The video is showing a Nitre Bluing process. That's a heat blue. He is useing the stump killer (potassium nitrate) as the 'salt' to melt and submerge the parts in.
At the appropriate temp the metal will turn color (straw, purple, pale blue, peacock blue, dark blue).
It's an old process and was generally only used for smaller parts by the factorys as someone pointed out.
Sodium Nitrate was the prefered 'salt' to use in the process. But most any will do the trick including Ammonim Nitrate.
The old handwritten instructions and recipe from the Winchester factory for their Nitre Blue still exists. It can be found reproduced in Madis's Book of the Winchester.
They used to mix a small % of manganese dioxide (?) in with the salt to lower it's melting point. That to enable different color ranges to be reached that the plain salt would not give them.
*** Any of these melt and works in the 600F range +/.
I use them up to 800F to be a charcoal blue look. ***
They are very dangerous to use for obvious reasons and a tiny spatter will burn through leather safety wear, and then your skin in a NY moment. I have the scars to prove it. It's like working with molten lead,,the smallest amt of moisture around and you have a molten explosion of the stuff all over you. Eye safety a must.
I use the process for small parts that mostly receive the Nitre Blue as a decorative or restorative finish. It is also most useful as a heat treat to draw newly made flat and V springs to the final temper.
They come out with the noted 'spring temper blue' color for a reason.
**His 290 degree claim for blue color wouldn't melt most lead solders. It won't turn steel dark blue.
Maybe it's 290C,,thats just under 600F,,that'd almost make it but not quite,,maybe why he was stuck with reddish tones at first and had to boost the heat.
Anyway, that process is but one 'bluing' process.
It is not the Rust Blue process,,not the Hot Salt process. Both of those different from each other as well.
For DIY home bluing I'd suggest Rust bluing either slow rust or Express. The most dangerous thing you deal with is boiling water.
You can mix your own Hot Blue Salts at home and do that process DIY.
IIRC it takes 8# of salts to mix 1gal of soln. Salts are a combination of Sodium (or Pottasium) Nitrate and Sodium Hydroxide (Lye)
But it's a long drawn out process, works/boils at 300F+, needs at least a couple of 'tanks' and heat source. Messy to do and clean up. The salts preciptate out of the tank with the evaporating water and then 'grow' on every surface in sight. They then attact moisture from the air and cause rust to metal and rot to any wood surfaces. The salt precipitate will destroy aluminum also.
Then what do you do with the stuff when your 1 gun blue project is over with..
