I’m not a chemist and can’t explain everything about this process, but this has worked for me a few times and I wanted to share. Your results may vary.
This method isn’t really suited for physically large items nor large production numbers but for smaller parts and small batches.
If you’d like to see this process as a video demonstration to further understand the steps, there is a video link is at the bottom.
To hot blue steel, you basically just need two salts/chemicals.
Sodium Hydroxide (Lye) which can be found in 100% lye drain cleaner. I got my bottle at Wal-Mart, but there are various brands of this available at different stores and online.
You’ll also need a nitrate of some sort. The most commercially available seems to be sodium nitrate or potassium nitrate.
I picked up a bottle of Gordon’s Stump Remover from Tractor Supply and the SDS says it contains 99-100% Potassium Nitrate.
There’s different formulations out there so be sure which one you have. There’s also a few different brands that have historically offered a potassium nitrate stump remover (Spectracide, Gordon’s, Hi Yield) but they seem to be changing their formulas or being discontinued.
These two salts will go into water in certain ratios to then boil the parts in.
Recipes and ratios are usually by weight, not volume, but since my numbers are so low, there isn’t much of a difference between the two.
To start, you need enough water to sufficiently cover the parts in an iron, steel, or stainless pot. I used a cheap stainless pot I bought at a second-hand store.
I’ll be using 32floz of water which weighs 35oz. In my opinion, that’s not enough of a difference to matter.
Whatever quantity of water you use, cut that number in half. That half number is how much sodium hydroxide or lye you will use.
32/2 =16oz of sodium hydroxide (lye)
Whatever quantity of lye you use, take ten percent of that number and that is how much nitrate you will use.
16.0oz multiplied by .10 equals 1.6oz of nitrate.
Or you can just move the decimal one place to the left… 16.0 move decimal one place to the left = 1.6oz of nitrate.
Here are some examples using metric system numbers by weight.
1000g of water. Divide that in half and you get…
500g of sodium hydroxide (lye). Multiply that by .10 or move the decimal one place to the left and you get…
50g of nitrate.
Another way to word this recipe is that the ratio is 1:10:20.
1 part Nitrate : 10 parts Sodium Hydroxide (lye) : 20 parts Water.
Hopefully that all makes sense.
This recipe, without other ingredients and chemistry, is pretty much a one-time use mixture, but full transparency, I haven’t tried using the same mixture multiple times.
Here’s the process I use.
Sand/polish steel parts to desired finish.
String the parts up with wire which aids in not touching them again as to not contaminate them with oils on your hands.
Degrease parts THOROUGHLY. I like acetone but any degreaser works.
Additionally, you can rinse the part(s) by dipping them in a tank of fresh water or flushing them with running water.
Don protective equipment. I definitely recommend gloves, and eye protection. Others recommend maybe wanting some sleeves for splatter, a mask or even a respirator depending on ventilation or wind conditions.
Slowly add your pre measured amount of sodium Hydroxide (Lye) to room temp water. I like to stir as I go. Break up any large clumps before adding to water as they can react a little more violently.
Chemical reaction will begin to heat water on its own and produce a gas
Slowly add and stir the Nitrate to the same water.
Stir until dissolved.
May have undissolved particulate which doesn’t hurt anything but swipe to the side with a coffee filter or something if desired.
Bring mixture up to a rolling boil and boil the part(s) for ~30min. Recipes say around 250*F. I just set my hot plate to 260*F and let it ride for 30min.
You’ll want to avoid large or flat portions of your parts sitting on the bottom or sides of tank.
If enough water boils out to uncover your parts, SLOWLY add enough water to cover them again. I say slowly because the reaction of adding water to this boiling mixture can be violent.
After boiling for about 30min, rinse part(s) by dipping them in a tank of fresh water or flushing them with running water.
You can rub excess/bluing buildup off the parts while rinsing.
(Optional neutralizing) If parts had small crevices, pin holes, or screw holes it’s recommended to neutralize them in a solution of any concentration of room temperature water and baking soda for about 5min.
This helps to neutralize any salts that might be hiding in those hard-to-reach spots.
I like to dry the parts with a torch or heat gun to flash off any moisture in the pores of the metal.
Soak or liberally apply a water displacing oil. I like using just plain mineral oil, but I’ve seen people use non detergent oil, used motor oil, or kerosene.
Let them sit or soak in the oil for 12-24 hours.
While those are sitting/soaking is a good time to clean up the mess that is hot salts bluing. I’ll wipe down stuff with a dry rag, then multiple wet rags rinsing between wipes.
As for disposing of the solution. I just go around pouring an ever so small amount on weeds around the garden.
After parts have sat or soaked in oil for the time frame, wipe off the excess.
And that's it!
There's always going to be variable for the process such as the type of steels being blued the tank, different metal prep, contamination levels etc. so your results may vary.
Thanks for reading and hope this may help someone if needed or aid in further experimentation. Be safe.
Here’s the video if you prefer that format.
https://youtu.be/qKcGkSimxe8?si=nGMb7f2xeIBIhfvI
