Mack M. Jones, from "War Department Education Manual, EM 862," 1944 quoting text of 1898: "Different grades of iron and steel may be distinguished by the sparks produced when ground on a grinding wheel. The higher the carbon content of the steel, the brighter and more explosive are the sparks."
PHOTOGRAPHER'S JOURNAL: After being forged and tempered, plough blades and other farm tools needed to be sharpened, but a good grindstone was also needed to sharpen many of the tools the smith used to work the iron. No blacksmith could be without a good grindstone. In an age before electricity a farm blacksmith needed a large stone that could develop significant centrifugal momentum.