How to Blacken Steel With Motor Oil

Blackening not only protects bare steel from rusting, but the process also yields a relatively attractive finish. You can achieve this finish with some pretty common household items. You will need:

• acetone
• clean rags
• motor oil
• pliers
• handheld torch
• heavy leather gloves, jacket (welding attire works great), and safety glasses
• fire extinguisher (just in case)
• metal or glass bowl (preferably one you don't care about)
• outdoor space conducive to mischief

DISCLAIMER regarding outdoor space and subsequent mischief:
must be outdoors
clear area of all flammable items
you will spill oil and sometimes that oil will be on fire. asphalt or concrete are ideal--a tarp probably won't cut it

Step 1: Prepare Metal

Using acetone and a clean rag, wipe off excess oil, rust, etc off of your metal parts. You might also use steel wool or a scotch brite if you need a little more abrasion. This would probably have been a good time to be wearing gloves. If you don't have any or choose not to wear gloves, be sure to thoroughly wash hands after this process. 

Step 2: Douse, Quench, Douse, Quench

This is where the process needs to move outside for sure. Hopefully you can get an idea of the setup from the photos in this step. I have annotated them for clarification.

Here is the process:
1. set up workspace. lay out rags to place your parts. I laid out two so that I could separate finished from unfinished.
2. wear safety gear and bring fire extinguisher outside with you...just in case
3. pour motor oil into bowl, about 2 cups were more than enough for me. It really depends on your bowl and your part. Make sure that you pour enough oil so that your part(s) can be completely submerged
4. light torch and adjust flame to something manageable
5. submerse a part completely in the oil and retrieve with pliers
6. hold part over flame, moving slowly as to evenly burn off the oil. The oil coating your part will ignite but will burn out soon. note that during this part you will drip oil on/around the torch. it may light on fire but should also quickly burn out. do not be alarmed unless flames grow
7. when oil has burned off and your part is a reddish color, drop it in the bowl of oil, or "quench" it in oil. 
8. retrieve the part once again with the pliers. dropping the part and retrieving with pliers may seem unnecessary, but if you hold onto the part the whole time with the pliers, you will end up not blackening the bit clamped between the pliers. If you want an even finish, you should drop and retrieve each time
9. repeat process 2 or three times per part to achieve desired finish
10. place finished parts on rag and let cool
11. wipe off with clean dry rag

References:

check out this great ehow article--which is where I learned a lot of this process!

Comments

[danielmerrick]

I have been using various methods to blacken steel and this one works well without any technical difficulties. I make various hardened tools for my use and for sale. When I first started hardening steel, I quenched my hot steel (844 C, 1550 F) in used motor oil because it was free. I was getting a dark black finish at first quite by accident. The motor oil is unpleasant to work with because you tend to get a lot of flames when quenching large parts, plus it stinks. It stinks even more when you burn it off in the tempering oven - definitely an outdoors job.

I decided to switch to canola oil because it has a higher flash point and ignition point and doesn't stink so much. It is also pretty cheap at Walmart in gallon jugs. It worked fine for hardening the steel but I didn't end up with the nice black finish I wanted. I tried cold blacking using products from several suppliers but it seldom works as well and requires additional cost and processes. I tried quenching in canola oil then tempering with a coating of other oils (linseed, WD-40, motor oil, etc.) to no avail. It seems to require the high heat of quenching.

I doubt that carbon as mentioned by others has anything to do with the blackening since the black material we are trying to make is an iron oxide (magnetite Fe3O4) and does not contain carbon. Magnetite doesn't rust and has is a nice black. To make black iron oxide generally requires a catalyst of some sort. Motor oil has various additives such as metal compounds (zinc, molybdenum) and alkaline additives such as potassium hydroxide. I believe that one or several of the usual motor oil additives act as a catalyst resulting in black iron oxide.

By the way, once you have your parts black, rub them with a tiny bit of boiled linseed oil on a rag which will even out the color and give it some luster.

[kev.oue]

Used motor oil will most certainly have a higher carbon content, this is a trick used by some ot the old timers. I still do it today, the blacker the better. this is for expierenced people only you shouldnt be letting your 12 year old heat treat parts in the driveway.

[yoyology]

Did you use fresh motor oil? Will used oil work?

[tselle]

I used old but unused motor oil. I think used oil should work but perhaps not as well since it may have less carbon to burn onto the metal. shouldn't hurt to try it at least on a bit of scrap! Let me know if it works!

[I_SLIPPED_ON_MY_BEANS]

Used motor oil will likely have more carbon not less

[corradini]

So sorry, and trying to be polite, but this is actually a pretty terrible method for oil-blackening/coating steel. (I know, I'm a professional knifemaker and also make a lot of my own tooling - and put extremely hot (1,600°F) steel objects in oil regularly - and can still count to 10 on my hands, and see with both eyes.) For one thing, the source "ehow" article (which links to another site) is to say the least questionable ("Jane Smith" - seriously?!?!) More importantly, dousing a part in oil and then hitting it with a flame is not a great idea, let alone a good controllable process. Especially when there is a much simpler, safer, and reliable method: use an oven. This is not only not a great method, it is actually potentially dangerous.

[YacoobM]

Hi. Did you use new oil or used oil?

[astrong0]

I'm making a Tsuba and a couple of seppa for katana I've been making. I've needed to do blackened steel to protect these parts but the vinegar and bleach mixture I've been trying to do just isn't working in the seppa. No clue why.

[Mindmapper1]

This is a great instructable but I must point out that it would be far better to have some sand to extinguish any fire or soak any spilt oil. NEVER use water or a water based for extinguisher.

[LordBenjamin]

I know this is an old post, and I hesitated to comment. But I feel like it should be mentioned that you MUST wipe off any oil and reheat the metal after you have repeatedly quenched it, or it will become brittle. If you're making a knife blade like I am, that's something to consider.

[wordofmetatron]

I just wanted to mention that this may seem like a hardening process that would lead to the piece becoming brittle but it wont.
Two reasons for this....

1. Per the instructions of heating it to a reddish color, the temperature reached at a dull red is low enough that this is more of a tempering process than anything else.

2. The fact that oil is used as the quench means that the process of cooling is slowed significantly, again making the quench more akin to tempering.

The knife you are making will need to first be hardened requiring a cherry red or even towards a straw yellow color prior to quenching depending on the mill spec. Then it will need tempering as well which adds a quality known as toughness or notch toughness to the blade.

This will allow the steel to keep a good edge and the blade won't snap at the first impact.

Since the OP is likely using sheet steel for this, my guess would be A36 or equivalent there is simply not enough carbon to form very much of the boundary martensite that causes brittleness.

As far as the added carbon that makes up the bluing and prevents rust adding enough additional carbon to allow for embrittlement, the temp is too low, the time exposed to the oil is too short.

I am of course making some assumptions here about materials but other than that the science is clear.

[DIY-Guy]

Simple, neat, very nice. Thank you. I have been looking for a simple blackening process for nickel and will try this when I have a torch handy.

[rdomunky]

Tried this out this weekend using Canola oil, since it's a branding iron that will touch food and I didn't want to mess with motor oil. Actually ended up working really well using a plain propane torch (didn't even get the steel red hot). The finish is a beautiful semi-glossy black and no signs of rust yet. Thanks for the recommendation (which you gave me at TechShop... should have told me you were the author of the Instructable!)

[Wit Knien]

I do not think the amount of carbon present will be the issue, it is more likely that the used oil has more impurities in it which could result in a lesser appealing surface finish. I would filter the used oil first to get rid of any larger particles.

Secondly I would suggest using isopropanol (rubbing alcohol) over acetone if you only need degreasing properties. Acetone is a pretty nasty solvent.

[arthurkanzler]

This has been a method of protecting Ferrous metals for years. Its actually called "oil blueing" and is a way of protecting the steel from rust and corrosion, I have most often seen it used in the manufacture of firearms.

[lime3D]

This is the basic technique for hardening steel, so be careful if you need your piece to not be brittle.

[nanosec12]

In your intro, you mention doing this work outside in a safe area, and that "asphalt or concrete are ideal--a tarp probably won't cut it"
I would caution against diong this over asphalt.....asphalt is a petroleum based mixture and can be damaged by the heat of the burning oil that my drip on it. Probably won't ignite, but will leave divots and depressions in the 'burnt' areas.

[audreyobscura]

This is pretty slick. (pun definitely intended) Good work.