Putting pure salt ( kosher or rock) on a piece of steel is an old blacksmith trick to determine when the metal is at the approximate temperature to quench. Rock salt melts at 1300F/700C. In the modern age, a magnet is easier to use, and tells you when the metal reaches about 1350F/735C.
A file ( assumed to be about 1095/W1) steel needs to be heated to about 100F/50C degrees higher than non-magnetic temperature ( around 1450F/785C). If being annealed, it needs to cool slowly after being heated to 1450F. If being hardened, it needs to be held at that temperature for a few minutes, and then quenched in a fast oil or brine. You will need, at the minimum, a propane torch ( Bernzomatic JT-7 torch will work) to do both the annealing and the hardening. Throwing the file in the coals is not a good system.
Tempering in the kitchen oven at 375-450F ( 190-230C) for one or two hours will be necessary after hardening ( or to bring down the too hard file to a usable hardness).
Understand that these methods will very likely not produce a good knife, and that it will take longer than a weekend.
zaph1:
From your posts to date, I would assume that you are a young father who is rather compulsive ( not meant in a bad way).
Knife making is a skill that takes time to do and time to learn. I would recommend that you read a book or two on knifemaking, read the stickies at the top of this forum, and then figure out if knifemaking is for you. It would be a good idea to post a query on this forum asking if any makers in the Altoona area would have you come over to learn a bit ( and thus you would get access to some proper equipment). Attending a hammer-in in your region will be a huge leap in learning.
Also, a lot of information is given on forums like this and others. Some of it is good advise for new makers,some of it is good for those who already understand the craft, and some of it is not good advise at all. Reading some books, and some tutorials online will help filter this somewhat.
This site has some of the best around here. If you have an attitude that you are asking questions to learn, and will listen, then you will get some good answers from professionals. If you are more interested in telling how and why you are going to do things your way....to those who do it all the time.....you will mostly get answers from people who have built only a few knives...or are working on their first knife.
Every one of the great makers on this forum made a "first knife". Most were pretty sorry looking. We all crawled before we walked. Take your time and learn the right ways. It will be safer and more enjoyable.
Final question, and it is more out of curiosity than related to knife making - how old are you, and what experience with explosives do you have?
