On Zinc.—Zincography differs only in a few details from lithography. All ordinary drawings may be made on zinc plates instead of stone, the materials and mode of printing being the same. The plates compete successfully with large stones on the scores of price and portability. The zinc employed is of the quality known as "best rolled vieille montagne." Irregularity of surface may be remedied by pressing. The coating of scale and oxide is removed by scraping, and the surface is then rubbed with pumice, &c., exactly like stone. All drawings on zinc are made on a grained surface, which is produced as in the case of stone, replacing the stone muller by one of zinc. When grained, the plate is washed first with cold and then with hot water, and dried rapidly. A once-used plate may be re-prepared by removing the ink by spirit of turpentine, washing first with water and then with strong alkali, pouring over a mixture of 1 part each sulphuric and hydro chloric acids in 24 parts water, washing again with water, and re-graining.
The drawing is made as on stone, and etching is then effected by one of the following mix tures :—(a) I pint of decoction of nut-galls, made by steeping 4 oz. in 3 qt. water: for 21 hours, boiling up, and straining ; pint gum solution, of creamy consistence ; 3 dr. solution of phosphorio acid ; (b) oz. nut-galls boiled in lf lb. water till reduced to 1, strained, and added te 2 dr. nitric acid and 4 drops acetic ; (c) decoction of nut-galls simply. After etching, the sequence of operations is washing off; gumming in ; drying by heat ; washing off with turpentine, without moistening or removing the gum ; rolling in till quite black ; sprinkling with water, and con tinuing to roll and sprinkle till the plate is clean and the work is rolled up. Printing is performed as with stone, using an ink containing weaker varnish, and exercising somewhat greater precaution.
Photographic Processes.—Since the modern development of photography (see Photo graphy, pp. 1532-44), a great number of printing processes, some remarkably simple, others more intricate, and many hearing a close general resemblance to each other, have been devised. These will now be noticed.
Aniline.—The process of W. Willis is founded on the action of bichromates on organic matter, the printed image heing coloured by means of an aniline salt.; it is extremely useful for copying plans and simple line-subjects. The operation is as follows :—Sized paper is floated in potassium hichromate containing a little phosphoric acid ; it is next exposed beneath a translucent positive, and when the image of the latter is clearly shown, it is sulojected to the action of aniline vapour. The result is that the parts shielded from the light by the lines of tloe positive are deeply
coloured (green, black, or reddish, according to the aniline salt used), while the other parts retain the weak tint of the reduced chromium oxide. In developing the print, it is exposed to the contact of the vapour from aniline dissolved in spirit of wine, the solution being placed in a basin, and heated by a spirit-lamp. The prints are fairly permanent after washing.
Poiterin'a Powder. mixture of gum arabie, sugar, and glycerine, with some senaitizing solution of potassium bichromate, is poured upon an impervious surface (e. g. a glass plate), and dried by warmth. Thus prepared, the plate is immediately exposed beneath a translucent positive for a few minutes. The parts affected by the light become hygroscopic, in proportion to the duration of the exposure, and intensity of the light, and any impalpable powder delicately brushed over the plate will adhere to the hygroscopic parts, according to their degree of moisture, thus forming a reversed copy. The developed image is coated with collodion, and transferred to paper unreversed, the soluble bichromate being washed out in the operation. Obernettor's recipe for the sensitizing solution is:--4 parts dextrine, 5 parts white sugar, 2 parts ammonium biehromate, 2-8 drops glycerine for every 100 cc. of water, and 96 parts water. The glass plate is sometimes previously coated with collodion.
Herschel 's Cyanotype.—This process is in very common use by architects and engineers for copying plans, produeing an image with white lines upon a blue ground. Sensitive paper is made by brushing it over with a solution of ferric oxalate (10 gr. to the oz.); it will remain good for years, if kept secure from light. The sensitive paper is exposed under the positive, and then brushed over with, or immersed in, a solution of potassium ferricyanide (red prussiate of potash), of almost any strength, by which the image is developed. The colour of the ground is deepened by subsequent washing with solution of potassium bisulphate. The ferric oxalate (peroxalate of iron) is prepared by saturating a hot aqueous solution of oxalic acid with ferric oxide (peroxide of iron). A better sensitizing solution may be prepared by mixing 437 gr. ammonium oxalate, 386 gr. oxalic acid, and 6 oz. water, heating to the boiling-point, and stirring in as much hydrated per oxide of iron as it will dissolve.