Photoengraving
It is then exposed to light—usually strong ultraviolet (UV) light—through a photographic, mechanically printed, or manually created image or pattern on transparent film.
A solvent is used to wash away the soft parts, laying bare the underlying material, which is then bathed in or sprayed with the acid or other etchant.
It can be used to make flat springs, levers, gears and other practical components that would otherwise be fabricated from sheet metal by cutting, drilling, jigsawing or stamping.
In this method, the metal (usually zinc or magnesium) is held face down and a mixture of nitric acid and a soap-like oil is splashed onto it.
The first[citation needed] photoengraving process was developed in the 1820s by Nicéphore Niépce, which used photoresist to make a one-off camera photograph rather than a printing plate.
His usual test subjects were paper prints of conventional engravings, and exposure was by contact under direct sunlight rather than by the use of a camera.
[citation needed] Very long exposures in bright light were required, but bitumen had the advantage that it was superbly resistant to strong acids.
The daguerreotype image consisted of a microscopically fine granular structure on the surface of a silver-plated copper sheet that had been polished to a mirror finish.
In some instances, very pleasing results were obtained, but exceptional skill and care were required and the very fine structure of the image limited the useful life of each plate to a few hundred prints at best.
Henry Fox Talbot is usually credited with the first workable process for converting a grayscale image into a varying structure of stark black and white that resulted in a reasonably durable printing plate.
As with other early halftone processes, the plate could not be combined with ordinary type, so for inclusion in a book or periodical each image had to be printed separately and either bound in or tipped in with an adhesive.
His process came into widespread use during the 1890s, largely replacing the hand-engraved wood and metal blocks that had previously served to provide illustrations.
