It consisted of three three-hour sessions. The first was a practice knife where we used mild steel (steel with about .16-.29% carbon) and the instructor taught us all the parts to making the knife. This was mostly review for me since I had previously taken the Simple Knives class there which was essentially the same thing.
The second session we made the real knife, using high carbon steel (about .30-1.70%. What we used was probably around .8-.9%) which is much harder than mild steel. It heats differently and fights back more when being smithed.
The most interesting part, however, was after we had finished hammering the blade, and we annealed it. The instructor had a basic understanding of the metallurgy behind much of this process, and so he told us about it. Some of it I knew already, but some was new. For annealing, we heated the blade up to the point where the whole thing was hot enough to change from a Base Centered Cubic crystal structure to a Face Centered Cubic structure, called austenite. At this point the steel also ceases to be magnetic. I don't know why, though I sure would like to. After we have heated the blade to this temperature we placed it into an ash bed and left it to cool overnight. The slow cooling allows the steel to form large grains, making it very soft.
In the third session we came in and removed our knives from the ashes. We wanted them soft so that we could grind and file them easily. So we proceeded to grind and file for quite a while. This is mostly what makes the blade, since you can't get it sharp enough to cut anything with a hammer. After lots and lots of grinding, when we have a decently sharp blade, we begin the tempering process. This starts with again heating the blade up to an austenite temperature, and then cooling it rapidly in a vat of oil. Not only does this make it very hard and brittle from the rapid cooling, but gives it a nice oil finish.
After this comes the main tempering. A really hard brittle knife isn't so good, since it can be brittle enough that dropping it on the floor might shatter it. Also it would be really really hard to sharpen. So we took large metal clamps, heated them really hot, and then held them against the back part of the blade.
At this point our instructor explained to us tempering colors. Steel will, starting at about 300 degrees, start showing colors. It starts with a light straw color, goes to bronze, then brown, then purple, then deep blue, then a bright teal color. It's pretty dramatic. His theory as to why this occurs is that a very very small oxide layer forms, small enough to be transparent, but still diffract the light, creating the colors. He's not sure about this, though, so it would be a cool thing to check. This is useful because it can tell us what temperature the blade is at. We hold the hot clamp on until the blade becomes bronze. We then quench it very quickly. The purpose of this is to leave the blade hard enough to hold and edge but soft enough to sharpen well and not break off. After the tempering comes some final sharpening, and then you have a finished knife!
The class was great, and lots of fun, especially since I learned not just how to make a knife, but much of the materials science behind it!
Below is a picture of the knife I made:
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