Brass screens, as well as screens made of other metals, are often used to cover furnace vents and, since they all have dampers, they do not block air flow. Home Depot, for example, sells a solid brass Gothic register that's popular with Victorian home renovators because of its vintage look.
Other brass screen options include an art deco style with geometric shapes, free-form patterns that resemble lace and a simple parallel line grate. Some are highly polished, while others have a satin finish. An Oriental style grate option also comes with an oil rubbed bronze finish. All have rust resistant damper boxes that open and close easily.