PHOTO: A wavy clay temper line on a Steve Corkum "Persian".
What Is It?
It goes by many names, there is differential tempering, differential hardening, selective tempering, selective hardening, zone treating, soft-back draw, and the list goes on and on it seems. By whatever name is used, it means that the blade has been heat treated to have a hard cutting edge and a softer "springy" spine. There are a few different ways to achieve this, so the term I use depends on the process I used. More on this later.
What Are the Benefits?
A blade in which the spine has been hardened to a lesser degree than the cutting edge will exhibit a great deal of strength and flexibility and still have exceptional cutting ability. This is assuming of course that the heat treating was properly done.
This is quite beneficial to larger blades which must be able to handle a lot of abuse. Prime candidates would be a bowie or camp knife which might be used for chopping up firewood, clearing a path through the woods, etc. This is also good for thinner blades that require quite a bit of flexibility. Some knifemakers do differential heat treating on all their blades.
One of the performance tests for a prospective Journeyman or Master Smith in the American Bladesmith Society is the bend test. The smiths blade is clamped vertically in a vice and must be bent 90 degrees without breaking. At first one might think that all you'd have to do is temper the entire blade to a spring temper. Well, the problem with this is that the cutting edge would be too soft to pass the cutting tests. Prior to the bend test the blade must cleanly sever a one inch free hanging rope approximately six inches from the loose end, in one movement. Next it must be able to chop a 2x4 in half at least twice, with no edge damage, and still be able to shave hair from the applicant's arm. This is achieved by differential heat treatment.
How Is This Done?
There is more than one way to get the hard cutting edge and tough, springy back. To cover the whole spectrum I say that my blades have been differentially heat treated. To be more specific as to the method used, I use one of the terms mentioned above. Some of the methods, as well as the terms I use to describe the methods are listed below.
This is the term I use when the transition was performed at the hardening phase of heat treating. This would cover the edge quench, clay coating, and torch methods. It is not my intention to give a full course in heat treating, but here is a brief explanation of the three methods.
Edge Quench - With this method, the whole blade is brought to critical temperature, then the cutting edge only is placed horizontally in the quench medium, and carefully rocked upward toward the tip of the blade. The steel exposed above the quench line will cool more slowly and therefore reach a lesser degree of hardness.
Clay Coating - First the back of the blade is coated with clay or refractory cement. The blade is then brought up to critical temperature and placed in the quench medium. The portion of the blade which is coated will cool more slowly and reach a lesser degree of hardness.
Torch - The cutting edge is quickly brought to critical temperature with a torch and then placed in the quench medium. This method is usually only used on smaller blades because it is difficult to evenly achieve the proper temperature on the full length of a large blade.
This is the term I use when the transition was performed at the tempering phase of heat treating. That is, the blade is fully hardened, then tempered, and finally the spine is drawn back with a torch. This is often done with the cutting edge placed in a tray of cold water. The depth of the water is usually set from 1/3 to 1/2 the overall width of the blade. The spine is carefully drawn back with a "painting" action of the torch on the spine.
As previously stated, this was not intended to be a lesson in heat treating. I have just scratched the surface to give you an idea of what differential heat treating is, where its application is beneficial, and some of the methods used.