Information AboutBluing (steel) |
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In contrast, the Red Oxide Of Iron (Fe2O3), does not occupy the same volume as iron, thereby causing the typical reddish rusting away of iron. Both "cold" and "hot" oxidizing processes are called bluing, but only the "hot" process provides any significant rust and corrosion resistance, and then only when also treated with an oiled coating. USAGE Bluing is most commonly used by gun manufacturers, gunsmiths and gun owners to provide limited resistance against rust while also improving the cosmetic appearance of the weapon, primarily by eliminating shiny reflections from the gun steel. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 0.0001 inches (2.5 micrometres). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts. New Gun s are typically available in blued finish options offered as the least-expensive finish, and this finish is also the least effective at providing rust resistance, relative to other finishes such as Parkerizing or hard Chrome Plating . Bluing is also used for providing rust resistance for steel parts of fine clocks and other fine metalwork. Bluing is often a hobbyist endeavor, and there are many methods of bluing, and continuing debates about the relative efficacy of each method. Historically, Razor Blade s were often blued steel. A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semi-conductor Diode junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in the Crystal Set AM radios which were often built by soldiers during World War II. HOT VS. COLD BLUING Bluing may be applied, for example, by immersing the steel parts of the gun to be blued in a solution of Potassium Nitrate , Sodium Nitrate , and water heated to the boiling point. Similarly, stainless steel parts of the gun to be blued are immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods is called ''hot bluing''. There are many other methods of hot bluing. Hot bluing is among the most effective forms of bluing, providing the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal. There are also methods of ''cold bluing'', which do not require heated solutions. Commercial products are widely sold in small bottles for cold bluing firearms, and these products are primarily used by individual gun owners for implementing small touch-ups to a gun's finish, to prevent a small scratch from becoming a major source of rust on a gun over time. At least one of the cold bluing solutions contains Selenium Dioxide , to accomplish the bluing. Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance. It does, however, often provide a very good cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis. Cold bluing is often applied by first cleaning the steel area to be blued with alcohol, allowing the area to air-dry, touching a cotton swab in the cold bluing solution, applying one or more applications of the cold bluing compound to the steel being blued to match the rest of the hot blued finish, allowing the area to dry completely, and then using a good grade of gun oil to rub onto the cold blued areas, overlapping with the original hot blued areas. Provided regular oiling and rubbing is done, this method will provide adequate protection against rust for many gun owners. Hot bluing and cold bluing kits and solutions are also sold commerically, for use by gun hobbyists. Large-scale industrial bluing is also performed using a bluing furnace. This is an alternative method for creating the magnetite (black oxide) coating. In place of using a hot bath (although at a lower temperature) chemically-induced method, it is possible through controlling the temperature to heat steel precisely such as to cause the formation of black oxide (magnetite) selectively over the red oxide. It, too, must be additionally oiled to provide any significant rust resistance. LIMITATIONS Bluing only works on steel or stainless steel parts for protecting against corrosion. Aluminum and polymer parts are not blackened by applying bluing. Similarly, aluminum is not protected against corrosion by applying bluing. Other techniques must be used for protecting non-ferrous alloyed parts on guns against corrosion. For example, aluminum gun parts may be protected against corrosion using either patented processes (e.g., or ) or similar trade-secret processes. Holster wear will remove hot bluing over long periods of use. TERMINOLOGY Some prefer to call thin coatings of black oxide by the name ''gun bluing'', and to call heavier coatings by the name ''black oxide'', but they are both the same chemical conversion process for providing true gun bluing. Historically, this process was also referred to as "browning" as many of the original finishes were brown rather than the safer-to-apply coatings that are blue and that became popular. (Many of the browning formulas, and the early bluing formulas, were based on cyanide-containing solutions, which are especially toxic.) The terms ''bluing'' and ''browning'' are interchangeable. REFERENCES A standard overview on black oxide coatings is provided in MIL-HDBK-205, ''Phosphate & Black Oxide Coating of Ferrous Metals''. Many of the specifics of Black Oxide coatings may be found in MIL-C-13924. This Mil-Spec document additionally identifies various classes of Black Oxide coatings, for use in a variety of purposes for protecting ferrous metals against rust.
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