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Phic Process (From a negative on glass)

From a negative on glass or paper a positive impression is taken on paper prepared in the following manner: Take sized paper, very smooth and even in texture, which coat with a solution of gum Arabic in pure water, saturated with bichromate of potash; it is known that bichromate of potash, in combination with an organic substance, such as gum, gelatine and starch, becomes insoluble in water after a certain exposure to light. The paper, prepared as above, is then exposed to light behind a negative, and when the photographic image is sufficiently developed, such parts of the gum impregnated with bichromate of potash as receive the rays of light become, insoluble, or partly so, exactly according to the gradation of tone in the negative emploved. The sheet of pre- | judgment and experience of the operator. A pared paper, with the photographic image thus printed, is placed face downwards on a lithographic stone, grained very fine, or polished according to the nature of the image to be reproduced, and previously arranged in a percussion-press (it can be done in a lithographic press, but the result is uncertain). Place several sheets of damped paper upon the stone over the photographic proof, and apply a heavy pressure; the water contained in the damp paper is pressed through the photographic proof, and dissolves the parts of gum remaining at liberty; the dissolved gum attaches itself to the surface of the stone. When the stone has remained a certain length of time in the press, sufficient to allow the small quantity of soluble gum in the dark shadows to attach itself to the surface of the stone remove the pressure, and withdraw the photographic proof carefully from the stone; a negative image is then visible in gum on the stone, with all the gradations of tone. Dry the stone, either spontaneously or by gently warming it; when well dried, cover the whole surface of the stone with greasy ink, which may be applied with a roller or otherwise; the greasy ink is thus brought into contact with all the parts of the stone untouched by gum; the coating of ink is then removed by passing through the lithographic press, by spirits of turpentine or otherwise, and all the gum removed from the surface by washing.

The stone is then rolled in with ordinary printing-ink, and the positive image appears in black; it is then printed as every lithographic drawing, but has the great peculiarity of requiring no etching, the gum having so far penetrated by pressure into the substance of the stone as to allow of a great number of impressions being taken off. Gum Arabic is preferable, but similar substances can be used instead, such as gelatine, dextrine, and mucilaginous solutions. In photozincography proceed as above described, substituting a zinc plate for a lithographic stone. In photographic engraving on copper, steel or other metal plates, the preliminary operations are the same as those employed for stone or zinc, excepting that a positive image on glass or paper should, in the first instance, be used instead of a negative. When the plate is withdrawn from the press, and the photographic proof detached from it, it should be well dried for the stone. The plate is then covered with a thin coating of varnish, and when dry may be soaked in water to remove the gum, or immediately plunged in a weak solution of acid, and etched in the usual way of etching on steel or copper.

Wherever the plate is protected from the varnish by the gum it will be attacked by the acid, and an engraving is produced, which is then printed as an ordinary etching on metal plate.

OSBORNE'S (PATENT) PHOTOLITHOGRA-