In the case of the bitumen process, the photograph is taken as before, the negative placed upon the zinc plate in the same way, and the image printed upon the bitumen. When this has been done, the plate is flooded with turpentine, and all the bitumen dissolved away, with the exception of that upon the image. The subsequent proceedings are as in the case of the albumen process, and need not be recounted.
It will be seen (if this outline can be followed) that the bitumen process differs from the albumen only in the composition of the ground (as an etcher would term it), but the quality of line is very different. The zinc plates used are cut from polished sheets of the metal, from one-sixteenth to one-eighth of an inch in thickness.
A well-etched block should feel sharp yet smooth to the thumb and fingers, as if it were cut. A badly etched or over-etched block has an altogether different feel: scratchy, and repulsive to the touch. Frequently it happens that by carelessness or mischance the process-man will over-etch a block; that is to say, he will allow it to remain in the acid-bath a minute or so too long, so that the upstanding lines become partly eaten away by the fluid. The result, when printed, is a wretched ghost of the original drawing. An over-etched block, or a good block in which the lines appear too thin and the reproduction in consequence weak, can be remedied in degree by being rubbed down with oilstone. This, if the lines are not under-etched, thickens the upstanding metal and produces a heavier print. But some of the smaller process firms have an ingenious, if none too honest, practice of pulling a proof from the unetched plate, and sending it along with the defective block. This can readily be done by inking up the image with a roller before printing, and then passing the thin plate of metal through a lithographic press, or through a transfer press, such as is to be found in every process establishment. Of course the print thus secured is a perfect replica in little of the original drawing, and looks eminently satisfactory. One can generally identify these proofs before etching by their backs, which have, of course, not the slightest marks of the pressure usually to be discerned upon even the most carefully prepared proofs of finished blocks. The surface of a zinc block sometimes becomes oxidized by the acid used in etching not having been thoroughly washed off. This may occur at once if the acid is strong, and then it generally happens that the block is irretrievably ruined; but if oxidation occurs after some time, it is generally superficial, and can be rubbed down. The process of oxidation begins with an efflorescence, which may be best rubbed down with a thick stick of charcoal, broken across the grain. But zinc blocks are frequently ruined by carelessness in the printing-office after printing. When the printing has been done it is customary to clean type and blocks from the printing-ink by scrubbing them with a brush dipped in what printers call “lye”—that is, a solution of pearl-ash—which, although it does not injure the leaden types, is apt to corrode the zinc of which most process blocks are made, if they are not carefully and immediately washed in water and dried. A block with its surface destroyed in this manner prints miserably, with a fuzzy appearance. The easiest way of protecting blocks from becoming oxidized is to allow the printing-ink to remain on them, or if you have none, rub them over with tallow.
12½ × 9. BOLT HEAD: A MISTY DAY.
Pen and pencil drawing, reproduced by the bitumen process.