Tempering in hot oil

[lastbrunneng]

I saw Sam Salvati temper a blade in a hot oil bath on the Man at Arms series. Post hardening and straightening, he immersed the sword in 350* oil.
Is this a common practice? What are the benefits? Are there any not-obvious safety concerns a person should know about this technique?
Very curious here.

 

[GHEzell]

If the oil reaches it's flash point you will have one heck of a fire. You would need to be very careful about temperature control.

 

[Stacy E. Apelt - Bladesmith]

There are folks who temper in oil...but it has to be done safely. Many oils have a low flash point. Tempering oils are mineral oil base ( about 95%). Cooking oils will not work well and can be very dangerous.

The benefits are excellent transfer of heat and ease of use. A table top deep fryer or modified turkey fryer with a PID control and safety shut-off would work WITH THE PROPER OIL TYPE.
The drawback is that oil is that the heat transfer is rapid, and blades should be pre-warmed before inserting or they may crack from thermal shock. The alternate is to put the blades in the oil at 200F and bring them up to the temper point along with the oil.
Another drawback is the oil isn't cheap.
As you know, several gallons of 400° oil is a big hazard all by itself.

I love Sammi-boy, but because someone did something in a video or demo doesn't make it a safe thing. Also, even though he is wild and crazy....Sam has a lot of experience.

 

[lastbrunneng]

I went looking through MSDS sheets and the first several mineral oils I found had flash points in the low 300 degree range, wouldn't want to use those! I have zero intention of replicating this though.
Since the heat transfer is so quick are the tempering cycles shorter?

 

[Bo T]

I believe, in the old days, a mixture of molten tin and lead was used. So a fairly uniform temperature and no radiant heating of the blade are advantages for oil tempering.

"Since the heat transfer is so quick are the tempering cycles shorter?"
From reading a slight difference in time at temperature for tempering has little effect. So a couple of minutes out of an hour would probably be inconsequential.