The “lost wax” (cire perdue) process has been used for millenia to produce cast metal sculptures, usually bronze, that are hollow. Making them hollow is essential, because otherwise, the piece would be enormously heavy, even if small, and too expensive to produce. The clever technique for producing a thin-shell casting can also, with a few additional steps, produce a master that can be reproduced at will.
Considering how important this process is in the history of art, I am amazed at how difficult it is to find a decent explanation of it. Without clear diagrams, the lengthy narratives of the process are impossible to follow, and among the many illustrations I found, all seem to leave out one crucial step or the other!
Just to keep my own thinking on the subject straight, I created my own diagrams. Click to enlarge them.