that the ferric chloride is partly in the ferrous state. If the precipitate is a slight one, it may be disregarded, as it is almost impossible to exclude the precipitate altogether ; a light blue colour would indicate that the solu tion is in its proper condition. To ensure the ferric condition of the iron it is ad visable to add a little oxalic acid to the ferric chloride (in the proportion of 1 to 2) before making the test. If the water used is hard, a little citric acid should be added to the potassium solution. Having ascer tained the condition of the potassium ferri cyanide, dilute a small quantity of am monium ferri-citrate solution, and test it with a few drops of the (b) solution. If a blue precipitate appears, the iron is in an improper condition, and should be rejected for another sample. The green ferri-aip monium citrate, if obtainable, is best. The solutions should preferably be prepared immediately before use. The paper may either be brushed over with or soaked in the solutions ; the latter method yields the more vigorous prints.