Best Way To Blue Steel? Cold Blue, Hot Blue or Rust Bluing.

G'day everyone,

In this video I want to show to show you a small experiment that I have conducted since mid January. At the same time that I nickel plated the tool holder, I also blued up several test pieces to test out various types of bluing. Bluing is a steel surface treatment, mainly used to cosmetically turn it black, but it also can provide some level of rust protection.

This is usually done by leavening it baths of very hot sodium hydroxide, which is very dangerous and I would discourage that process to be done in the home shop.

However there are several other methods that can give similar finishes and corrosion protection. These include hot oil bluing, cold bluing and rust bluing. However each of these have associated pros and cons.

Timestamps
0:00 - Intro into bluing
2:50 - Cold Blue
7:50 - Hot Oil Bluing
11:33 - Rust Bluing
13:52 - Corrosion Resistance Tested
14:41 - Hot Caustic Bluing

[b-lazr]

A couple notes that have helped me with rust bluing. 1. Light coating with the rusting solution you mentioned, then let it dry with a fan or blow dryer. 2. I will let it boil for up to 10 minutes if it isn't converting well. 3. I use very fine steel wool or even denim to card off the surface after boiling (carding wheels can be purchased). 4. I only use distilled or RO/DI water to boil and I change the water frequently because it becomes contaminated with the rusting solution. 5. After I repeat the process many times (up to 10), I card the part one last time and soak in kerosene overnight. The bluing becomes much darker and more stable after the kerosene. Then it is safe to oil it with something heavier. If you skip the kerosene and oil it right away with something containing detergents, you may run into issues. I hope this helps anyone, I've gotten some really great finished from this process, even darker than what I get from my parkerizing setup.

[borisj]

Method I have found to be the best is dipping the part cold into molten potassium nitrate. By the time the part has come up to the temperature of the nitrate it is done. Gives a rich S&W dark blue if the part is polished up before treating. The colour can be adjusted by adding a pinch of cement colouring.

[TheUncleRuckus]

The best things I've found for Cold Bluing is a _Syringe_ and a small _Foam Paint Brush_ or alternatively a wad of Raw/Unspun Cotton like Cotton Balls.
The Syringe lets you apply the solution directly to the Foam Brush so you don't have to worry about wasting any as you can just add any unused Cold Blue back to the bottle, and the Foam Paint Brush holds and applies the solution much better than a regular bristled brush does. 👍👍

[thepenultimateninja]

Have you looked into parkerizing? It's easy to do in the home shop, and the chemicals are readily available and reasonably safe to handle.
The finish is extremely corrosion resistant and durable.
The only downside is that it does add a very small amount of material to the part, so it can affect the fit of parts with very fine tolerances. Not an issue for most tools though.

[ericlondon]

I am a gunsmith. I have done the hot caustic hot bluing. It is a long an complicated and expensive set up with specialized equipment. Mine was heated by propane, of which it used a lot. Most all alloys of steel came out with a rich deep black, some came out splotchy, very ugly, even odd colors like green and pink ! Stainless steels have their own special mix of bluing salts. I blued so many in my area, that it became unprofitable to keep renting the propane tank for just one or two a year. Do not get into hot caustic bluing unless you can justify a continued need.
There are specialized tool blackening ( cold blue ) chemicals that can be bought that do far better than the one in the video.
Also, there are special paints that can be air brushed on, and baked in an oven.

[David-hmic]

Huge fan of your channel here. I have some experience as a gunsmith and in teaching gunsmithing classes so would like to offer some additional thoughts.

As others have mentioned, Parkerizing is a very viable alternative. It is best done to steel that has been blasted with aluminum oxide first, then degreased and Parkerized. That will give it the texture that promotes oil retention. Zinc Parkerizing resists corrosion much in the same way that hot dipped galvanizing does, by attaching zinc to the steel. It’s a simple process done at about 160°-170°F/71°-77°C. When the part being Parkerized quits bubbling, it is done. It’s easy, relatively inexpensive and an excellent method of preventing rust. It is not suited for precision surfaces.


Cold blue is just a change in surface color; nothing more. It's caused by the very toxic selenium in the solution. You're correct that the brand you used is not the best. "Hot oil blue" would be a better description for that process. Gunsmiths use the terms hot bluing and caustic bluing interchangeably. Cold bluing can be improved somewhat by gently warming the piece to around 140°-150°F/60°-65°C before applying each coat.


Your “super slow” rust bluing method was done far too fast. For firearms the steel is allowed to rust overnight in a humid environment before boiling and carding. The extended rusting time permits deeper penetration of the iron oxide.

Having done a lot of hot caustic bluing in a correct environment, it is not the best method for home use. The correct temperature is actually 201°C/395°F and is controlled by the concentration of salts in the solution. Contact with hot bluing salts will result in severe heat and chemical burns and it can destroy eyes.

[kaminmiller]

I've had good luck with Parkerizing steel parts, also known as Phosphate coating. There are 2 main types, Iron Phosphate and Manganese Phosphate. If you see a recipe for phosphating that calls for Manganese Dioxide (by harvesting it from alkaline batteries) then it's is not Manganese Phosphating, and you end up with a light gray Iron Phosphate coating. Manganese Dioxide is non-reactive in most acids.

I worked with a chemist friend before he retired, and we found a method that works. If you can get the materials, then great.
You need a solution that is 1% phosphoric acid, 0.2% nitric acid, and 0.5% manganese carbonate (MnCO3). In a stainless container I heated 1.5L of water to 95C, then added the phosphoric acid, the nitric acid, then the manganese carbonate. When dissolved and the temperature stabilized, I submerged the parts in the solution for about 20 minutes. The part was then dried, and coated in oil for a nice dark gray to black finish.

[sandvichdays]

Look up beyond ballistics, its backyard ballistics second channel and he made several videos on bluing and goes into more detail. It may help if you still need info.

[cameronalexander]

Gun smiths used to use "rusting cabinets". They'd place barrels & actions in wooden cabinets & and then through the boiling and polishing process.

[Azmodon]

Former R&D director at a cutting tool fab shop (my main job was reverse engineering cutters for GM / Bombardier / Honda etc) All of our bluing was cold - we didn't brush it on, we had a tupperware container of the stuff and the parts got submerged and agitated. The liquid will gradually turn from blue to green, to a bit yellow. A 300-400ml container would last hundreds of parts / more than a month of constant use, it just might take a few extra seconds. Once the solution starts to go neon (green / yellow), finished parts will have some brown sludge form on them (over the black) that can be rubbed off during oil application. As mentioned in other comments, parkerizing should be the go to for longevity without as sensitive a need for constant oiling (the phosphorous of the surface treatment when exposed to moisture it will release phosphate ions into the water producing phosphoric acid, which inhibits oxide formation, and converts iron3 oxide(brown rust) into iron2 oxide (black rust) which is much more soluble and washes off). It still needs to be oiled to make it last, but the magnetite / selenium foam of 'black oxide' trying to hold oil just doesn't compare.
- Just checked, completely forgot you can soap parkerized parts lol, basically galvanizes the thing.

[yearsofanythingisenough]

Everyone of those types of blueing must be cared for or rust will form. I gave them up long ago. My go to metal finish for steel is parkerizing. If you can heat water you can parkerize. The military has required it on all their small arms in both world wars. It is extremely resistant to wear through on high points of frequently handled parts unlike blueing and hides machining marks really well. The finish is a matte grey and if you prefer black you simply add a little cold blue to the solution. It stores indefinitely in plastic jug(s) until you want to warm it up and use it again. Parts should be degreased, but they need not be polished as the finish is matte. You can even sand blast the parts if you want the parkerizing to get a death grip on an item for generations to come. Check it out. You won’t be blueing any more.

[douglasharley]

to my eyes, the rust bluing looks the best... *old school!* 🤩👍

[olm]

Great video. I have found "Oxpho Blue" works much better than Birchwood Casey's cold bluing compound. Keep up the informative content. Also, heat the part in boiling water rather than using a torch. It will give a much more even heat and hence a more even finish. Thanks.

[highdesertdrew]

I've done both caustic as well as nitrate process. I've also done a few other heat related processes. FWIW, the caustic process isn't as nasty as it sounds, it's roughly ~350F. I used to have a mix of potassium nitrate, sodium nitrite and sodium hydroxide with a very tiny amount of water in a stainless steel pot on a hot plate I would use for dipping parts. It is definitely the superior method as the molten salt pretty much saponifies any remaining oil, and converts any oxide to Iron II oxide. It doesn't outgas, you just want to have some careful controls on how things are going. If you want to try it yourself you can use a stainless steel or cast iron frying pan, fill that up with about an inch of sand, and then put a small stainless steel pan in the middle full of your bluing salts. It takes quite a while to get up to temp with the extra mass, but it also thermo-regulates itself and you can use a kitchen gas flame for doing that. Read up on it, check out some of the chemistry vids out there, it's much less exciting than it sounds.

The other process is parkerizing, which is done at boiling water temperatures, but uses phosphoric acid, manganese dioxide, and some steel wool. It's harder to make it look even, and requires the parts to be degreased and bead blasted before processing. It does involve boiling acids, but it's not as exciting as it sounds, just do it outside.

[joewhitney]

Great video and narrative on the different effects and processes. The comments add a lot of additional information.
Thanks for sharing everyone.

[Hyratel]

The one you skipped was Bright Bluing. See Clickspring, it's actually a type of iridescence. Bright blue finish, it's a bonded oxide using carefully controlled heat. As witth hot oil blue, not suitable for heat treated parts

[rjsprojects]

Cold blue is my usual go to. For small parts I usually add a little to a dish and roll the part around or submerge it.

[JETHO]

Im a gunsmith that quite frequently does hot caustic bluing. Id like to have seen you do it as well. Its also not very difficult to do as most people think it is.

[TechGorilla]

I like to keep a small piece of Scotch-Brite coated in a little 3 in 1 oil. When I get a small rust spot on a tool (or when they need a bit of a clean), I just give them a scrub with the oiled pad and a paper towel. Job done. In my pea brain, the Scotch-Brite helps dig the oil in to the metal and leave a protection.

[satyajitOWall]

I would be curious to see how bluing compares to applying a rust converter. For example, at 12:38, after you've uniformly oxidised the part, you can then just brush it with rust converter (NOT rust remover) to convert the Iron (III) oxide into an inert layer that chemically bonds with the underlying metal surface. Rust converters are mainly tannic acid or phosphoric acid based, with additives that promote adhesion.