Hi,
During the first half of the 18th century barrels and iron /steel hardware were temper/fire/ blued, charcoal blued, painted with asphaltum (russetted), or left bright. Below are examples of each treatment. First, left bright. Polished iron (not steel) is amazingly resistant to rust:
Next is fire or temper blued simply by heating the object to 575-585 degrees F. the screw heads are fire blued:
Here a lock is fire blued:
Temper bluing is attractive but it fades over time to gray. Recent research by Richard Colton showed that the detachable pans and all external lock screws on French trade guns were often fire blued while the rest of the lock was left bright. Scottish iron stocked pistols were also fire blued, which has faded away on almost all the surviving examples.
Packing steel and iron parts in charcoal to limit exposure to air and then heating the pack to 800-900 degrees imparts a deep blue we call charcoal bluing. Below are examples of charcoal bluing on a high-end gun.
Charcoal bluing of this quality is only achieved by bringing the metal surface to a high polish and heat treating, cooling and rubbing with oil and rottenstone, then repeating heating and rubbing until the color is deep, mirror like, and pretty durable. Charcoal bluing that looks uneven and spotty is because the metal was not polished sufficiently first and heat treating cycles were not repeated. There are many examples of original guns that exhibit that level of bluing.
Rust browning using acid solutions was developed in the 18th century and became fairly common on British guns by the 1780s. It seems not to have become popular in America until about 1800. Below is a 1784 painting by Ralph Earle of a British sportsman with his fowler. The barrel is obviously browned:
Here is typical rust browning on an English gun styled from the late 1770s:
Here is a high-end example achieved by a high polish on the barrel and a very slow rusting solution.
It was a very durable and rust resistant finish and showed the patterns in stub twist and Damascus barrels very well.
During the latter half of the 18th century, British makers also preserved the case hardening colors as a decorative treatment and coloring for iron parts
Color case hardening was beautiful but not very durable. It was often painted with lacquer to preserve the finish as long as possible.
Perhaps as early as the 1790s but definitely during the early 19th century, gun makers discovered they could rust brown parts and then boil them in water to produce a deep and very dark bluing (rust bluing). Here is a high-end example:
This was a very durable finish and survived well into the 20th century as a common finish.
Depending on the quality, trade guns from the first half of the 18th century were likely finished in the white with more expensive versions occasionally having fire or charcoal blued components. During the later half of the century, browning probably became more common on better quality trade guns along with the older methods. During the 19th century, I believe there are trade gun examples showing the uses of all methods, including color-case hardening. I expect it all depended on price.
dave