This formula will produce a durable blue/black color.
I came across this formula many years ago in a gun magazine, I believe that this formula was used by P.O. Ackley.
Per US gallon of soft water.
5lbs of caustic soda.
2.5lbs of 33% ammonium nitrate fertilizer. (sodium nitrate or potassium nitrate can also be used in the same quantity)
If you are making any solution of caustic soda, DO NOT use an aluminum (aluminum) container. Caustic soda eats aluminum. Use steel or cast iron (stainless steel works just fine).
Before you even think about starting, you will be messing with stuff that is very hot and will give you a very nasty chemical burn. Wear rubber gloves (buy your own, don't use your wife's if you value your wedding tackle) and a face shield. A waterproof apron would be good to have. Have on hand a large bucket of water just in case you get some on you. This bucket is for immediate use and just to tide you over while you get to shower. Just get in and start showering, boots, cloths and all, then strip in the shower. You need to flush that caustic soda off of you.
Just be respectful of what you are messing with and you should be all right, this is not that difficult.
You need to mix the witches brew outside. The kitchen stove is not the place to do this, unless you want to have the whole family stay in a hotel for a couple of days while the air clears in your house.
Here is how it is done.
Get your water boiling and start adding the caustic soda a bit at a time. As the concentration of caustic soda increases you will have to turn up the heat to keep it boiling. The purpose of the caustic soda is to raise the boiling point of the water.
Now we add the ammonium nitrate fertilizer. There is going to be a large volume of ammonia fumes released when you add the fertilizer, get up wind of the boiling water. Make a wind with a fan if you have to. If you think pealing onions is bad, just get a whiff of the ammonia fumes.
Add the fertilizer very slowly because there is quite a reaction as the ammonia is released, too much and the pot will boil over (you need to keep the pot on the boil while adding the fertilizer). Once all the fertilizer is in, keep the pot boiling for about half an hour as there is more ammonia will be coming off.
Storage when not in use. A plastic bucker or container with a snap on lid is best because the caustic soda will clime the side and spill over the rim.
The bluing its self. You will need 3
suitable sized steel pots. One with fresh clean water for rinsing
One for the bluing salts and one for degreasing in.
The process is degrease, blue then rinse. Done in a very well ventilated area (outside) and don't forget your large bucket of water just in case of an accident and your gloves and face shield.
Degreasing. Remove any obvious grease and oil with a cloth and varasol, engine degreaser, what ever you have handy, even paint thinner.
Mix up some weak caustic soda solution in the degreasing tank. With wire hangers suspend you parts to be blued in the degreasing tank, small parts can be put in a wire basket. Boil this up for about 5 minutes. Then transfer to the bluing tank. Do not touch any of the parts with your bare hands or you will get finger prints in the bluing. A clean pair of cotton gloves work just great here.
Bluing. Do not blue anything that has been lead soldered, the bluing will eat the solder. The parts to be blued should be covered but closer to the top of the tank. Towards the bottom are precipitates that have a hard greenish glassy appearance and are the devil to get off without marring the blued surface. You have to bash at them with hardwood or plastic to break them up. Sometimes a thumb nail will pry them off. Extended rinse boiling does not seem to help.
Bring the bluing tank to the boil. If you are lucky the steel will turn blue as the bluing solution boils. It probably will not. If the parts start to turn blue before the solution boils, the solution is boiling at too high a temperature. Add (soft) water to dilute the solution and lower the boiling point. DO NOT add water to boiling bluing salts you will get a STEAM EXPLOSION. Only add water to cold salts, better to add a little to much water, you can always raise the boiling point by adding caustic soda
If there is no bluing at all then add caustic soda to increase the boiling point of the solution. With adding water or caustic soda, a little goes a long way, use in moderation. The bluing needs to take place while the solution is boiling. Boil for about 10 minutes. The solution should be boiling at about 300 degrees F.Rinsing. Transfer the blued parts to the rinsing tank, boil for 5 minutes to get the steel good and hot. When the parts are removed they will dry almost immediately. Let the parts cool and apply a light coat of, (1st) water displacing oil, (2nd) soluble oil or (3rd) WD40.
* Soluble oil is used in machine shops as a coolant when mixed with water, use the un mixed oil.
You will probably only have one heat source to boil your different pots on, not a problem.
The finish. Polished parts look better than "as machined" parts. Sand blasted with a fine grit gives a nice mat finish. Glass beading give the finish I like the best, it is neither mat or black chrome looking, more like a satin finish. Very nice.