Scientific American Supplement, No. 312, December 24, 1881 eBook

This eBook from the Gutenberg Project consists of approximately 122 pages of information about Scientific American Supplement, No. 312, December 24, 1881.

Scientific American Supplement, No. 312, December 24, 1881 eBook

This eBook from the Gutenberg Project consists of approximately 122 pages of information about Scientific American Supplement, No. 312, December 24, 1881.

As soon as the plate is dry, a positive cliche of the drawing to be reproduced is laid upon it, and the whole exposed to the sun for a minute, or to the electric light for three minutes.  The reaction produced is the same as with the citrate of iron, but much quicker; the exposed parts are no longer hygroscopic, but in the parts protected by the lines of the drawing the sensitive coating has retained its stickiness, and will hold any powder that may be passed over it, thus producing a very clear image of the drawing.  The coating being excessively thin, the little moisture it holds and the powder applied suffice to break its continuity, especially if the powder be slightly alkaline.  If the rest of the surface were sufficiently resisting, the plate might be bitten at once; but light alone is not enough to produce complete impermeability:  the action of heat must be combined with it.  The plate is, therefore, placed on a grating, with wide openings, a large flame is applied underneath, and it is heated till the borders where the copper is bare show iridescent colors.  The sugary coating thus becomes very hard in the exposed parts, but under the powder it is broken, porous, and permeable to acids.  The surface is then covered with the biting fluid, which is a solution of perchloride of iron at 45 deg.  Baume, and after few minutes’ contact the plate is engraved.  It only remains to clear off the bichromated sugary coating which forms the reserve, and which, being hardened by the heat, resists ordinary washing.  It is removed perfectly by rubbing the surface with a hard brush and warm potash lye; the plate is then ready for printing.  Sometimes it may be necessary to give several successive bitings, or to use a resinous grain; in such cases the various methods of the engraver’s art are employed.

B.—­Photo Engraving for Half-Tones.—­To reproduce by engraving the image of any object, a portrait, or a landscape, the gradation of tint is obtained by repeating three times in the following manner the operation A, just described: 

The copper plate being prepared as before, it is exposed to the light under a positive, and given a long exposure, say four minutes, in the electric light.  The sugary coating hardens under the whites and the lighter shades—­it only remains tacky under the blacks.  The positive cliche is removed, the plate powdered, and bitten; the blacks alone come out.

The plate is cleaned, then coated again with the sugary preparation, and exposed a second time under the positive, care being taken to preserve an accurate register, which may easily be done.  The second exposure is not so long as the first—­say two minutes, and gives the image of the middle tints and blacks.  The plate is powdered and bitten as before, bringing out the middle tints, and, at the same time, giving greater depth to the shadows.

In the third operation, the plate is exposed still less to the light—­say one minute.  The high-lights alone harden; the light shades, middle tints, and the shadows remain permeable.  After powdering and biting, the plate is finished.

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Scientific American Supplement, No. 312, December 24, 1881 from Project Gutenberg. Public domain.