Many handicraft shoemakers use shards of broken glass as abrasive tools to scrape leather and other shoe materials, especially in narrow areas or to smooth out the marks of a coarser abrasive, like a rasp.
Making
Scrapers can be made by scoring panes of scrap glass with the corner of a triangular file or other hard tool, then snapping along the scored line over the edge of a bench or another fixed block. The edge at the break is the edge used for scraping.
Carreducker
https://carreducker.blogspot.com/2014/07/breaking-glass.html
- smooth surface after rasping
- picture-frame glass ideal
- 2-3mm, for breaking cleanly
- nick with another piece of glass or a file
- place the nick over a sharp edge
- lean the glass forward about 45 degrees
- press forward and down with thumbs, slightly twisting wrists
- takes very little effort
- should get a curved break
- use the concave edge to scrape