If you want to be a metalsmith you’ll have to become familiar with solder, which is the substance you use to fuse two metal surfaces.
The word solder traces back to the Latin word solidare, meaning “to make solid.” That’s what you use solder for: to make something solid. In this case you are joining two metal surfaces so they appear as one solid piece. The word also has a verb form, also solder, to describe the act of joining the metal pieces. You might use solder, for example, to solder a metal piece on a computer's motherboard.