When making a line etching, an acid or mordant is used to etch the image into the plate.
A copper plate is covered with a layer of acid-resistant wax called a hard ground. The artist creates the motif by scratching through the wax, exposing the copper, with a hard steel point. The softness of the ground compared to copper, makes it easy to obtain a free flowing line. When the plate is submerged in an acid bath, the exposed lines will start to etch. The acid “bites” only the areas of the plate, where the ground has been removed, leaving the rest, untouched. Depending on how long you leave the plate in acid, the lines can be fine and thin or deep and course. The plate is filled with ink and the surface wiped clean before it is run through the press.
Historically, line etching made it possible for artists and amateurs alike to make intaglio prints without learning the skill of engraving, but for a long time the technique was regarded as less refined than engraving.