Caustic (hot) blue

[Swede]

This is a topic that gets discussed fairly often... how to blacken or blue steel. Usually seen on firearms, but also frequently seen on steel tooling, such as AXA toolholders and such. When the metal is shiny, the finish can be beautiful. It also does protect a bit, at least better than raw carbon steel. Many of us, knowing that the correct hot, caustic blue is a bit of a chore, are satisfied with the smelly (and frankly, lame) wipe-on blues.

I finally had a reason to give this process a shot. I had made a propeller hub from steel, and I wanted the blue for both contrast and protection. While I was at it, I figured I'd blacken some nicer tooling pieces I'd made in the last year.

The part of interest is upper right... 12L14 steel.

This process I am going to describe is really practical for small parts/pieces. To blacken large things requires a LOT of chemicals and a LOT of heat, but anything that'll fit into a liter pyrex container can be blackened easily in a home shop, with little fuss. And the results are both excellent, and easy to do!

What you'll need:

Simple Green (or some other strong aqueous cleaner-degreaser)
lye (Sodium hydroxide; "Red Devil" brand lye works, sold as a drain opener. Be sure it is white and pure, no funky colored crystals.)
potassium nitrate ("Stump Remover" in the U.S., sold at hardware stores)
distilled water
container
heat source
thermometer that can read up to 300 f./150 C.

Prepare the blue salts. The mixture is (by WEIGHT) 2 parts lye, 2 parts water, 1 part potassium nitrate. Fill a beaker or any suitable container (NOT aluminum) with 500ml water, 500 grams. Weigh out 500 grams lye. Slowly add this to the water; it'll get very hot.

Obligatory warning: lye will melt flesh and/or blind you. Goggles, gloves, etc. This is a process for "big boys."

Turn on the heat... bring the solution up to maybe 130 C. The boiling temp is raised due to the lye. Slowly add 250 grams potassium nitrate. There is no noticeable reaction. Stir with a glass rod to dissolve.

In the mean-time, you can prep your parts. Wipe them down with acetone or similar. Then, what I did (and it seemed to work well) was put them in a pot, added pure Simple Green, and heated this to a near boil. Let them soak for 15 to 30 minutes at least. Grease and oil will be stripped. This is VERY important... surface prep and degreasing is 90% of the battle.

When the blue salts reach 275 F / 135 C, they are ready.

The parts should already be hung on stainless wire. Remove them from the degreaser, and dip/swirl in clean, distilled water. Shake excess water off. Then, hang them in the hot blue pot.

 

[Swede]

Hack your watch. The reaction takes time. Keep the heat between 135 C to no hotter than 145 C. A good heat source is a plug-in hot plate or similar. While the parts are cooking, get your rinse water ready... any container that can hold a liter or so of water.

After about 5 minutes, it should be noticeably black. After 15 to 20 minutes, it'll be done. Rinse them off in your rinse tank, then give them a thorough washing, paying attention to screw holes and such. Here are the results:

The other round object is a mandrel for a lapping plate, and the long item is a holder to grind/sharpen 3/32" carbide graver squares. All of them came out better than I could have hoped.

After the wash, you can improve the finish by buffing vigorously with an oiled rag, or 0000 steel wool. The finish can handle aggressive polishing, if desired.

Overall, this is a process I'll reserve for deserving components, as it is a bit tougher than just smearing on some cold blue, but it was far easier than I thought it'd be, and the results exceeded my expectations. I hope someone gives it a try... it's easy.

 

[maverick]

Very nice write up. The parts look great. Do you think it would work on 303 stainless or cast iron?

 

[Swede]

Stainless will definitely not blacken. Cast iron should respond well, but a scrap would be a good idea.

Each type of steel has a different characteristic. Generally, the higher the carbon, the better it works. 12L14 is one of the worse steels, and it still came out great. The little tool was annealed O-1, and it blackened much deeper and quicker than the 12L14.

 

[tel]

At last! A hot bluing recipe where I have all the materials on hand! Just gotta try this one, hopefully over the weekend. Thanks Swede, and a karma point!

 

[Swede]

Thank you sir! I think this is just about foolproof if the steel is degreased properly, and the hot "Simple Green" did a good job. Good luck with it! ;D

 

[bezalel2000]

Great result :bow:

I'm Just curious, does the appearance of the blued O-1, change during hardening ?

or does it keep that nice crisp black look?

Bez

 

[Swede]

Great result :bow:

I'm Just curious, does the appearance of the blued O-1, change during hardening ?

or does it keep that nice crisp black look?

Bez

I used the O-1 as is, annealed. I'm pretty sure any blue desired would have to be done after heat treatment. The treatment itself would destroy any blackening done prior to it.

 

[Allan819]

Thank you Swede for all that very useful information. Can we reuse the blue salt many time? How long can we keep it?

 

[steamer]

Thanks Swede! Great write up!

Dave

 

[Herbiev]

Excellent post. One to keep in my recipie book :bow:

 

[Ken I]

Swede, thanks for the post, great write up.

One point from my own experience - the boiling point should be 135-145°C (140°C optimum)

If it boils within this temperature range then the saturation is just right.

If its lower than 135°C you wait for some of the water to boil off and the boiling point increaces. This it is going to do anyway and the boiling point will continue to rise - this is where the problem comes in - you have to add water to bring it back down again.

It doesn't work simply by maintaining 140°c on say a solution that would boil at 160°C - it must boil at 140°C for the saturation / reaction to be just right.

Since the supersaturated solution is at 140+°C - adding water will be very dangerous - it can explode in your face.

It can be done but you need to be very careful and wear all the mandated protective gear.

For limited use such as we require, it is safer to let it cool down to below 100°C before adding water and then bring it back to the boil.

As you pointed out - when making up the solution, add the caustic to the water - NEVER the water to the caustic - it will blow up in your face due to the reaction generating enough heat to flash into steam.

Can we reuse the blue salt many time? How long can we keep it?

As long as its in a suitable container - it keeps and can be reheated time and again - you do need to cover it as it tends to cause corrosion around about it.
When cooled it pretty much solidifies - wet mushy crystals - depending on ambient conditions it can absorb moisure - but it can also dry out - either way you want to keep it tightly covered.
Reheating a semi-solid mass requires a bit of care, if you think it needs more water (dried out) do so at the start - but adding very small quantities very slowly - same caution for mixing fresh above - and always wear gloves and a face shield.

Ken