Sunday, June 22, 2014

Nitre Bluing


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.....

 What I got was this mess.


I was thinking it might have been from oil left in the pot to keep it from rusting.....in my haste I forgot to clean the pot first
I prepared a screw anyway, figuring what the hell, I might as well try it.
I swirled it around in the sludge for a couple of minutes and then plunged it into a jug of water

This was the result
It did indeed blue the screw, but it is more blue/black then the "peacock" blue that we want. I am going to clean out the pot and start over with some fresh potassium nitrate.




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
I use Acetone to degrease & clean the parts
First I tested the solution with a finishing nail, we got an electric blue color!
 So I went ahead and dunked the screws one at a time. After a few minutes in the solution and a quick quenching in water, I got excellent results!

It is hard to see from the picture, but we have a nice electric blue color. I still think the heat was too hot, and I will continue to experiment (and buy a high temp thermometer), but I think I am on the right path now.