Photogravure
This process was developed in the early 1800s. Photogravure enables a photographic image to be converted to an etching, and is one of the finest means of photographic reproduction available. It combines the details of a photograph with the richness of an etching. The image, a photographic positive is exposed to photo-sensitized gelatin tissue, and an aquatint is also applied. This gelatin tissue is then bonded to a copper plate, and the image is developed in water. The plate is then etched in 4-6 different strengths of acid (ferric chloride). The water content of the acid dissolves the gelatin tissue, and the acid then bites the metal. The plate is printed as for an etching. . Photographic and drawn images can also be combined in Photoshop and used here.
Direct Gravure
The process is the same as for photogravure, but traditionally the image source is a drawing on a transparent paper such as Mylar. Photographic and drawn images can also be combined in Photoshop and used here.
Etching
A metal plate (copper or zinc) is covered with an acid resistant wax. An etching needle is used to draw the image into the wax resist – the drawing reveals bare metal under the resist. The plate is placed in acid, and the acid bites into the metal where it is exposed. The longer the plate is in the acid, the deeper the acid will bite, and the darker the line will print. The plate is inked, and the ink is pushed into the grooves bitten by the acid. Excess ink is removed from the plate, damp paper is placed over the plate, the plate is passed through the printing press, and the ink from the plate transfers to the paper. An etching has a sharper and cleaner line than drypoint, due to the action of acid on the metal plate.
Aquatint
Aquatint is often used as part of the etching process. Aquatint enables tonal areas to be added to an etching plate. The plate is covered with powdered rosin, the rosin is fused onto the plate over heat. The plate is immersed in acid, and the acid bites between the microscopic dots of powder. The longer the plate is left in the acid, the deeper it will bite, making the aquatint darker.
Photopolymer
For this process a thin metal plate with a light-sensitive polymer coating is used. As in the preparation for photogravure, an image (photograph or drawing or a mix of both) on transparent paper is exposed directly onto the plate. The plate is immersed in water for about 1 minute in order to develop the image. The plate is dried and exposed to light again to harden the image on the plate. To print the plate the image is inked as for an etching. A piece of matt card, cut to the same size as the plate is inserted under the plate on the press bed. This enables easier printing and gives the classic plate indentation associated with intaglio prints.
Lithography
Lithography (stone writing), developed in the 1790’s, relies upon the antipathy between grease and water. Flat slabs of limestone from a (now empty) quarry in Bavaria are exclusively used for stone lithography. The surface of the stone is ground with grit (coarse to fine) to create a smooth surface on which to make the image. The image is created using lithographic pencils, crayons and washes – all of which combine grease and black ink. This range of drawing/painting materials enable a wide variety of marks to be made on the stone. The process of drawing deposits greasy marks on the stone. When the stone is to be printed, water is sponged over the surface, and a roller charged with (greasy) ink is rolled over the stone. Where there are the marks of the image, the ink stays on the stone, where there are no marks, water repells the ink. Damp paper is placed on the stone, and is passed through the press, transferring the ink to paper.
Drypoint
A sharp needle-point (drypoint) tool is used to draw into the surface of a metal (usually copper) plate. The drypoint tool throws up a burr as it is drawn across the metal. When the plate is inked, the burr catches the ink, which gives a drypoint its characteristic soft, velvety line. Excess ink is removed from the plate, damp paper is placed over the plate, the plate is passed through the printing press, and the ink from the plate transfers to the paper. Each pass through the press lessens the burr on a drypoint, so editions are usually small.