Folks had to expect me to put my nose it this one, they probably also expected me to be my typically verbose self, so here we go
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From all the great opposition among bladesmiths to commercial quenchants I have encountered in my travels and talks, I am convinced that the quenching process is profoundly more complicated than most knifemakers understand (which is too bad since it is the most critical moments in the creation of a knife). The greatest error most folks make in judging a quenchant is by simply asking how fast can it cool things. If this is all that mattered, why not just use brine and detergent for everything?
There are three distinct phases to a quench (there actually is a fourth but it can be irrelevant to this conversation) and each of those stages must be taken into consideration for what you are trying to do with the steel.
You quench you blade into a liquid quench medium and immediately you go into the first phase The vapor jacket/blanket phase. The 1400F+ steel will immediately bring any liquid it contacts to its vapor point, forming a insulating jacket of gas all around the blade. This very undesirable insulating effect happens to coincide with a cooling range in the steel that is most critical to cool the fastest if you want to fully harden the blade. There are a few ways to defeat the vapor jacket, the first being the chemistry of the liquid itself. You want a liquid that has the highest boiling point and vapor point, so that less of ti will go into gas. You may also want to mess around with its surface tension qualities so that the jackets breaks up easier. One way to physically beat the jacket is
Agitation, that is physically force fresh liquid into contact with the blade.
In just a short while the blade will cool enough for the vapor jacket to become unstable, as liquid intermittently makes contact with the hot steel, a violent boiling begins- The vapor collapse phase . This stage results in very rapid, and, unfortunately, very uneven cooling of the steel. This is when scale and other debris can be blown off the surface from the violent mini explosions of gas.
Anyone putting hot steel in water will notice how when it initially enters there is a calm before the storm (the vapor jacket phase) then as the water bites the steel there will be the growling, hissing, boil (the vapor collapse phase), and then there will be a trailing off to the final stage- The liquid cooling phase. This is the point where the steel is in complete contact with the liquid and the heat extraction via conduction and convection is entirely unhindered. It is also the part that coincides with the formation of martensite, so the less uneven stress, the better.
Lets just take a look at good old water, and examine its qualities. It has a very low vapor point (212F.) so as soon as your blade goes in, you have a huge vapor jacket problem, and when this jacket collapses, the low boiling point results in an incredibly violent and uneven cooling. When you reach Ms the cooling is still very quick, in fact, the jacket is still collapsing through at least half of martensite formation. There should be little wonder that water quenching often results in warped and cracked blades.
The ideal quench would form no vapor at all, cool incredibly fast until Ms (martensite start) is reached and then cool very gently and evenly. Science is still working on it for every type of steel, but I can confidently say that they have come darned close and it is readily available.
Now lets look at some other considerations. Aside from vapor points some liquids have better thermal extraction rates than others. Some can handle being scorched with hot steel over and over again much better. Thermal breakdown and oxidation become considerations in the oil mediums. Whats in it? Will it stain or discolor your steel? Will it stain or discolor your lungs? Will it burst into violent flames and put you in the hospital as soon as you put steel to it? How expensive is it to use?
Commercial quench oils take all of this into consideration (at least the good ones do) and I highly recommend them. I have worked with plenty of home made concoctions before I quit playing the eccentric alchemist/bladesmith role and just bought some good quench oil, and there simply is no comparison. All of my Rockwell numbers evened out to a very predictably high range, and my next batch of quench oil will be just the same as the last, without trying to figure out what I all mixed in that winner potion. My blades came out very clean, I could even watch the entire cooling process below the perfectly clear amber liquid. Interrupted quenching for a martempering effect became so much easier. It is safe to say that I am sold.
Most of the home made quenches that I have played with and have heard described, tend to deal with distortion in a far too simplistic and tragic way- just slow down the cooling
This invariably results in the formation of fine pearlite, a condition that results in the standard line my blades seem O.K. so my quench is good enough. Fine pearlite can be pretty darned hard to detect without a microscope and can seem quite hard- but it is not martensite and it will not perform at the same level as it. And with the new standard of knife performance being how well it bends, blades with extra pearlite in them really shine, it is really too bad when you think about it.
Sorry, I didn't even bother to look at the topic setting questions, I do that too often.. how rude 
