The frizzens on old flintlocks were often carburized, meaning they were made from iron, put into a container containing scraps of material containing carbon and heated to absorb the carbon and then quenched. There's a clickspring video where he makes files that way. It's possible these were made in the same way.
case hardening isn't a reliable way to make a flint striker. Many new files are case-hardened and don't spark well, old files on the other hand tend to be thoroughly hardened and do spark well.
@@waveman0 you will definitely get a crack. This is because god hates humanity. But the reason I sometimes work it a bit cold because I work outside and when the sun shines it becomes difficult to even see how hot steel is. It just looks cold but if you are in shade it is obvious when the steel is cold or not. The camera picks this up better than my eyes. But yes your right
