The copper plate to be engraved is attached to the positive pole of a suitable battery and placed as anode in a solution of sulphate of copper, or in water acidulated with sulphuric acid.
In suitable cases, different degrees of depth may be given, putting parts of the anode and cathode nearer together or further apart.
Grove etched daguerrotype plates with a single pair of Grove or Bunsen cells, by inserting the plate to be etched, and a platinum plate of the same size, in a wooden frame having two grooves a quarter of an inch apart.
The daguerrotype having been attached to the battery as anode, and the platinum plates as cathode, the frame is immersed in a suitable vessel filled with
| Hydrochloric Acid | 2 | parts. |
| Distilled Water | 1 | part. |
Contact is made for about half a minute, after which the plate is removed from the acid, washed thoroughly with distilled water, then placed in a solution of hyposulphite of soda or ammonia, and the deposit removed from the surface {115} with gentle rubbing with cotton. It is again rinsed with distilled water and dried.
MORDANTS FOR ZINC.
The comparative cheapness of zinc would give it an advantage over copper or steel for engraving or etching with the graver or point, but it does not seem to be recommended for these purposes. It is hard to cut with the graver, and, though it bites easily, it is not suitable for fine work. Another defect is that it will not stand a long impression; but this may be overcome by surfacing the plate with copper. The principal uses of this metal for printing purposes are for surface printing or zincography in the same manner as lithography, and for the process of biting in relief, and zinco-typography or Gillotage, now so largely employed as a substitute for wood blocks. It can also be engraved very delicately in the same style as engraving is done on stone, through a coating of gum.
The etching fluids for zinc are of two entirely different kinds: first, mixtures of gum and weak acids used for preparing plates for zincographic printing in the lithographic press, or for preliminary inking preparatory to being bitten in relief by the Gillotage process; and secondly, mineral acid, more or less dilute, used for biting in relief and ordinary etching.