I follow quite a few knifemaking and blacksmithing groups on Facebook, and have learned a great deal from them. When a TTAK commenter asked a question this week about making a knife from steel from an old safe, I actually knew the answer as this is a common question I have encountered.
If one wants to make a knife from mystery metal, it is important to test whether or not at the end of all of your hard work, the material you will use will be able to be hardened to a durable edge. For this you use a spark test.
Spark testing is a method of determining the general classification of ferrous materials. It normally entails taking a piece of metal, usually scrap, and applying it to a grinding wheel in order to observe the sparks emitted. These sparks can be compared to a chart or to sparks from a known test sample to determine the classification. Spark testing also can be used to sort ferrous materials, establishing the difference from one another by noting whether the spark is the same or different.
Spark testing is used because it is quick, easy, and inexpensive. Moreover, test samples do not have to be prepared in any way, so, often, a piece of scrap is used. The main disadvantage to spark testing is its inability to identify a material positively; if positive identification is required, chemical analysis must be used. The spark comparison method also damages the material being tested, at least slightly.
Without a positive identification, certain steels may still be difficult to heat treat, but if one can determine an inadequate amount of carbon at the start, it will save a lot of headaches.
I am sure that there are knife-making readers who can elaborate on this. I am appreciative of anything you all have to contribute on the topic.