ENGRAVING, HELIOGRAPHIC. The processes of heliographic engraving have not yet been brought to sufficient perfection to be worthy of more than a few brief remarks in this place.
M. Nicephore Niepce availed himself of the property possessed by bitumen of JudEea, of being rendere,d insoluble in the usual menstraa by the joint action of light and oxygen. A metal plate was coated, in the dark, with a solution of bitumen in a solvent, dried, and exposed to light under an eng,raving, or in the camera. It was then washed with a solvent, which removed the bitumen where light had not acted, and allowed it to remain where light and oxygen had rendered it insoluble. The bare metal thus ex posed was then etched with an acid, and an engraving produced.
This process was followed up by M. Niepce de St. Victor, and some improvements made in it, which will be found described at different times in his communications to the French Academy of Sciences.
Mr. Fox Talbot, a few years ago, obtained some little success with a rather different, and, we imagine, a better process. He coat,ed a steel plate with a mixture of gelatine and bichromate of potass, and exposed it to light under a negative. Where the light acted on this mixture, it reduc,ed the salt of chromium, and caused it to combine with the gelatine, thereby producing a compound which was insoluble in water. The plate was then washed with water, which removed the unaltered gelatine only. A picture was in this way produced upon the plate, which was etched by a solution of bichloride of platinum, which attacked only those parts where the metal was not protected by the gelatinous compound.
MM. Salmon and Garnier have suggested a process of helio graphic engraving different from either of the preceding, and which appears very ingenious.
" If a polished plate of brass, previously submitted to the action of iodine vapour, is exposed to diffused light, and then rubbed with wadding, charged with globules of mercury, the following pheno mena will be observed—the plate will not be amalgamated, the mercury refusing to attach itself wherever the iodine has been influenced by light. If, instead of proceeding thus, the plate is covered in places with an opaque body, and we then try to amal gamate it as before, it will be observed that the mercury takes perfectly on the parts where light has not acted, while it refuses to adhere in other places. This property, discovered by MM. Salmon and Gamier, indicate,s the possibility of reproducing photographic images upon a plate of brass. A negative on glass, or paper, is applied to the iodized bra,ss plate, and left in contact from ten minutes to two hours in diffused light. On mercurializing the plate, the mercury is seen to adhere to those parts which correspond to the blacks of the negative, leaving the other parts intact. If an
ink-roller is then passed over, the untouched parts take the ink, while the mercurialized parts do not, so that the picture is black upon a white ground.
" An etching may then be produced by first dissolving out the mercury by means of a solution of nitrate of silver acidified with nitric acid, and then biting in the plate still deeper by acid alone. If, on the other hand, an engraving is wanted for printing in a lithographic press, the plate is immersed for a few minutes in a galvanic bath charged with chloride of iron, so as to deposit a thin layer of metallic iron in the places previously occupied by the mer cury, and where the brass is now bare, namely, on the lines of the drawing. The utility of this deposit of iron will soon be perceived. The brass plate being removed from the bath, the greasy ink is dis solved off with spirits of turpentine. The entire plate is then exposed again to the vapour of iodine, and rubbed with wadding bearing globules of mercury ; whence it results that the plate, as before, acquires a white colour, from the amalgam of mercury ; but as this metal does not amalgamate with iron (for mercury is preserved in iron vessels), a mere gentle rubbing of the plate removes it from the places covered with iron, that is to say, from the drawing itself ; so that after the second operation the drawing has the lines covered by a thin layer of iron, while all the rest of the brass plate is coated with mercury.
" Things being in this state, the ink roller is passed over the plate ; only the drawing itself takes the ink, while those parts coated with mercury repel it. This is just what was required. As many impressions as desired may now be printed, only taking the precaution to rub the plate afresh with mercury after a certain number of impressions have been thrown off. If it be wished indeed, the two last operations might be omitted : it would suffice to wet with water the plate once inked with greasy ink, as done by the lithographic printers ; under these circumstances the parts free from the drawing would be isolated by water, whicn would prevent the greasy ink from touching them.
" Supposing now that instead of a plate to be printed in a litho graphic press, it is desired to have one that may be printed with letter press, the following will be the mode of procedure : Taking the plate at the moment when it is about to be immersed in the galvanic bath, a preparation of gold should be substituted for the salt of iron, and a thin layer allowed to deposit over the drawing (it will take the gold because it resists the action of the acids better) ; the plate is then inked and eaten away all round the lines, which will be preserved by the gold, the surrounding copper only being attacked, so that the drawing itself will be found in relief."