How to colour steel with heat (hot oil finish) is a simple but effective way of achieving a desired colour of choice.
The colours are fantastic and often come out in a beautiful two-tone shine. There is something really rewarding about this technique. Not only does it give you an alchemical feeling of satisfactionbut it also gives the metal a finish to stop corrosion.

Step 1: Preparing Your Materials

You will need:

-2x containers (one of them preferably being metal for the oil)

-oil

-unleaded petrol/Acetone/Terpentine/Rubbing alcohol or any good degreaser

-metal object/s (I used mild steel, Please note this method should NOT be used on tempered/hardened steel as you will effect the properties of the metal)

-something to hold your object with i.e. pliers, mole grips,...

-heat source

-protective glove (optional)

-wet towel on hand for safety

In Explanation:

-An old metal container (I know, a plastic container is pictured but always use a metal one) filled high enough with oil to completely submerge your steel object would be ideal. You can experiment ultimately what oils to use. Try it on some unimportant pieces first. I found through my bluing adventure that different oils do act slighty different but ultimately everything I've used has done the job. For this Instructable I'm using thread cutting oil. I have also had good results with 5w/30 motor oil, I've also used two-stroke oil with good results. It's worth noting that there can be some additives in motor oil that release hydrocarbons when burnt which is a known carcinogens to the body (in other words: Bad Stuff!)

Your container should be relatively larger than your object.

-An old container filled with your chosen degreaser (unleaded petrol...)

-Your desired polished metal object/s (very important!: Make sure it is made from Mild Steel or Titanium. You can not do this method with Aluminium because it won't reach the temperatures required for bluing, it will just melt. Trust me, I'm speaking from experience :) )

I have cut and polished 4 pieces of mild steel to demonstrate different colour stages. The object on the left is a custom mild steel potentiometer knob!

-Something to hold your piece with, make sure it's appropriate to hold it with so it doesn't get too hot in your hands. It would be a good idea to get some gloves on.

-A heat source. That can be a pen torch (pictured), a blow torch, a heat gun, a gas hob (which I am going use) or anything that will provide you with enough heat.

Please note: As you are working with heat, please be smart, use your common sense, do the bluing process in a well ventilated area and keep a wet towel to hand.

Step 2: Cleaning Your Metal

For the process your metal should be nicely polished for the best results.
The more shinier the polish the more your end result will glow.

I can't stress how important the degreasing step is! Do NOT wipe/touch your metal with anything after degreasing as the whole purpose of cleaning was then for nothing. Even a fingerprint will affect the consistency of the colour.

Now securely hold your piece with whatever you have chosen.

In my case I drilled/threaded the metal piece and made a holder with a threaded end. This is not necessary as long as you can hold your item appropriately and not cover where it is to be heated.

Next wash your piece in your chosen degreaser for a minute or so to get rid of all grease/fingerprints that was on the metal.

Then remove it from the degreaser, don't be tempted to wipe it off with anything. Just let any residual matter evaporate/dry.

Step 3: Bluing

Now you can start.

Apply heat to your object and heat up gently and equally.

As I'm bluing round pieces I slowly and evenly rotate the piece over the flame to equally disburse the heat.

Be patient in the first few minutes as the metal needs to reach the temperatures where it changes colour!

Once you see it turn golden, the process has started and colours will change fairly quickly!

Sometimes at this point, I come just a fraction away from the heat to go through colours a bit slower.

It will first turn gold, then orange, then red, purple, blue and then dark grey!

Step 4: Finishing

Once you get the colour you want, go outside to your oil container and fully quench your object into the oil!

Note: I know a plastic container is pictured but always use a metal one!

Bear in mind not to touch the walls of your container as you don't want to disturb the process of what's happening!

Remember the oil is probably gonna release some smoke/fumes once the piece is quenched so as said before: please do this step in a well ventilated area.

Once it's cooled down, take the object out of the oil and place it on a piece of paper/cloth and let it set for about 20 minutes. Alternitavely you can leave it in the oil bath to stabilize.

Then wipe off all excess oil and look at your beautiful finished object!