engraving that uses acid on a metal surface is known as etching.
etching is also used in the printmaking industry to carve an image into a metal plate with acid. once the acid is applied to the metal it bores into the surface, leaving behind lines and rough areas.
the process of etching is believed to have begun with daniel hopfer of augsburg, germany in his profession as an armor maker. it took a while for the process to be applied to print making.
both copper and zinc have a thin coating resin layer that is resistant to acid, which makes them ideal metals to use for etching. a coating can be made of a waxy material for use with metals that do not naturally have a coating. to make the engravings and lines clearly visible through the resin the metal plates are usually smoked. the metal is then scratched with a sharp tool which exposes the metal surface without penetrating it.
after the design has been etched onto the metal plate it is placed into an acidic solution which will only attack those parts of the plate which are exposed. the process of immersing the plate into the acid is called a bath. the plate is taken out of the bath several times, and then place back in until it has been etched to the desire depth.
to prevent the acid from penetrating the metal any further a coating of varnish is applied to the metal. you will get darker prints from the lines that are exposed to the acid the longest. there are some etchers who manually apply the acid to the plates surface rather than using a bath.
when making a print the varnish is removed and then ink is heated and placed onto the plate. after the ink is applied the excess is removed so that ink only remains in the depressions created by the etching process. the plate will then be covered with a soft, moist paper and placed in the etching press.
etchers will sometimes need to use modifying techniques in order to get the right image. removing any unwanted lines is one way that etchers will modify their products. they first make a trail print, and then burnish or modify the plate to correct it.
each plate can only make a few prints, as prints will look different in various stages of the printing process. after the desired number of prints are created the plates are usually destroyed by the etcher.
different methods of etching will produce different effects. items tend to have the look of a pencil drawing when they are created using soft-ground etching, while items created using aquatint with look more like a wash drawing. hard-ground etching is usually used in combination with aquatint.
the delicate art of pictorial etching is said to have evolved in germany as well. this method has its origins in burin engraving, where artists would etch on iron and gives us the earliest evidence of artistic etching.
wet and dry etchings are the most popular methods of etching these days. a container or bin filled with a solution that dissolves the metal is used for the easy process of wet etching. a mask is required so that the solution will only etch the desired parts of the metal plate. this means you will need to find a masking material that will not be destroyed by the etching solution. this method works great for etching thin films.
there are three main categories of dry etching: reactive ion etching (rie), sputter etching, and vapor phase etching. in rie gases are mixed with plasma in order to break gas molecules into ions. these ions will then react to the metal surface as the ions are accelerated.
gases are also used with sputter etching, but the ions are not. the simplest dry etching technique is vapor phase etching. in this technique the metal plate is placed into a chamber where gases are then introduced. a chemical reaction occurs at the metals surface interacts with the gas molecules, causing the design to be dissolved into the plate.
silicon dioxide etching, which uses hydrogen fluoride, and silicon etching, which uses xenon diflouride, are the two most popular types of vapor phase etching.