I know the difference between mild steel and high carbon steel during a spark test means high carbon steel puts off sparks that fork and look like fireworks while the mild steel tends to have much duller less impressive sparks. My question to all the way more experienced blacksmiths on here is... has anyone found a good rule of thumb for identifying different high carbon steels from spark tests? I use almost exclusively salvaged metal in my forge and I know different metals prefer different heat treats. I know springs from cars are spring steel, tire irons are tool steel etc. but a lot of what I use is scrap from a metal fabrication shop and I have no idea what most of it is. Any tips for identifying scrap metals would be most appreciated.