Carbon steel Case Knives
Case hardening is the process of adding carbon to steel surface and heating it to harden the exterior of the blade. The insides of the blade remain soft though. I've tried bluing/tempering on some of the kitchen knives that I never used. The problem with the knives was that their blade was very thin. That's why I couldn't make their intended use. This thin blade actually made the process quite quick. The source for the heat was a gas stove because that's what I could find quickly. You cannot use candle flame because candle isn't a combustible source. If you heat it on a candle, you'll just get the knife blackened because the carbon and soot from the incomplete combustion gets stuck to the blade. Gas stove uses petroleum gas which is fully combustible and hence a blue flame.
The first time the blade was quite short. Also the blade carried my finger prints all over them. I couldn't move the knife on the flame as I wanted without getting my hand burnt. This resulted in uneven and bad looking pattern with lots of smudges and finger prints on the knife.
This time I took a knife with a longer blade and cleaned it up nice. Then I heated the knife until red hot with a slow pushing and pulling motion (sort of stabbing in slo-mo). Because the blade was very thin, it heated up rather quickly. After heating it red hot, I dipped the blade in cold water for a good 20 seconds. This immediate cooling of the metal after it's been heated red hot hardens the blade and makes the colors come out. This time around, the finish on the blade was better with more even colors on the blade and almost zero smudges.
Notice that the tip is blue. This is because the tip was the most exposed part of the blade during the whole heating process. The top of the hilt is made of plastic and it deformed slightly during the heating process. Don't heat for too long though or else the blade will turn slightly slate grey in color which does not look good.
I am not a blacksmith nor a professional knife guy. I like the way some CS:GO knives looked and decided to try to temper them. This knife is not perfect. My knives look more of a 'Fade' than 'Case Hardened' but the concept is pretty much the same. For Case Hardening, I've heard people use ash and clay mixed with just enough oil to make a thick paste and applying it over the areas where you want most coloring. I haven't tried it so I dunno for sure (though I will try it soon). Take care and wear gloves while heating the knife blade because it gets pretty hot! Don't forget to cool the blade after heating it.
I tried the oil and ash method mentioned above and the results were great! What I did was this- I made a paste out of burnt incense stick and some powdered charcoal with machine oil. I applied small blots of it on the blade of my third kitchen knife and heated it like before. The paste on the blade sizzled up when I heated it on my stove and the result is this...
Featuring Kitchen Knife | Case Hardened!
I applied the paste on some parts but next time around I am going to evenly apply the paste to all of the blade.