There are a variety of mechanical and chemical / conversion finishes available for bronze and brass metal that can give a wide range of appearances to the material. Mechanical finishes tend to be more cost effective whereas chemical finishes (such as a statuary bronze finish) can be expensive and difficult to protect, but provide a beautiful appearance.
The more cost effective of the finishes are the mechanical finishes applied with abrasives. A brushed finish (a sample of which can be seen second from the right in the first of the adjacent images) is applied on a wide belt sander. The grain can range from a coarse grain applied with a 80 to 120 grit belt to a fine grain that is smooth to the touch and applied with a 320 to 400 belt. Another mechanical finish option is a mirror polished finish. This finish (illustrated in the photo to the left at the far right in the first adjacent image) is applied by first sanding and then polishing the material and is sometimes called a #8 finish. Both of these mechanical finishes are often protected with a clear lacquer that can be applied wet or is sometimes powder coated, depending on the degree of durability required by the application.
The chemical / conversion finishes for achieving a statuary or architectural bronze finish are anything but simple. First, there is the language with which companies will specify finishes. We regularly hear statuary bronze, architectural bronze, oil rubbed bronze, aged bronze, US10A, US10B and more. All of these terms do have meanings, but the meanings have become muddled by inappropriate usage over the years. The second source of complexity is the variety of processes used to achieve these finishes. And finally, there is a wide range in appearances of the final product. As an example, the three bars at the top in the picture in the top right accompanying this article are all statuary bronzes applied with either a different process or with some highlighting.
To start with process, finishing houses will generally pre-treat the material with either an abrasive or a cleaning agent or both to create a uniform surface for applying the oxidizing agent. The oxidation is generally achieved through the formation of a surface film of cuprous oxide or copper sulfides. The oxidizing agent is sometimes applied with heat, but more frequently without. When the material is oxidized, it has a darker deep brown or black appearance. To achieve an even darker appearance, fabricators will sometimes mix dark pigment in with the clear coating. To achieve a lighter appearance, the fabricator would highlight the piece by cutting back the conversion coating with an abrasive. Both the spraying and highlighting are hand finishes and require skilled craftsmen to apply. If you have a specific light or dark finish in mind, the best way to get what you want is to bring your provider a sample of the finish or at a minimum a picture.
Once the color is applied there is then a question of how to protect it. One option is a US10B finish, where no coating is applied and the material is allowed to age naturally. The difficult here is that any scratches that penetrate the surface film will expose the underlying yellowish material. The alternative is to apply a clear coating to the material. There are a wide variety of clear coatings ranging from wet lacquers, acrylic lacquers, nitrocellulose lacquers, clear epoxy coatings, urethanes or clear powder coatings. These coatings are selected on the basis of a combination of durability and cost. Maintaining the coatings can be difficult as cleaning agents can remove the coating and repairing scratches that do penetrate the surface coating can require refinishing the entire piece.
A different cost effective alternative to achieve a statuary bronze appearance is to use a specialty powder coat. While it is difficult to match the finish exactly, the technology in powder coating alternatives is improving and can also be a very durable coating for the material. Another possibility is bronze anodizing an aluminum base metal. Both of these are lower cost options vs. working with the expensive brass material.