WOOD, PHOTOGRAPHING ON.—There are a variety of processes for photographing upon wood, and for ordinary decorative processes any of them can be used, such as the carbon transfer process, the powder process, transferrotype, etc. But the principal object of producing images upon wood is now usually for the purpose of the engraver. Photography is now rapidly taking the place of the artist on wood. Of the many processes of photographing upon wood only a few, however, can be utilized for this purpose, as they are nearly all defective in one point, and that is that the block becomes wet during the operation. Engravers' boxwood is generally very sensitive to moisture, and to wet a block is generally to spoil it.
The two following methods are given in " Spon's Workshop Recipes": (r.) From a negative of the subject desired make a clear thin positive upon glass by the wet collodion process. The positive should be of the proper size on clear glass without the sub stratum. Tone and fix as a transparency, and lay in a dish of water containing a small percentage of sulphuric acid to loosen the film. The film will soon become so loose that it can be easily stripped from the glass and transferred to the block. To do this safely, lay on the film a piece of wet albumenized paper little larger than the glass, press out the bubbles and surplus water carefully, then turn back one corner of the paper, and it will come off bringing the film with it. Have the block smoothly whitened with Chinese white in gum water, and the surface slightly damp. It is now easy to transfer the film to the wood and remove the paper, when the block must be allowed to dry spontaneously.
(2.) Another way to print on wood is by a sort of photolithographic process. Coat paper with a thin uniform coat of gelatine in warm water. Dry, and float a short time on a weak solu tion of potassium dichromate in water. Dry again, and expose under a negative till all the details are visible. Roll the entire surface of the print with a printer's roller, charged with litho graphic transfer ink thinned with spirits of turpentine. Soak the paper in a dish of tepid or
warm water until the ink can be removed by rubbing gently with a soft sponge. All the ink, except the lines composing the picture, can be removed: when the print should be laid face downwards on the whitened block, and subjected to a heavy pressure in an ordinary letter press. The paper can easily be removed by wetting the back.
Besides the importance of wetting the block as little as possible, there is another very im portant point in this branch of the art, i.e., nothing must remain on the surface which is capable of clogging up the point of the engraver's tool, nor must the chemicals used have any deleterious effect upon the wood, causing it to become friable, by which the fine delicate lines crumble and give way under their construction.
Among other processes that have been recommended for photographing upon wood for engraving purposes may be mentioned the following: In Ives's process thickly-salted albumen is prepared by beating up the whites of six eggs with 8o grains of ammonium chloride. It is then allowed to stand, and afterwards filtered through flannel. A little of this is poured on to the center of the wooden block and mixed with some dry, pure white lead, the mixture is then rubbed into the wood with the ball of the hand. When quite dry it is sensitized by floating on a 6o-grain solution of silver nitrate for five minutes, blotted off and placed before a fire to dry. When the block is dry and cold it is fumed for half an hour or so with ammonia, and printed rather deeply under a reversed negative. When printed the block is floated on a large dish of water for an hour or so, changing the water several times, and it is then toned and fixed by floating for half an hour on a ten per cent. solution of hypo, made strongly alkaline with sodium carbonate, and containing a little gold chloride. Finally float for three or four hours on constantly-changed water. If the wooden block is found to be very porous it should be coated two or three times with the albumen solution.