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gelatine, relief, film, negative and exposed

PHOTO-RELIEF ENORAVINO.—Processes in which, by photography and subsequent man ipulation, a printing surface is obtained, which stands above the general surface, and receives the ink in the same manner as type. Wood-cuts come under this heading. The Woodbury pro cess is often termed a photo-relief one, although here the image itself is in relief, the printing block being an intaglio plate.

PHOTO RELIEFS.—Images in relief pro duced by photography, and largely employed in photo-mechanical printing processes. The sim plest method of making a relief is based on the peculiar property of dichromatized gelatine to become insoluble when acted upon by light. If we take a glass or a sheet of paper coated with a thick film of dichromatized gelatine and expose it beneath a negative, those parts situated be neath the transparent parts of the negative image lose their solubility, while the parts not affected retain it. The remaining parts or half-tones of the negative allow it to be affected in different degrees, according to the amount of their trans • parency. If the exposed gelatine film be laid in cold water, only the soluble or unaffected parts will swell up, the insoluble portions remaining the same. By this means we get a true relief. From this a casting may be made, and electro type printing blocks produced.

If the exposed film be laid in hot water, however, we have already explained that the un affected parts will dissolve away. In Fig. 344 we are able to see in a clear manner the action that takes place. A represents a negative, portions being opaque, others transparent, and parts varying in opacity ; these are the half-tones ; B is the dichromatized gelatine film after exposure under the negative, the dark parts representing the action of the light in rendering the gelatine insoluble. If this be placed in cold water the unexposed portions will swell up above the others,

as shown in C. If, however, it be laid in warm water the soluble parts will then dissolve, and the relief will then have the appearance of D. It will be noted that here the half-tones are lost ; they have no support, and consequently are easily detached and torn away, as shown by the dotted lines. To prevent this, however, a new support is given to the gelatine, or the film may be coated on a collodion or other transparent film, and exposed on the reverse side under the negative. We then get a result as shown in D.

If we compare C with E we shall see that by the two processes, i.e., the applica tion of cold and hot water, we get two op posite kinds of reliefs ; in the first the un exposed parts stand out in relief, but in the latter case it is those which have been exposed to the light.

If a gelatine relief be pressed into lead, any number of images on pigmented gelatine can be made from the mould thus produced. This is the basis of the Wood burytype process. For other photo-mechan ical half-tone processes the image must be broken up into a fine grain. This is some times done by putting the gelatine relief into fine sand. For detailed instructions in relief making see Woodburytype.