echoppe

Callot is credited for inventing the echoppe etching needle, this needle has a slanting oval area at the end and in a similar way to a fountain pen will allow etchers to swell and fill lines in a similar manner to engravers. (London Fine Art Studios)

The echoppe, a tool with a slanted oval section, is also used for “swelling” lines. The plate is then dipped in a bath of acid, technically called the mordant (french for “biting”) or etchant, or has acid washed over it. (Articulate Ink)