loupe

CONT: Procédé de la cive ou boudine : (...) Le plateau [de verre] terminé gardait en son centre une sorte de lentille plus épaisse, la loupe ou boudine.

CONT: Crown glass method: ... After this disk is broken from the pountil, leaving a bull's-eye in the center, it is annealed and cut into panes.
OBS: It was the centre part of a large pane of crown glass.