Bluing
The blueing consists of the generation of a superficial layer of magnetite, ferrous-diferric oxide (Fe3O4), around steel parts to improve their appearance and prevent corrosion.
History
The origins of blueing are somewhat uncertain. What was known is that heating steel to a bluish tone and immersing it in oil considerably increased its resistance to rust.
It appears that the bluish-black appearance of iron oxide, similar to the plumage of a turkey, is what gives the process its name.
Definition and types of bluing
Bluing consists of generating a superficial layer of magnetite, ferrous-diferric oxide (Fe3O4), around the steel parts to improve their appearance and prevent corrosion.
The wear and tear of bluing, although unsightly, is one of the "lesser evils" that most often affect ferrous metal.
On the market there are several products for cold bluing, valid for small touch-ups, the truth is that for large surfaces and especially in restoration processes it is not enough.
There are two methods of bluing: acid (cold) and alkaline (hot). Acid is undoubtedly the method that provides the best quality, durability and appearance, but it requires a long time to achieve the desired result. Alkaline bluing is obtained by applying chemical products that provide a highly adherent and durable surface oxidation. On the other hand, alkaline bluing is much easier to achieve and in a very short time, which is why it is the method usually used in the restoration of small pieces that have lost their original bluing.
Dip bluing
The pieces resulting from this type of bluing take on a bluish coloration, which is why it is also known as bluing.
This is the immersion of metal parts in a mixture of potassium nitrate and sodium nitrate at temperatures between 310 and 350 °C. After carrying out this immersion, the pieces are washed with hot soap, which ends the immersion bluing process.
Bluing by heating
This bluing process is carried out in ovens at temperatures of 400 °C. To start the process, it is necessary to carry out a thorough cleaning of the metal parts to remove rust and dirt from them. They are then coated with a 25% asphalt solution, although an oil-in-gasoline varnish solution can also be used. After this step, the metal pieces are placed on an iron mesh and placed in the oven for around 10 minutes, which ends the process.
Bluing can be done on sheet metal, but is most often used on gun barrels and other firearm components.
The importance of a good bluing installation
Bluing is a very effective method to prevent metal corrosion as long as it is carried out in suitable facilities for such processes. When bluing plants are poorly designed or poorly installed, a series of setbacks can occur, such as: inefficiency of the process, production of deficient parts, breakage of the resulting parts, no protection against corrosion or production of toxic elements. For this reason, it is necessary to have advisers who have the necessary knowledge to carry out the installation of bluing plants.