At the time when copper engravings were used exclusively in the Institute, the plates were made by the galvano-plastic process with the aid of a twelve-cell, zinc-silver battery (system Smee). Now such plates are only made for the patrices in heliogravure. The last daguerreotype plates in the American market were also galvano-plastic.
Improvements introduced in the mechanical arts in modern times have also been introduced for the generation of the electric current in electrolytic operations. Magneto'electric and dynamo-electric machines have been substituted for the hydro-electric cell, by which the liberation of injurious gases and interruption of the work is entirely avoided, and more uniform and accurate work obtained. Machines for continuous currents are preferred with wire twists of considerable diameter, and wire of but little resistance, so as to generate a current great in quantity but of little intensity.
(6) Mantal, director of the stereotype foundry of Dupont's printing house, describes as follows the process of converting a lithographic or copper plate print into a typographic block. The composition to be reproduced is drawn with a crayon or pen upon a lithographic stone, which undergoes all the preparation necessary for a proof upon transfer paper. It is then transferred to a plate of properly planed zinc, which has been washed with a solution of soda or potash and dried with a rag. The transfer is made just as if it were a question of an impression upon stone. Care is taken to see that the fine lines of the drawing are all reproduced, and, if they are satisfactory, gum water, alone or with the addition of a decoction of nutgalls, is passed over the surface of the zinc. The gum combines with the zinc, and renders it proof against the contact of fatty matters.
After the plate has remained under gum for a little while, it is washed and then inked with thick ink by means of a lithographic roller, just as would be done for pulling a proof from stone. Then, by means of a cotton dabber, resin in impalpable powder is dusted over the entire surface - although finely powdered bitumen may likewise be used. This resinous dust adheres to the oily parts, solidifies them, lodges in all the interstices formed where the inking has been slight, and forms a protecting envelope against the penetration of the acid. Care is taken to remove all the superfluous resin.
The edges and bottom of the plate are now covered with lac varnish or a solution of bitumen, after which it is immersed in a bath of water containing 5 per cent, of nitric acid. After remaining in this for 20 minutes, it is taken out and gently rubbed with a piece of soft charcoal - an operation which, by removing the first layer of ink, allows the beginning of the conversion of the drawing into a typographic plate to be seen.
This first biting in is usually very slight. If it has proceeded regularly, a second inking is given before immersing the plate in the bath again for another 20 minutes. Upon being taken out the second time the ink is removed as before, and the plate is examined to am whether the acid has done its duty. Then a third inking is given, and the plate is immersed again for 20 - 25 minutes.
At every biting in, the strength of the bath is increased 2°-3° by the acetometer. It is rarely the case that a fourth biting in is necessary. The trough containing the bath is of oak lined with either guttapercha or sheet lead. It is fixed upon a pivot that allows it to be given a continuous rocking motion while the plate is immersed. This agitation is indispensable in order that the acidulated water shall constantly flow over the plate and carry away the salts of zinc that are formed.
The transfer of the drawing from stone to the zinc plate is effected in a lithographic press. Only line drawings are treated by this process.
The zinc plates are prepared by specialists. Moreover, if it be desired to write, draw, or make a transfer upon a zinc plate, it is essential that the latter shall undergo various preparations, such as polishing, scouring, etc. If these operations have been properly performed there will be, obtained good typographic plates that it will be only necessary to mount upon wood after the whites have been routed out. Finally, the blisters are removed with a graver, all the inequalities are straightened out, and all the small defects observed are remedied. As for typographic plates derived from an engraving on steel or copper, instead of making a drawing upon stone, the engraving is transferred thereto, and from this is pulled a proof upon India paper, which is transferred to the zinc plate. (Chronique Industrielle.)