Collotype Processes. — Several methods of collotype printing have been described under Photography (p. 1543). Impres sions may be obtained in a lithographic pre,s, but the form shown in Fig. 1169 is especially adopted to this process.
Edwards' Heliotype.—The most im portant of the many modifications of the collotype process is the " heliotype " in vented by Ernest Edwards, wherein the great advantage consists in toughening the gelatine film by means of chrome-alum. Il is method is briefly as follows :—The solution of gelatine and biehromate, with the due pmportion of chrome-alum, is poured upon the previously waxed surface of a carefully levelled glass plate, and dried, when the film is readily detached. The latter resembles a piece of thick paper, and may be similarly handled. After exposure in eontaet with a negative, the film is placed on a plate of zinc or pewter under water, mid firmly attached by passing an indiarubber " squeegee" sharply over the surface of the film. The piloting tilm on its plate is s.aliol in water sufficiently long to remove the superfluous bichromate, to prevent the further action of the light, and is then ready for the press. This is preferably on the vertical principle, such as the Albion printing-press. The inking possesses peculiar featurea a very stiff ink may be used to give the deepest shadows, and this may be followed by a thinner ink, even ono more or less coloured, for the half-tones, thus producing a bichromatic effect in a single printing. The time occupied in drying the film is 21-36 hours at 32° (90° F.) ; 1500 copies have been successfully taken from one plate ; one man can print 200-300 copies daily ; for very long numbers, it can hardly compete with lithography in price, but for moderate numbers, the cost is very small.
Capt. Waterhouse has introduced a modified process, as follows :—The sensitive film is laid on flat copper plates, finely grained on one aide. After levelling on the drying-apparatus, the plates are washed with warm water, and coated on the grained side, while still wet, with a mixture of 15 grm. Nelson's opaque gelatine, and 1 grm. powdered potassium bichromate, in 100 cc. water,
adding 4 cc. formic acid when the first are dissolved. This is applied like collodion, and the excess is poured off. The coated plates are replaced iu the dryiag-apparatus, and covered over. In about 2 hours at 50° (122° F.), the films dry with a fine, even, glossy surface, perfectly free from streaks and waviness. It is hest to let the plates harden for a day or two before use.
Bibliography.-11. Fitz-Cook, ' Graphotype : a process for producing from drawings blocks for surface printing ' (Jour. Soc. Arts, Vol. xiv., No. 681, London : 1865); J. Pratt, 'Machine for Type-writing' (Jour. Soc. Arts, Vol. xv., No. 754, London : 1867); E. Edwards, Photography in the Printing-press : a description of the Heliotype process ' (Jour. Soc. Arts, Vol. xix., No. 962, London : 1871) ; J. L. Ringwalt, American Encyclopaedia of Printing ' (Philadelphia : 1871) ; J. R. Johnson, 'Improvements in the Manufacture of Printing Types ' (Jour. Soc. Arts, Vol. xxi., No. 1061, London : 1873); Type-writing Machines' (Jour. Soc. Arts, Vol. xxiv., No. 1213, London: 1876); J. Husnik, Das Gessnamtgebiet des Lichtdrucks ' (Vienna : 1877); T. Mac Kellar, The American Printer ' (Philadelphia : 1878); J. Krtiger, ' Die Zinkogravure ' (Vienna : 1878); J. Husnik, Die Heliographie' (Vienna : 1879); F. J. F. Wilson, 'Stereotyping and Elec trotyping' (London : 1880); E. Bigmore and C. Wyman, Bibliography of Printing' (London : 1880); F. J. F. Wilson, 'Printing-machines and Machine-printing' (London: 1880); W. D. Rich mond, ' Grammar of Lithography ' (London: 1880); T. Bolas, Modern Autographic Printing Processes' (Jour. Soc. Arts, Vol. xxviii., No. 1417, London : 1880); F. Noble, Principles and Practice of Colour Printing ' (London : 1881); G. Field, Grammar of Colouring' (London); P. B. Watt, Hints on Colour and Printing in Colours ' (London); Prioters' Register ' (London: 1872-); Printing Times and Lithographer ' (n. s. London : 1875-).
(See Ink; Photography.)