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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. 
<p>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.</p>
<p>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.</p>
I just wanted to mention that this may seem like a hardening process that would lead to the piece becoming brittle but it wont.<br>Two reasons for this....<br><br>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. <br><br>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.<br><br>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. <br><br>This will allow the steel to keep a good edge and the blade won't snap at the first impact.<br><br>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.<br><br>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.<br><br>I am of course making some assumptions here about materials but other than that the science is clear.
<p>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.</p>
<p>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!)</p>
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. <br> <br>Secondly I would suggest using isopropanol (rubbing alcohol) over acetone if you only need degreasing properties. Acetone is a pretty nasty solvent.
This has been a method of protecting Ferrous metals for years. Its actually called &quot;oil blueing&quot; and is a way of protecting the steel from rust and corrosion, I have most often seen it used in the manufacture of firearms.
This is the basic technique for hardening steel, so be careful if you need your piece to not be brittle.
Did you use fresh motor oil? Will used oil work?
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!
In your intro, you mention doing this work outside in a safe area, and that &quot;asphalt or concrete are ideal--a tarp probably won't cut it&quot; <br>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. <br>
This is pretty slick. (pun definitely intended) Good work.

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