I was always interested in bluing, although it seemed like a difficult task. There are many ways to blue steel; cold bluing, hot bluing, bluing in an oven, using a torch, and many different bluing solutions. It all seemed like a mess to learn at first, but it’s actually very simple.
In this article, I will talk about torch bluing, which is my preferred steel bluing method whenever possible. It’s amazing for bluing any small steel object unless it’s made of stainless steel.
Necessary Equipment for Torch Bluing
The two most obvious requirements are a torch and oil. If you don’t have a propane torch, buying one with the hose is best.
Most torches attach directly to a small tank which isn’t practical when you have to get the precise temperature and heat the object evenly. Another thing to look for is the head shape. You want a narrow head like on the torch I linked above.
You will need mineral or vegetable oil, which is used to burn the color into the steel permanently.
What Oil To Use for Bluing Steel?
When it comes to oil, most people say any oil works. However, I wouldn’t say that. Any oil will do if you want mediocre bluing results. You want a mineral oil without synthetic additives to get a clean, evenly colored surface without a mosaic of different shades.
Synthetics cause oil not to burn, which leaves uneven spots and even mixed colors when used to blue steel.
I used Castrol GTX Classic 20W-50 Conventional Motor Oil, and even though it has some additives, they are not synthetic, so it works well.
Vegetable oils can also work, although they are extremely different from one another and require some experimenting until you find a good one.
Steel Tempering Colors
You can get a variety of colors by heating the steel to certain temperatures. The gaps between those temperatures are not that wide, meaning you have to heat the whole steel object evenly without letting one part get hotter than the other unless you want mixed, rainbow-like colors.
The steel attains these colors just from heating before it’s put in oil, so you can see what temperature and color you will get at any point during the heating process.
How to Blue Steel With a Torch
Getting good bluing results and clear colors isn’t easy. To get consistency, I like to follow a 4 step process.
Polishing steel before bluing is not essential, but it’s important if you want smooth colored surfaces after bluing. How much to polish or grind depends on the condition of the steel object. Sometimes it’s not even necessary.
Unless I’m bluing something new, I like to grind and polish a bit to remove any imperfections on the surface. Thorough grinding is necessary only if there is any rust.
I use 400-1000 grit sandpaper to wet-grind the rust and then continue through 2000-4000 to wet-polish. It’s not always necessary to go all the way to 4000 grit; you might be satisfied with how the steel surface looks before that.
Most people skip cleaning and later wonder why their bluing is spotty and ugly. Even your skin oil can mess up the color if you touch the steel with your fingers.
To simplify the cleaning, I recommend 99% isopropyl alcohol, like the one I linked before. The percentage must be as high as possible; otherwise, you may as well skip this step.
Why is 99% isopropyl alcohol so important?
Every cleaning agent leaves its particles behind after cleaning. Try washing anything with a detergent; the surface feels different before and after, even if it was completely clean. 99% isopropyl alcohol cleans amazingly and evaporates completely, leaving nothing behind. Diluting it or using a lower alcohol percentage leaves too much moisture on the surface that can mess up the surface while the steel is being heated.
Heating Steel With a Torch
Properly heating the steel is the most important part of the bluing process. It’s not as straightforward as grabbing the torch and bursting the steel with the flame.
The key to good results is heating the steel evenly and slowly.
Heating evenly is self-explanatory. All you need to do is rotate the steel object or move the torch around all the time to heat all its parts as evenly as possible.
The importance of slowly heating steel, especially at the very start, is where many people make mistakes. Steel is an alloy, meaning it’s comprised of several metals. These metals react to heat differently on a molecular level. My observations showed me that certain steel types tend to create discoloration spots on the surface when heated too quickly. Not to mention, the steel can weaken by extreme heat.
Dipping Steel Into Oil
Dipping the steel into oil after heating is the easiest part, yet the most dangerous for you.
The first mistake is putting the heated steel object into the oil while being too close. Make sure to use long pliers or a wire to hook the object and dip it into the container with oil because the oil might boil and even catch fire on the surface, depending on which oil you use.
Vegetable oils boil and catch fire more easily, which is another reason why mineral motor oils are my preferred choice.
It’s enough to leave the steel in oil for 10 minutes, although I prefer up to 30 minutes to be safe.
Bluing Steel in an Oven vs. Torch
A torch is great for bluing because you can control the heat and see the color while doing it. Oven, on the other hand, requires more knowledge and experience. However, it has its benefits.
The oven is great for bluing because it can heat larger steel objects more evenly. To do that, you need to have an oven designed for that. Typical ovens won’t heat evenly and can’t reach most of the needed temperatures.
This post is about torch bluing instead of oven bluing because ovens are simply not practical for an average or a beginner user.
Bluing Steel With Ferric Chloride
Ferric chloride is typically used to cold blue steel. However, it has its uses in hot bluing as well.
It might not be easy reaching higher steel temperatures for a beginner, especially with larger steel objects. That is where ferric chloride comes in.
Ferric chloride has the effect of producing high-temperature colors with low-temperature steel. For example, you can get a nice dark blue color which you typically get at 550F (290°C), at a much lower temperature.
My tests show that the needed temperature is about 100F lower than it’s shown on the tempering color scale.
The problem is that it takes a few tries and errors until you can figure it out because it changes the color once it’s put into the ferric chloride and oil mixture.
There are many different ways to mix ferric chloride with oil. I used a 1:4 ferric chloride to oil ratio with the oil I always use, and it worked great every time.