Nitre Bluing is not the same as Hot Salts Bluing (although heat and salts are used). Nitre bluing is more closely related to heat treating or heat bluing.
The parts are suspending in a salt solution that contains Potassium Nitrate, Sodium Nitrate and other chemicals (Brownell's recipe also contains Sodium Nitrite, other recipes include Manganese Dioxide) that is heated to 600 degrees. The colors achieved are the result of the heat and the salts combined.
In Hot Salts Bluing the color is the result of the chemical rusting, the heat helps with the melting of the salts and the conversion process, but the temp is less than 1/2 what is used in Nitre Bluing.
Just to clarify, Potassium Nitrate is also known as "Salt Peter" and is one of the ingredients in traditional black powder (along with Sulfur & Charcoal). Potassium Nitrate is also used as an ingredient in the brine for making Corned Beef as well as other foods.
Sodium Nitrate is a fertilizer and food preservative. It is used in the production of many kinds of foods, most notably hot dogs and sausages.
Manganese Dioxide is a powdered metal, used in parkerizing and for pottery glazes.
None of these are used in the construction of fertilizer bombs like the one used in the Oklahoma City Bombing. That ingredient was Ammonium Nitrate (I just wanted to be clear on that).
One word of caution, nitre bluing should not be performed on critical parts like some hammers, firing pins, triggers and other parts that are heat treated. The 600 degree temperature in nitre bluing may anneal these parts and take away their heat treatment, making them prone to premature wear.
The Colt Single Action Army Revolver below has the cylinder pin, trigger and screws done in nitre blue (click on the picture for a larger version)
This color can be achieved by heat tempering, but the process is difficult to master. See my write up here.
As usual I didn't want to spend a bunch of money on the "over the counter" pre-made stuff.
So I went looking for a recipe that I could make up myself, I found a few, some even said to just use straight potassium nitrate. Some suggested adding 10% Manganese Dioxide (the same stuff used in parkerizing). Others suggested a mixture of Potassium & Sodium Nitrates.
I needed a reliable way to get the salts heated to the correct 590 degrees.
I bought a Lee lead pot like this one:
I bought some industrial grade potassium nitrate from a pottery supply store. Five (5) lbs cost me $10 including tax. You can find it online, but shipping prices might make it expensive. I already have Sodium Nitrate and Manganese Dioxide from my other refinishing processes.
I decided to try straight potassium nitrate 1st. If I don't get good results I can add the other ingredients.
So I put the Potassium Nitrate into the lead pot and plugged it in.....
I prepared a screw anyway, figuring what the hell, I might as well try it.
This was the result
So, I cleaned out the lead pot, and tried a different recipe. This time I used 2 parts Potassium Nitrate, 1/2 part Sodium Nitrate and a pinch of Manganese Dioxide. I used an empty chew can for my measuring cup (very scientific).
When I heated it up I got the same gooey mess
After the solution heated up, the gooey mess settled down and I went ahead and cleaned up the screws. These are for my Arisaka Redux project
You cannot anneal steel at 600 degrees Fahrenheit even if it is a high carbon steel. Please check this sort of information before publishing it on your website.
ReplyDeleteYes you can.
DeleteYou can temper it not anneal it
Deletethanks for your comments, perhaps "annealing" is the wrong term to use when discussing the removal of surface heat treatment.
ReplyDelete600 degrees will indeed bring the hardness of your metal parts down a few points. It’s tempering the steel just a little. 600 for 30 min isn’t all that critical but I wouldn’t do it to my bolt head on a rifle...
ReplyDeleteThanks for your info on bluing. I'm going to give it a try soon with my Lee melting pot. I've had a lot of success with fume bluing and rust bluing but want to try using your mixture for small parts.
ReplyDeleteThere is a common misconception about annealing metal. True annealing, done to soften hardened metal so that it may be re-machined, occurs at temperatures approaching 2000 degrees F and requires soaking of the part in that heat for at least 1 hour per inch thickness of material. "Tempering" is done at much lower heats and is done to reduce the hardness of steel and increase its toughness. Tempering ranges are typically 400 degrees to 800 degrees F.
Modern gun steels are primarily alloy steels that are very precisely heat treated for "hardness" and then Tempered to reduce their hardness and increase their "toughness". When you reduce the hardness of steel you get a corresponding increase in the toughness of the steel. The "toughness" is a measure of the steels ability to deform without failing. Hard steels fail in a brittle fashion, tough steels fail by stretching. In a firearm you always want maximum toughness. Modern alloy steels temper in the 800 F range, bluing will not affect them.
Older firearms used mainly low carbon steels, many of which (Mausers, Arisakas, Springfields,enfields...) were case hardened. Heat bluing these guys will not impact their hardness in the least within normal temperature bluing ranges. Most of the older firearms made from low carbon steel, casehardened or not, will not be affected by temperature ranges involved with blueing. Trigger notches, sears, and parts intentionally made very hard may be affected a little bit if you hot lead blue or blue with some method that gets above 400F and holds it for a long period of time but it is a simple matter to use some casenite to re-harden these items if needed. Probably would be a good idea to touch up these spots on an old gun anyhow.
The thing to remember is bluing temperatures can only "Temper" steel, they do not "Anneal" it. Bluing below 400 F and for short durations (15-30 minutes) will not effect temper at all. If you know the type of steel in your firearm then look up its "tempering" temperature, as long as you stay below it in your work you will not change its temper.
None of the above applies to springs, they harden and temper in many heat ranges and durations, keep them out of the bluing unless you know how they were originally heat treated and tempered.