| 摘要 |
Metallic surfaces, such as comprising copper covered with a thin protective coating of chromium, are electro-chemically treated, e.g. by subjecting the surfaces to anodic oxidation or dissolution or cathodic reduction or deposition, using a potential difference to displace the ions through a photographic film emulsion layer previously applied to the surfaces, e.g. by conventional rotogravure processes, giving a non-uniformly permeable resist whose permeability varies over a continuous range. In electrolytic etching, the metallic surface, which may be a printing block with a coating of silver or bichromated gelatine matrix previously exposed through a photographic film, constitutes the anode and is immersed between copper cathodes in a hydroscopic electrolyte, either employed alone or in the form of concentrated solutions such as LiCl, CaCl2, MgCl2 ZnCl2, FeCl3, CuCl2, KOOC.CH3, NaHSO4, KSCN, Na2Cr2O7 and CrO3. This electrolyte may b inert to the metal in the absence of an electric current, or may be capable of chemical attack and effecting electrolysis in which case a "positive" or "negative" potential difference is applied to increase or decrease the intensity of the treatment. Dialysis of the gelatine may be slowed down by adding tanning products such as formaldehyde, tannin or glyoxal to the bath. The process may also be applied to the decoration of aluminium objects, the manufacture of medals and matrices for gramophone records, and the engraving of punches. |
