Metal in the living space: why brass tarnishes and stainless steel does not

Brass tarnishes, and that is not a flaw

Brass is a copper alloy: at least half copper, up to forty per cent zinc, depending on the purpose small proportions of other metals. The copper in it reacts with the oxygen in the air and with whatever else is floating in it. The surface becomes darker, first golden brown, then reddish, over time greenish.

This process is called tarnishing and is a form of oxidation, not dirt. It cannot be stopped, only delayed. How quickly it happens depends on humidity, sweat from hands and the room – there is no figure for it, and anyone who gives one has guessed.

The shine can be brought back with acid: lemon juice with salt, vinegar, ready-made brass polish. What matters is drying thoroughly afterwards; otherwise the freshly bright surface tarnishes again within days. And every polish removes the patina completely. That is a decision for a particular look, not a care step that would need to be done regularly.

Clear lacquer: what it can do and what it costs

The usual way to stop tarnishing is a continuous layer of clear lacquer. As long as it is intact, it works reliably – no oxygen, no oxidation.

The problem begins when it no longer is. At a chipped or worn-through spot the metal continues to tarnish, while all around it stays bright. Instead of an evenly grown patina, a patchy pattern develops, and that can only be remedied by removing all of the lacquer.

That is why many high-quality pieces are deliberately left unlacquered. An open surface changes, but it changes evenly everywhere.

Stainless steel: stainless is not rustproof

Stainless steel contains, by definition, at least 10.5 per cent chromium. This causes something remarkable to happen at the surface: instead of a layer of iron oxide, that is rust, a dense, firmly adhering layer of chromium oxide forms. It is called the passive layer, is only a few atomic layers thick and completely invisible.

Its best property is self-healing: if it is mechanically damaged, it immediately rebuilds itself in the presence of oxygen. A scratch in stainless steel is therefore a visual problem, not a corrosion problem.

Even so, brown dots do appear, and they are almost always taken for rust. In about ninety-nine out of a hundred cases, however, it is not the stainless steel that is rusting but something lying on it: tiny iron particles from the air, from brake dust, from construction work. They settle and rust when damp. This is called surface rust, does no harm to the material and can be wiped off. It should still not be left, because the passive layer underneath suffers in the long run.

Chrome-plated does not mean made of chrome

A shiny chrome-plated surface is a stack of layers. First copper goes onto the base metal, nickel over it, chrome at the very end. The proportions are surprising: copper can be twenty-five to forty micrometres thick, nickel about fifteen – and the chrome layer itself under one micrometre.

That is enough because electroplated chrome is very hard, harder than almost anything in the living space that knocks against it. Functionally, even two to five tenths of a micrometre suffice.

What matters in practice is the layer beneath. Under the chrome lies nickel, and nickel allergy is common. As long as the chrome layer is intact, this does not matter. At a worn-through spot, for instance on a handle or an edge, there can be skin contact.

Powder coating: the most robust colour on metal

In powder coating, colour powder is sprayed electrostatically onto the workpiece and then baked on, depending on the powder at between 140 and 210 °C, usually at 180 to 200 °C.

The result is thicker than a coat of paint: up to 150 micrometres compared with around 80 for a stove enamel finish. The layer is correspondingly more resistant to scratches, abrasion and impact. For furniture frames, table legs and anything that gets knocked by a vacuum cleaner or the toe of a shoe, that is the decisive difference.

A side effect that is rarely mentioned: the process works without solvents, and excess powder is collected and reused.

Anodised aluminium: the layer grows inwards

In anodising, nothing is applied. The top metal layer of the aluminium is converted electrolytically into oxide – so the protective layer grows into the component rather than on top of it.

This has two consequences. First, the dimensions are preserved, which matters for components that are plugged or screwed together. Second, the layer cannot chip off, because it is not a coating but the material itself. It is considerably more resistant to corrosion and wear than bare aluminium.

Care: dry, soft, without abrasives

The basics are the same for every metal surface: dust with a dry cloth, if needed wipe over with a slightly damp cloth, then dry. Moisture that is left standing is the accelerator for brass, and for stainless steel the condition for surface rust.

Abrasive cleaners and scouring pads have no place on any of them. They leave a network of fine scratches in which dirt settles and which no polish can get out again. On coated and chrome-plated surfaces they additionally destroy precisely the thin layer that makes up the look.

With unlacquered brass, it is worth deciding before you start: either polish regularly and keep the bright finish, or do not polish at all and let the patina grow. Both look good. Only switching between them produces patches.