This is a good discussion and a lot of good advice being shared here because the gentle and gradual methods of heating the oils that are being offered are much better (both for the oil and yourself) than using massive super heated objects to do the job. I do understand and still do just heat up a large piece of steel to accomplish the task in a pinch (on the road doing demos), but I am always careful to keep the smoking to a minimum and never allow flames. While the very act of quenching steel in excess of 1400F is the purpose of the oil, it also is what much of the special engineering in these products was aimed to overcome the destructive effects of.
If you are using the appropriate amount of oil for the volume of steel you are quenching, there will not be a rise in quenchant temperature as it will be doing its job effectively. Using a large block of overheated steel intentionally exceeds the quenchants ability to cool and thus heats things up, this is not a problem so long as it is kept under tight control. But the vast amounts of vapor and oxidizing by using very high temperatures in mass will eventually shorten the life of the oil, and flashing it certainly will (not to mention what breathing all that unnecessary smoke does for your lifespan).
A gentle warming provided by the devices mentioned here are just the ticket to accomplish the task without the above mentioned issues. Since I only oil quench my smaller blades, as I have mentioned before I use a large portable roasting pan for my quench container, then all I have to do is set the dial to the desire pre-wa ming temp and go, and it comes with its own handy and stylish cover.
Remember that unlike other slower oils, you don't want to heat it above say 120 degrees anyway. Oils have a "sweet spot" and the listed range for #50 is 70-130F IIRC. Tough Quench is like 120-180 and it is fastest at around 150 and starts to slow down above or below that. The only time I have to heat #50 is in January down here.
