What is the powder blacksmiths sprinkle on metal?

Could you please enlighten me on a matter related to the craftsmanship of blacksmiths? I'm quite curious about the powder they sprinkle onto metal during their work. What exactly is this powder, and what is its purpose? Does it serve a functional role, enhancing the quality or durability of the metal, or is it simply aesthetic, adding a finishing touch to the finished piece? Moreover, is this powder a naturally occurring compound or a manufactured product specifically tailored for use in smithing? Could you provide a brief description of its composition and how it interacts with the metal? Thank you for your time and assistance in satisfying my curiosity.