Most of the time, the correct action is none.
These materials are chosen because they change. Below is how to look after each one, and — more usefully — when to leave it alone.
Bare brass darkens where it is handled and stays bright where it is not. The dark film is copper oxide and it protects the metal underneath. If you like how it is going, do nothing at all. This is the intended outcome.
A cloth and any non-abrasive brass polish, worked in small circles, then buffed off. Under a minute for a knob. Avoid abrasive pastes and steel wool, which leave a scratch pattern that shows once the piece darkens again.
Dishwashers, bleach, and ammonia-based cleaners. Ammonia strips the oxide unevenly and can leave a mottled surface that has to be polished out entirely.
Vegetable-tanned leather arrives pale and stiff and will mark easily at first. This is the material working. Carry it, do not baby it — the marks even out as the hide darkens.
Once or twice a year, a small amount of neutral leather conditioner worked in with the fingers and left to absorb overnight. Less than you think. Over-conditioned leather goes limp and will not hold an edge.
Blot, do not rub, and let it dry away from direct heat. A radiator will cook the hide and it will not recover. Once dry, condition lightly.
Wipe with a barely damp cloth. Re-oil when the surface starts to look dry or pale rather than on a schedule — a thin coat of any food-safe furniture oil, left twenty minutes, then wiped back hard.
Because the finish is oil rather than varnish, a scratch can be sanded locally with fine paper and re-oiled without stripping the whole surface. This is the advantage of oil and the reason we accept its lower resistance.
Keep dry. A trace of light machine oil on a cloth, wiped over a blade or point occasionally, is enough. Rust that has started should be taken back with fine abrasive and then oiled.
Treat exactly as the surrounding material. Do not pick out an engraved line with a brush or a pin — the contrast between the darkened line and the polished field is the effect, and cleaning it out flattens it.
Send a photograph.
If you cannot tell whether something is ageing or actually damaged, we will tell you honestly.