Manufacturer and builder 4, 1871

Soon after the invention of photography, efforts were made to transfer photograph to blocks of wood, and to engrave them afterwards like ordinary cuts. Thu advantage of this process is twofold: first, it is not necessary to make the drawing; and secondly, the photographic image is a much better copy than any hand drawing, especially if the copy is to be reduced in size. In regard to the process of transferring, it has not as yet met with any great difficulties, though images could not be very well fixed, and were liable to become too dark. A further and more serious inconvenience made itself apparent in the old process of preparation. The block became so much softened and impregnated with salts that the artist was unable to execute his work with ordinary facility; in a word, the wood became fibrous, and not capable of being well cut. At a more recent date, the attempt was wade to paste a skin of collodion with the photograph on the block, and then to cut it. It happened, however, that the skin would become partially loosened, which made this method completely useless.

The difficult problem has finally been solved by Mr. Leth, in Leipzig, whose method is in detail as follows: In order to preserve the wood from the penetration of water, during the subsequent operations, the wood-block is coated on its reverse and on its four edges with varnish or wax. After this, the perfectly polished cutting surface is grounded in the ordinary manner. Wood-cutters generally use for this purpose white-lead, zinc-white, or blanc-fixe, with gum-water; but for the purpose under consideration gluewater is sufficient. That grounding is accomplished by means: of rubbing with the finger and the ball of the hand true til the surface is uniformly white and dry. This surface is hereupon immersed in a somewhat concentrated solution of alum, which will coagulate the glue, the superfluous liquid being thrown off by swinging the block about. The block is then allowed to dry. The photographic image is not produced on the wood block, but on a glass plate which is coated with a solution of bichromate of potash mixed with gum and honey.

By laying upon the letter a positive photographic copy of an image, and subjecting it to the action of light, an indistinct picture is obtained, but by spreading burned soot or another dustcolor upon it, it appears perfect; and if a proper shade has been selected, can scarcely be distinguished from an ordinary photograph. The parts of the chrome salt solution which have been affected lay the light fail to take the dustcolons, while they adhere on the parts whichateo been protected against the action of the light, anal thus the image is brought out. The latter is now to be transferred upon the wooal block. This is done by cawing the picture with collodion, and after that putting it into dilute nitric acid. Thereby tho chrome salt Is decomposed and the still soluble gum washed off, while the dust-colors remain on the collodion. The cohesion between the collodion skin and the glas, is at the same time removed. If the glass is immersed in a large vessel with sugar-water, the skin may be easily removed by the fingers. The skin now floats in the sugar-water. It is made to cover the wood block by immersing the latter in the water, so that its prepared surface comes just below the fleeting collodion skin. When the latter lies without folds or bubbles in the proper position on the prepared wood surface, the block is withdrawn from the water and placed on edge for drying. In order to dissolve and separate the collodion skin, the surface of the picture is now sprinkled with ether. The dust-colors, which represent the image, however, remain upon the ground-surface, and after the ether has evaporated, the block is ready for cutting. In regard to the shade, it does not depend upon the chemical process, but upon the dust-powder itself, which may be taken of any tint. Any picture taken from nature may be transferred in this manner, and if engraved, will show a similarity to the original not easily attainable.

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