The two small mirrors set inside the crest are the most fiddly pieces of glass on this frame, and they are cut last.
They sit in openings that are not regular shapes. Each opening is defined by the carving around it - a scroll on one side, a leaf on another - and no two are identical, because the crest is carved by hand rather than repeated from a mould.
So each is templated individually. A thin sheet is pressed into the opening and scribed against the carving as it actually is, and the little plate is cut to that scribed line rather than to a drawing.
Fitting matters as much as cutting. A small plate wedged tight into an opening surrounded by carving is a plate under pressure, and pressure is what cracks glass at a point. So each is bedded on felt with a small clearance all round, and held with turn fixings from behind rather than pinned.
The reward for the trouble is at the top of the mirror. Two bright openings in the crest catch ceiling light and read as air, which keeps a heavily carved head from looking solid.