Hi Philip,
Like many of the responses already posted - broken glass was used to
scrape wood; unlike a metal scraper accompanied by the use of a dressing
burnishing too, a preferably flat piece of broken glass was very efficient.
The use of glass in this manner is old, infact, old English books on
carpentry/wood working use the term "glass paper" as opposed to the later
description "flint paper", "sand paper" etc.
Hope this helps.
Russell