An alternative method is to use single solu tions, one for sensitising and the other for developing the faint image, obtained by printing in the usual way, to the desired blue colour. The sensitising mixture is as follows : Ferric ammon. citrate (green) I ro grs. 220 g. Uranic nitrate . . • 35 „ yo „ Water . • . . I oz. r,000 Paper is coated with this mixture and printed in the manner already described. The faint image is developed to its full strength by placing in— Potassium ferricyanide 22 grs. 44 g.
Water . . . . I oz, Lax) ccs.
The print is completed by washing in water. This process is more rapid than the one first described.
The blue-print processes are used for printing upon fabrics and for the making of blue trans parencies for window decoration. For the latter it is necessary to use a plate coated with gelatine to serve as a vehicle for the sensitiser.
Toning Blue-prints. —Blue-prints may be toned to several other colours, but the various formulae published are uncertain in their action on home-made papers, two samples of which are seldom alike ; they answer better with com mercial blue-print papers. Before toning, wash the prints thoroughly. Green.—Make a satur ated solution of ferric protosulphate, acidify with sulphuric acid, and dilute with an equal quantity of water. Immerse the blue-print until the required tint is obtained, wash well, and dry. A weak solution of sulphuric acid (acid 4 drops, water i oz.) will also give the print a greenish tinge. Lilac.—For lilac-violet, immerse in a hot solution of lead acetate, or a cold solution of borax. A 2 per cent. solution of potassium sulphocyanide (ro grains in I oz. of water) gives a pink-lilac tone, after obtaining which blot off superfluous solution, expose to strong sunlight, wash, and dry. Greenish Black.—Dissolve 3o grs. of borax in I oz. of water and add sulphuric acid drop by drop till the solution just reddens litmus paper ; next add a weak solution of ammonia till the red colour begins to change, and finally add 4 grs. of catechu, shake well and
filter ; tone, wash, and dry. Brownish Black— Add 6 drops of liquor ammoniac to I oz. of water, immerse the blue-print, and allow to remain until the colour has vanished ; then wash and place in water I oz., tannic acid g grs., in which the bleached print gradually assumes a brown or brownish black colour ; wash and dry. Purple Brown.—Add 3o grs. of tannic acid and gr. of pyrogallic acid (or even less) to I oz. of hot water, immerse the blue-print until toned to a lilac, rinse, and place quickly into a weak solution of caustic potash (potash 8 grs., water oz.) wash and dry. Black—A good black is difficult to obtain ; success depends upon the quality of the negative and upon the depth of the blue-print. The deep shadows tone to a rich black, but there is a falling-off in the half tones. Of the many formulae, Lagrange's is the best, but one of the most troublesome. Rinse the print in distilled water and, in a yellow light, bleach in a silver nitrate solution (9 grs. in I oz. of distilled water). Wash well in distilled water, expose to the fumes of ammonia, and afterwards develop with an ordinary ferrous oxalate de veloper ; the print may then be washed and dried. Grey to Red.—Print darker than usual, wash for ten minutes, and immerse in a solution of copper nitrate (24 grs. to I oz. of water) to which a little liquor ammoniac has been added cautiously, a few drops at a time, until the pre cipitate first formed is just redissolved, leaving the liquid a deep blue. This bath turns the blue-print mauve, then grey, and after a time red. Prints dry more blue than they appear when wet. The bath does not act well on prints showing great contrasts, since by the time the dark parts have turned grey the half-tones and lighter tones will become red. Most of the tones obtained by the above methods are unsatisfactory. (See also " Window Transparencies" and " Fab rics, Printing on.") Bleaching Blue-prints.—Instructions are given under " Drawings Made from Photographs."