I am just an electrician doing experimental work with a Stubblefield coil. From what Ive read the most common steel is 1018. If Stubblefield used a common bolt I assume this would fall into the 1018 realm. Would the 1018 iron core be better cold rolled or hot rolled for the magnetic collapse as well as the least amount of magnetic retention? Should I use a different type of steel? I have an unknown steel 4 inch 3/4" sample rod that I put on 2 magnetron toroidal magnets overnight. I then put it up against an old color monitor and it showed no magnetic field at all. I did the same with a galvanized iron wire and that does show a magnetic field. Would this be a good test for iron cores or should an electromagnet be made to see if it retains the magnetic field? Thank you for any help!