Brass is an alloy of about 70% copper and 30% zinc. Sometimes small amounts of other metals are added. The usual colour is a pale yellowy-gold.
In homes, brass is commonly used for door and window furniture. Brass items are usually solid but modern fixtures and fittings are often plated brass over steel. Test for this with a magnet.
Heating a metal containing zinc can cause zinc oxide fumes or dust. Exposure can cause irritation of the eyes, nose, and throat and a flu-like illness called metal fume fever.
Should you clean tarnished brass? The normal practice be to keep it shiny. However, there are some risks to cleaning brass and other metals. Firstly, you will tend to lose an attractive patina that is itself a protective layer. Secondly, if the metal proves to have been plated, it is easy to damage that top layer.
Lacquer may wear or peel off in some areas. This leaves the exposed brass to tarnish, while the rest may remain bright. Old, damaged lacquer can be removed with acetone, preferably by immersing it.
Painted brass can be stripped with a standard chemical stripper.
If you wish to remove tarnish, mix 1 level tablespoon of vinegar in 0.5 pint of hot water. Dip fine wire wool in it, and scrub the metal lightly. When it is clean, rinse the fitting in fresh water and dry it carefully.
To clean a fitting in situ, make a protective shield from card, cut to fit round the fitting.
Note that polish is usually slightly abrasive, but some types leave a protective layer.
If you do not wish to build up a patina on the brass, finish with a clear acrylic lacquer, or better, a wax polish.
To age brass, expose it to ammonia fumes; avoid splashing ammonia on the brass otherwise you will get a spotted effect. You need strong ammonia; try a chemist.
The brass must be clean and de-greased. Depending on the size of the item, suspend it over ammonia in a glass jar, or place it in a plastic bag with ammonia-soaked rag.
Protect the brass with a wax polish afterwards.