PHOTOGLYPHIC ENGRAVING, the name given to a new art of engraving by the action of light, which was patented by Mr Fox Talbot on the 21st April 1858. A plate of steel, copper, or zinc is covered with a thin film of the following solution:—
The photograph or object to be engraved is then laid upon this film, and screwed down in a copying-frame. After exposure to light, solar or diffused, the plate is taken out, and a little finely-powdered gum copal carefully and evenly spread over its surface. The copal is then melted by holding the plate horizontally over a spirit lamp. Muriatic acid saturated with peroxide of iron, forming perchloride of iron, is the etching liquid employed, and is put up in three bottles:—
No. 1 is a saturated solution of the perchloride in water.
No. 2 consists of 5 or 6 parts of No. 1 in water.
No. 3 consists of equal parts of water and No. 1.
A little of No. 2 is first poured on the plate, and spread with a brush. The liquid will penetrate the gelatine wherever the light has not acted upon it, but cannot penetrate the parts upon which the light has acted, and "it is upon this remarkable fact that the art of photoglyptic engraving is mainly founded." Mr Talbot's specification, containing minute details of the process, has been printed in all the photographic journals.3
Mr C. J. Barnett proposed, in March 1858, another method of engraving on metals, which he calls photo-metallography, and in which galvanism regulates the etching, and prevents the gelatinous coating from being injured by removing the gas bubbles.4 (D.B.)