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Blackening

What does blackening/black oxide actually mean? And what is it from a chemical point of view? Is it possible to fix it when poorly made? And what's the use of blackening?
 
Blackening (or black oxide) is a chemical treatment of ferrous materials with steel coating. It is widely used to increase corrosion resistance and to minimize light reflection.

What is blackening/black oxide from a chemical point of view?

For stainless steel, black oxide – from a strictly chemical point of view – means the item is pickled in hot baths of salts, acids or lye (e. g. NaOH). The thin layer on the surface is then converted into magnetite (Fe3O4) which gives the item a darker finish: black, brown or even dark blue.
 
However, blackening or black oxide is not the kind of procedure that changes the colour of the item coating by colouring or applying a colour. On the contrary, this process can be defined as chemical passivation of the item's coating. In addition, the layer created by blacking process is not more than 1 µm thick. In other words, you won't spot any change of size with the naked eye.
 
Impregnated by oil, this protective coating layer gives the item extra protection against corrosion.

How is it done?

In our latitudes, powdery preparation known as Brunigal is used as a standard. However, the very first step before the process of blackening can be started is thorough degreasing. This is usually done in a hot bath with surfactants (tensides) – surface active agents – by immersing the items in the boiling solution of Brunigal. 
 
Right after that, thorough rinsing in water takes place. The steel is then treated with diesel oil or paraffin. Konkor-type oil can be used for further conservation.
 
Another fact to consider is that every object with threaded or blind holes has to be blown through before the process of conservation gets started. If not done properly, long-term exposure to residues in the blackening bath can cause severe corrosion.
 
In our latitudes, powdery preparation known as Brunigal is used as a standard. However, the very first step before the process of blackening can be started is thorough degreasing. This is usually done in a hot bath with surfactants (tensides) – surface active agents – by immersing the items in the boiling solution of Brunigal. 
 
Right after that, thorough rinsing in water takes place. The steel is then treated with diesel oil or paraffin. Konkor-type oil can be used for further conservation.
 
Another fact to consider is that every object with threaded or blind holes has to be blown through before the process of conservation gets started. If not done properly, long-term exposure to residues in the blackening bath can cause severe corrosion.
 
Of course, various kinds of surface defects or uneven colouring may occur during the process of blackening:
  • Grey: uneven greyish discoloration may appear in case of using nitride steel (where the surface was hardened by saturated nitrogen). This can be fixed by giving the surface a gentle corundum spray.
  • Red: red colour is a sign of chrome overdose. This can also be removed by spray or – before the process of blackening gets started – steel crumblings abrasion.
  • Brown: brown tint is a sign of depleted bath polluted by iron.

What are the most common methods?

One thing to remember before we get started: it's the process of blackening that gives a steel product additional qualities. Compared to other methods, black oxide provides better protective and aesthetical effect and also mechanical resistance. However, there's a dark side of blackening process, which is complicated ecological destruction of sediments and depleted baths.
  • Blacksmith finishing (carbonation): a simple process – get the object heated first and then immerse it in an oil tank (biological or mineral). The surface gets covered by a thin layer of carbon, imperfectly burned oil. This carbonated surface has to be wiped thoroughly in order to remove oil residues and carbon fragments.
  • Cold black oxide: this method uses a solution of selenic acid with copper sulphate in alcohol which give the surface specific grayish or greeny tint. There's a problem though: these solutions are highly toxic and that's the reason why this method is not commonly used in the Czech Republic.
  • Special blackening methods: unique markers (usually sprays or black markers) are used to create a black finish or various kinds of retouches. These markers contain substances which work as corrosion inhibitors (anti-corrosive substances).

Modern use of blackening

This method is widely used in mechanical engineering (especially in the production of precision tools), but also serves as a basis for painting. Blackening is not just a traditional way to finish steel that protects firearms from corrosion, it reduces glare that could reveal shooter's location, which is highly applicable for hunters and armed forces.

How to repair black oxide finish?

It's not easy to fix severe damages like rust or scratches. First of all, damaged surface cannot get fixed by painting! Alcalic reaction on the surface causes the paint not to adhere.
 
So what a modern man or woman longing for a nice black finish can do? There are two possibilities: cold black oxide, which is a short-term solution and brings lower quality, or preferably - asking a professional who can repeat the whole process precisely.