Which of the following methods is NOT used to test the hardness of a metal?

The pyrometer is a device used to measure high temperatures, often in metal processing and manufacturing contexts, but it does not measure the hardness of a metal. Hardness testing refers specifically to methods that assess a material's resistance to deformation, scratching, or indentation. In contrast, the Mohs Hardness Test, Brinell Hardness Test, and Rockwell Hardness Test are all established methods specifically designed to evaluate the hardness of various materials, including metals.

The Mohs Hardness Test ranks minerals based on their scratch resistance, while the Brinell and Rockwell tests involve applying specific loads to determine how much a metal deforms. Therefore, the pyrometer does not fit within the category of hardness testing methods.