RELIEF PHOTO-ENGRAVING. This covers all those processes by means of which a surface is prepared like that of a wood engraving, the lines, points, and small surfaces which are to be re produced upon the paper standing up in low relief from the background which is cut away. The level surfaces of these lines and points must be exactly true and uniform, all in one plane, except where it is deliberately lowered in one place or another in order that the block at that point shall press the paper less firmly and the ink impression show pale in contrast with other and blacker parts.
One of the most difficult of these processes is that called commonly the Celatin Process, or, to distinguish it from others also depending upon gelatin, the 'swelled gelatin process.' It is an expensive process and one requiring the great est care to employ it successfully, but its results are not very markedly inferior to that of photo gravure which is not in distinct line. In fact, a good print made by this process may he often taken for a photogravure print of somewhat in ferior excellence, without clear black and white, with a too uniform texture and a cloudy render ing of details A sheet of glass is coated with a film of gelatin sensitized with of potash and exposed under a black and white negative. The film is then soaked in water and swells up where the opaque lines corresponding to the whites of the design protected the film. while the surface of the film corresponding to the black lines of the design do not swell. From the surface so produced a matrix is made, and from this another in a material such a- Nvax, and. again, an electrotype of copper is made and mounted upon a block to give it the requisite stiffness and thickness.
The Woodbury type. the collotype. the arto type, the heliotype, the alberttype. and many simi lar processes are all based upon the properties of certain resinous and glutinous substances. and their changes when submitted to light. Asphal tum, albumen. and gelatin are the three used.
The Half-Tone Process has resulted from the invention of the mechanical art of ruling lines of extreme fineness. of perfect uniformity in width, and at absolutely equal distances from one an other. This, which was not practicable until within comparatively recent year-, has now been developed so that plates of glass can be ruled in this way with minutely incised lines. two hundred
or even three hundred to an inch. When two glass plates. each ruled with lines of this sort. for in stiince, diagonally across the plate. are brought together in such a way that the two sets of cross one another. the result is. as seen by trans mitted light, a fine network of or diamond shaped mesh. The half-tone process depends upon this 'screen.' as it is called. The nycrative is made by light passing through this screen. placed at a suitable distance in front of the sensitive plate. and the result is an image broken up into minute dots, easily seen with a magnifying glass. There is no absolutely con tinuous gradation of tint; but everywhere a series of VC11 dots which constantly in crease and decrease in darkness (iv intensity, con ditioned upon the relative lights and darks of the subjects to lie reproduced, and which are small enough to produce a general eficet of uniform gradation. This negative reversed is placed in centao with a carefully prepared copper plate, the zinc- is also ...sed that has been coated with a preparation of tisloglue, biehromate of ammonia, albumen. and water. It is then exposed to the light and afterward washed in water. and those parts protected from the light by the dense por tions of the negative will be washed out of the half-tones in due proportion. After burning in— that is. heating over a gas stove—the plate is all ready for etching, which is done in the case of a copper plate with perchloride of iron, or ith zinc nitric acid.
The half tone process requires less artistic s1.11-c and ability than, for instance, photogravure, but it requires the neatest handling. the most perfect delicacy of treatment, and it is often necessary fora good block that the acid should be applied in several consecutive 'rebitings' be fore the surface is perfect. In much of the finest work of today the highest lights and delicate tone gradations are very skillfully retouched by an experienced wood-engraver. :Nitwit of the re engraving by hand, however,. is clumsily done, and adds to the shortcomings of poor plates.