192 ANNUAL OF SCIENTIFIC DISCOVERY. 



black manner, capable of reproducing with greater or less fidelity to 

 the impression on the photographic proof, on glass or on paper, (taken 

 as the point of departure) the original object. No one has, as yet, 

 succeeded in preserving the charm of the original image, engendered 

 in the dark chamber, but the details are transmitted with considerable 

 fidelity. 



Claudet Process. The following process for engraving from da- 

 guerreotypes has been recently patented by M. Claudet. 



The process is established upon the following facts, which have 

 come to the knowledge of the inventor : 



1. A mixed acid, composed of water, nitric acid, nitrate of potassa, 

 and common salt, in certain proportions, being poured upon a daguer - 

 reotype picture, attacks the pure silver, forming a chloride of that 

 metal, and does not affect the white parts, which are produced by the 

 mercury ; but this action does not continue long. Then by a treat- 

 ment with ammonia (containing already chloride of silver in solution 

 is preferable for this operation), the chloride of silver is dissolved and 

 washed off, and the metal being again in its naked state, or cleansed 

 from the chloride, it can be attacked afresh by the same acid. This 

 acid acts better warm than cold. 



2. As all metallic substances are soon covered, when exposed to the 

 atmosphere, with greasy or resinous matters, it is necessary, in order 

 that the action of the acid upon the pure silver should have its full 

 effect, for the surface to be perfectly purified : this is effected by the 

 employment of alcohol and caustic potash. 



3. When a daguerreotype picture is submitted to the effect of a 

 boiling concentrated solution of caustic potash, before being attacked 

 by the acid, the state of the surface is so modified that the acid spares, 

 or leaves, in the parts which it attacks, a great number of points, 

 which form the grain of the engraving. 



4. When the effect of the acid is not sufficient, or, in other words, 

 if it has not bitten deep enough, the effect is increased by the follow- 

 ing process : Ink the plate as copper-plate printers do, but with a 

 siccative ink ; when the ink is sufficiently dry, polish the white parts 

 of the plate, and gild it by the electrotype process ; then wash it with 

 warm caustic potash, and bite in with an acid, which will not attack 

 the gold, but only the metal in those parts which, having been pro- 

 tected by the ink, have not received the coating of gold. By these 

 means the engraving is completed, as by the action of the acid alone 

 it is not generally bitten in deep enough. 



5. To protect the plate from the effects of wear, produced by the 

 operation of printing, the following process is employed. The sur- 

 face of the plate is covered with a very thin coating of copper, by 

 means of the electrotype process, before submitting it to the operation 

 of printing ; and when the pellicle or coating of copper begins to 

 show signs of wear, it must be removed altogether, by plunging the 

 plate in ammonia, or in a weak acid, which, by electro-chemical ac- 

 tion, will dissolve the copper without affecting the metal under it ; the 

 plate is then covered again, by the same means, and is then ready for 



