There is a lot of information on the internet about steel bluing processes and types, but I’ll try to summarize in one single post.
Steel bluing or steel oxidation is a chemical process that creates a protection layer on the surface of the material, which behaves like a corrosion resistant coating.
It’s often used in firearms parts in order to (partially) prevent rusting and also to give a better finish.
Lets see some of the most popular types of bluing, what is the exact process and check their pros and cons.
Note: before any type of bluing, it’s mandatory to degrease/clean the part with isopropyl alcohol, acetone or similar product, using gloves, so you won’t left fingerprints on the part.
Cold Bluing
A good example of cold bluing product is Ballistol Quick Blue, but there are many products available from different manufacturers.
It’s very simple method and perfect for home usage.
After the cleaning process, simply use brush or a cotton pad and add from the liquid to the part. It takes instant effect and you can easily see the result. You can add multiple layers for even finish.
After you are happy with the result, remove the remaining bluing product with dry towel/wipe.
Then you need to oil the part with whatever you have available – gun oil, motor oil etc and keep the part like this overnight. It will stop the process and will prevent from further corrosion.
A good rule here is that you should always use separate container for the bluing fluid, because if you put a used brush into the original bottle, it can make it unusable for later usage.
Pros:
– easy to apply
– can be used to blue only specific sections of the part
Cons:
– less durable compared to the other methods
– it’s very hard to blue all sides if the part is with a complex shape
Hot Bluing
This method is also suitable for home usage and it’s relatively easy to apply.
It’s a two step process:
1. Heat the part to around 300-450°C (really depends on the steel) until it gets evenly darker, but don’t overheat as it creates defects on the surface. You can use gas burner.
2. As it’s hot, put the part entirely in oil (can be new or used motor oil, cooking oil etc.) and keep it like this for several minutes.
Pros:
– easy and fast method
– cheap – with some luck you don’t need to buy anything
– better wear resistance compared to cold bluing
– complex parts are not a problem as the heat and oil reaches all sections
Cons:
– should not be used on already hardened parts
– cannot apply to a specific section of the part only
– in my experience hard to always get even dark
Chemical hot bluing
Not sure if there is a specific name, but basically this method uses a bath (usually hot) with a mix of some chemicals.
There are way too many variants, depending on the desired finish and material, so I’ll share the two common variants I’ve seen to be used from small shops here.
Note: this method is not really suitable for home and it’s mandatory to use hands and eyes protection. Some of the materials could need a license to obtain also. Please check you local regulations first and do on your own risk.
Materials for black matte finish:
– 1L of water
– sodium hydroxide (caustic soda) – 600ml
– sodium nitrate – 150ml
Materials for black glare finish:
– 1L of water
– sodium hydroxide (caustic soda) – 600ml
– sodium nitrite – 200ml
Slowly mix the ingredients, ideally in a stainless steel container.
Heat the mix up to around 130-135°C, which is around the boiling temperature.
Put the steel part and keep it inside for 30-40 minutes.
Then remove the part and clean good with hot water.
Apply some oil to stop the corrosion process and keep it overnight.
Pros:
– best wear/corrosion resistance compared to other two methods
– better overall finish
– complex parts are not a problem as well
Cons:
– complex, can be dangerous, needs a good attention, also materials are not widely available
– cannot apply to a specific section of the part only
– takes time