Gun barrels and old hunting weapons burnished dark blue, black parts of measuring or optical instruments and various artistic steel products such as gratings, ashtrays, fireplace equipment, cast iron or steel fittings often have a beautiful black appearance. It is often necessary to retrofit a missing part and give it a colour afterwards. The dark or even black colour is given to cast iron and steel artificially by bathing it in appropriate solutions. This is called colouring or chemical dyeing and consists in the creation of a black or dark blue oxide layer on the metal surface. This layer plays not only a decorative role. Thanks to its chemical passivity, after additional greasing it quite effectively protects steel against corrosion. The quality and appearance of the layer depends primarily on the degree and thoroughness of preparation of the object's surface, which consists of grinding, polishing, degreasing and etching. If we want the object to shine after blackening, we must not forget about polishing. Unpolished objects will be dull after blackening, which is sometimes desirable. 
[001] Blackened Mauser Kar 98K. The treatment increases resistance to corrosion and also reduces the possibility of light reflections from the weapon. The simplest way to blacken steel is the so called flame blackening. Mechanically cleaned object is heated in the furnace or over the burner to about 200°C and then rubbed with a cloth on a stick dipped in vegetable oil (e.g. cooking oil). When blackened in this way, a very large amount of pungent smoke with an unpleasant smell is produced. - sodium hydroxide NaOH 400 g - sodium nitrate NaNO2 600 g The solution must first be heated to 140°C only then will it become liquid, because at room temperature it is thick and crumbly. The already prepared objects, suspended on an iron wire, are immersed in the hot bath for 30 minutes and frequently moved. To obtain a glossy black coating with high corrosion resistance it is best to use a two-stage blackening. Heat the bath in an iron or steel vessel to 140°C and immerse the blackened steel exhibits or products in it for 10 minutes. In order to thicken and consolidate the coating, the objects without rinsing are transferred to the vessel with bath II. The temperature of the bath is 155°C and the duration of blackening is 35 minutes. After finishing blackening in bath II the objects are carefully rinsed with water, dried in sawdust and lightly greased with oil or petroleum jelly. Well done two-stage blackening not only gives steel objects a shiny black colour but also increases resistance to corrosion. The resulting solution is then saturated with a cotton wool swab and rubbed thoroughly onto the steel objects, after which the precipitate must dry and the solution must be applied to the surface. The item must then dry and be kept in steam for 30 minutes. A thorough rinsing in water, drying and greasing are the final steps of this treatment. The aim is to be able to choose the most suitable solution and blackening parameters, depending on the type of steel. 
[002] Partially burnished, handmade knife. Brunifying is a special, most noble type of steel blackening, used mainly for finishing the surfaces of expensive, precise objects, both new and antique. In this process, as in previous ones, a thin oxide layer is formed which gives the object an aesthetic dark blue colour and also protects it from corrosion. Bruising is a very tedious process, requiring great care and clean workmanship, but it produces very good results. It is used especially when replacing missing elements of old weapons. A very carefully polished, cleaned, thoroughly degreased and etched object is rinsed with denatured spirit, and after it dries it is moistened with a tampon dipped in the following solution: 
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