I've been planning this experiment for ages and I'm delighted that it was an overwhelming success. Since the time I made this steel lunch box three years ago I knew I wanted to heat color it blue. I have done knife-making in the past so I was familiar with the coloring of steel and wanted to expand it to a more aesthetic use beyond a functional one. My procrastination was equal parts not making the time and being intimidated by the experimental risk of it. Will I break our oven? Will this take hours of monitoring? Will this work at all? Yet, every time I saw that lunchbox I dreamed of how much better it could be. Now, with quarantine at hand, I had all the time in the world and nothing to lose. Well, I guess I could lose the oven... but I'd have the time to get a new one!

This is a proof of concept that you can use your home oven to heat patina or temper steel objects (stainless steel is a different animal so this is regarding mild steel).

Using heat to color metal is a cool and satisfying effect. It's usually done with a torch and a careful eye. However, if you have a large surface area to cover its hard to get the color be even because the colors change quickly with the focused heat of a torch and you'll most likely end up with patchy rainbows. That's where your oven comes into play, it essentially becomes a metal sous vide!

An oven will allow you to hold the correct temperature for the time it takes for the metal to get to that heat bracket for the color you want. Hypothetically, it wouldn't be able to over heat and lose your target color. Even with a less than perfect oven you should be able to leave it in there indefinitely as long as your oven doesn't exceed the color temp you want.

There are a million factors that I can't know everything about: gas oven versus electric, convection versus non, oven size, etc. I can, however, tell you my experience and hopefully you can adjust accordingly. With a little tweaking I'm sure this can successfully work with any high temp home oven.

Supplies

An oven that'll go to about 550F is preferred (usually rated as a high temp oven, also great for pizzas)

Mild steel metal object to color

Denatured alcohol or rubbing alcohol

Wires for hanging

Gloves for preventing oily fingerprints

Optional: clear coat like a clear spray paint (I used EverBrite)

Step 1: Prepping Your Metal

You'll want your metal to be at its final stage of your project. Get all sanding, filing, and buffing done first. You won't be able to do any sanding or buffing after you temper the metal in the oven without taking the color off so get that mirror polish on it now! The color only sets into the metal a couple of microns so plan accordingly.

I got my lunchbox cleaned up and opted to stay with a satin finish.

Its also important to have your metal be clean of any oils and grease as that'll affect how your metal will color. Right before you are going to put in it the oven give it a wipe down with some denatured alcohol or rubbing alcohol. That'll remove any fingerprints or other oils that got onto the metal from handling it. Don't use anything water based as it doesn't evaporate like alcohol does and will likely find crevices to settle into and start rusting.

It can also help to handle your metal with rubber gloves while you are wiping it down and any time after handling from the alcohol cleaning to avoid putting new oily fingerprints on it.

Step 2: Tiny Science Lesson

While I am using this heating process to make my lunchbox look pretty, the coloring of steel has a functional process.

Steel is amazing in that is has a color code for what the crystalline structure of the steel is doing. Each color indicates a specific temperature and each temperature indicates a flexibility/hardness of the steel. So the straw-colored range indicates a harder steel but its more brittle. The higher range temperatures in the blues and purples are softer but more flexible and creates a spring steel.

While this mostly applies to high carbon steels it kinda works on a smaller scale for mild steel. This is mostly for me to geek out about how interesting steel tempering and coloring is for a minute, it doesn't really affect the outcome of most projects. You should pick your temperature based on the color you want based on prettiness.

If you are interested in more of the specifics of this I'd recommend looking into knife making to learn more details about annealing, hardening, and tempering steel.

I found a couple of different temperatures listed but they are generally in the same range. I was conservative and aimed for anything in the purple to blue color range so 540F seemed like a safe bet for my needs. If you are looking for precision, consider testing your oven with an oven thermometer first.

If you don't have a high temperature oven you should still be able to get some colors in the lower orange temperature range.

Step 3: Time to Oven-ate!

I was worried that if I put the metal on a tray or directly on the cooking rack that it'd change the how the metal heated up, I didn't accidentally want grill marks on my steel! This may have been over cautious of me but I went ahead and suspended the metal using wires in places where you wouldn't be able to see the contact points.

Now, hypothetically, if everything in the oven is heated to the same temperature it shouldn't matter. I wanted to play to safe and optimize my chance at success. I want to experiment more with this and see if sitting the steel on a tray makes a difference.

I will say though, make sure your oven racks are clean! Some tiny food crumbs (probably pizza crust bits) fell on my metal as I was hanging it and the resulting grease discolored my metal where those little crumbs had fallen.

Here's the steps I took:

I did not pre-heat the oven, I wanted everything to come up to temperature at the same time (I also didn't want to have to fiddle about hanging my lunchbox on a hot rack in a hot oven).

I set it for a hour but as a good experimenter I monitored it closely and saw it hitting the color I wanted after about 30min. Thicker metals would probably take longer (I'm excited to experiment with rebar in the near future so I'll let you know then). It shouldn't matter how long it sits in the oven as long as the temperature doesn't get hotter than your intended target temp.

Steel does hold onto a surprising amount of moisture so don't be alarmed if you see the metal smoking/steaming as it comes up to temperature.

Once I saw the color I wanted I turned off the oven, cracked the door open, and let it come down in temperature for about an hour until it was cool enough to touch.

You'll want to keep your hands gloved while handling your metal if you want to clear coat it to ensure good adhesion.

Step 4: Protect Your Metal

Fresh out of the oven my lunchbox looked amazing with a nice satin blue finish. Good deep, even, color all around (except for the stainless steel hardware and aluminum rivets you'll notice). It is possible that the stainless steel might start to color if left in the oven for longer but my cursory research lead me to believe that stainless needs to get to about 800F to color. My oven certainly can't do that.

Since you are working with mild steel you'll want to protect your project both from rust and from the color getting rubbed off.

This coloring process doesn't do anything to protect the metal from rust. There is a chemical process called 'bluing' that does create a protective layer but that is different from tempering color ('bluing' actually turns the metal black or gray, not blue).

Oil can help prevent rusting but won't do much against the color steadfastness
A clear spray paint, like rust-oleum's rust inhibitor, is a good option
EverBrite is thicker and perhaps better for something functional

I opted to use EverBrite as my protective coating as I want to actually use my lunchbox and the resin coating should help avoid everyday wear and tear from removing the color for a good long while. You'll notice my satin finish turned into a glossy finish and it really deepened the color! Now its more of a deep purple instead of a glowing blue. It also depends on the lighting and the angle you view it at.

While I may not have experimented with every possibility, I was able to successfully get an even color on my large sheet metal lunch box and I'm excited to turn everything I can fit into my oven blue!