Well, this subject I have been giving some series thought to off and on since i started doing clay quenching when i got my first propane forge 6 months into my 3 years and 7 months of smithing experience and to improve my understanding i thought i would share my current understanding of quenching steels. If I get something wrong please correct me. as i understand it the action of forming martensite is universally stressful regardless of the method of quenching do to the speed at which it forms. that speed being roughly the speed of sound which is why we can hear the dreaded ping of doom. Just for ease of discussion, we are going to put on some rose-colored safety glasses and assume each quench is using the ideal quenchant for the steel as I am not qualified to judge what is best for every steel, so I am just focusing on the methods of quenching and we will not be including stainless steel as i have not HT more than one blade made of stainless. So instead I am going to go threw the quenching techniques that I am aware of and list from what I think is the gentlest to the harshest. Notes: I am not going to touch the subject of molten salt quenching as I do not have the equipment to perform this and can not make first-hand observations about this method of quenching. I please ask that someone who does this type of quench regularly please make some observations about it. I have yet to try and likely am not going to try interrupting quenching eather however my understanding is that its a method of full hardening a blade.

First, any experienced Smith knows that if you get your steel too hot before quenching it you're likely going to get at least one crack, the photo I am sharing was a clay quenched blade that i got too hot and the damage is a result of my failed attempt to break it over my anvil. More information on the picture will be shared a bit later in this post. Also Regardless of the quenching technique I use, I always thermal cycle 4 times letting it cool to black but not to room temperature each time and quench on the 5th cycle of heating if I am doing a clay quench i let it cool to 399F then dip it in water before applying clay. I do not have an HT oven and this is how I chose to thermal cycle, however, I also rarely get warps on the rare occasion that I do a threw hardening.

my current understanding is that I feel the gentlest form of quench is clay quenching and the reasons for this is because it both limits the amount of steel that will make the transformation from Austenite to martensite and also it also slows down the speed of the quench medium. However you need to cover the flats, spine, and front of the riccoso, or you're going to have a greater risk of the blade cracking. Going from memory for the quench speed of water I believe it's rated at 5-7 seconds, however, with clayed blade water becomes a rightly a 7-11 second quench similar to the speed of AAA. depending on how thickly I applied a coating of clay I have seen Parks 50 go from a 7-9 second quench speed to somewhere between 30 seconds and 2 minutes. I have also seen a 30-second water quench. However, in doing these really thick clay coatings I also learned that the laws of physics apply in the fact that the heat is drawn upwards as the roughly 1470F-1500F steel and clay hits the medium your using which in turn causes it to boil (in water creating a steam jacket that slows hardening) that causes the heat to rise to the surface which means the exposed steel on the blade should still cool at the rate for the quench medium and the insulating effect of the clay causes your steel to cool at a much slower rate. this delayed cooling prevents the formation of martensite thus lessened the stress on the steel to only the exposed steel. you know your blade is as cold as it will get when your medium stops boiling. also if you're using the right steels like 1075, 1084, 1095, and W-series you get a simple hamon. you get more complex hamon by applying ashi lines. However, I have only just started falling down this rabbit hole so I can't say anything other than to experiment with applying them as that is what I have started to do. However, Picture 2 has 2 simple Hamons I have made using NJSB 1075. the larger blade was taken up to 1000 grit well the smaller blade was taken up to 2000 and has been scratched from use, both are what I call brut de polish blades, likely better know as Differential polish. however, I like brut de polish better as it just sounds cooler to me.

There is another benefit to using clay especially if your judging temperature by eye and that is you will know martensite has formed when parts of the clay are falling off the blade after the quench medium stops boiling. However, you risk subjecting the blade to auto tempering or greater stress if you pull your entire blade out before it's temperature has fallen down to around 399F which I think that's the temperature that martensite becomes stable and martensite stabilizes at the speed of sound. also, I did a proof of concept experiment were quenched 2 1095 blades ground to match each other as close as possible, made from the same 1/8th bar, thermal cycled 4 times, and quenched into parks 50 only difference being 1 was clayed one threw hardened. the clayed blade did not warp well the threw hardened blade developed 2 warps. I concluded that eather the added mass of the clay or the act the martensite transformation being transferred into the clay causing it to loosen the clay prevented the warping of the thin steel.

Also, I need to state that Japan did not invent the technique of the clay quench, they just brought it to the form of high art. the technique originated in china spreading east and west. Japan was a vassal state of china around the 800's AD according to the book A Study of the Eastern Sword the technique made it to was the middle east.

I find this poor damaged blade that was not to be, fascinating and is why I may be Bias to clay quenching. This badly cracked blade is a clayed 1095 blade that was about 5/16ths of an inch thick and my current understanding is it would need water to harden, however, I screwed up and got it too hot and I already had it in my coolant water bucket before I realized it was too hot and the end results speak for themself. When I screw up this bad I do not temper, I break the blade over my anvil by striking the anvil with the flat of a blade. 2-3 strikes and the only damage was what you see, the silver circles are cracks. this is also why I chose to use clay for the Journeymen smith test over blue backing...however I may blue back the blade as that will not damage the hamon.

The second gentlest quench I feel is the edge quench as this only subjects the edge of the blade to the transformation of martensite. However, I only advocate the method were you heat only the edge of the blade then fully immerse it in your chosen quench medium as the step method is just plain dangerous in my eyes. this method is generally thought done with an oxy-fuel torch which I have not done yet, however, I have done this technique using a coal forge. You can just heat the edge by running it threw the coals well running your blower. I brought this second method up because when I took a bladesmithing class back in march my instructor did not know this was a possibility. however, I am unsure if this simple method can make a hamon as the steel used in the class was 5160 and also have yet to get the blade polished up. the blade was 5160 and I have yet to get the polish up to a high enough grit to see if a weak hamon formed. However, both The Art of the Japanese Sword: The Craft of Swordmaking and its Appreciation and Knives 2019: The World's Greatest Knife Book state how to do the clayless method to get a hamon far better then I can.

as I understand quenching the harshest is the a threw Harding as it subjects the entire blade to the austenite to martensite transformation. this can lead to warps or cracks if the blade is not evenly heated.

so this is my current understanding of the techniques of quenching and I feel that quenching a clayed blade is ultimately the gentlest on the steel that I have preformed. so please ask that my more experienced colleagues correct any flaws I have in my understanding of the quench.