This broken glass is called "cullet," and is carefully swept into piles and kept in bins for use in the furnaces.
"How To Write Special Feature Articles" by Willard Grosvenor Bleyer
The price of the same glass thrown under the bench, and known as "cullet," is L1 per TON.
"Stained Glass Work" by C. W. Whall
They can be traced by cullet heaps and broken-down furnaces, and by their names, often mutilated, recorded in parish registers.
"Encyclopaedia Britannica, 11th Edition, Volume 12, Slice 1" by Various