<P> These processes were later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor - intensive . They are still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas . Rust bluing is also used on shotgun barrels that are soldered to the rib between the barrels, as hot bluing solutions would dissolve the solder during the bluing process . </P> <P> Large scale industrial hot bluing is often performed using a bluing furnace . This is an alternative method for creating the 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 selectively over the red oxide . It, too, must be oiled to provide any significant rust resistance . </P> <P> There are also methods of cold bluing, which do not require heat . 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 an adequate cosmetic touch - up of a gun's finish when applied and additionally oiled on a regular basis, however, rust bluing small areas will often match and blend better and wear better than any cold bluing process . </P> <Table> <Tr> <Td> </Td> <Td> This section is empty . You can help by adding to it . (May 2017) </Td> </Tr> </Table>

When should a cold blue finish be applied on a rifle