Thursday, July 05, 2007

Brass Corrosion Inside a Mechanism

"Brass" is an interesting material and may be comprised of copper, zinc, tin, lead and other metals in what we refer to as an alloy. Various alloys of brass have different properties and amounts of strength, plasticity, density, reactivity, etc. Brass and its more heavily tin-laden cousin Bronze are often thought of as relatively corrosion resistant and rust free. But brass can and does corrode under certain conditions, perfectly illustrated here in the shutter mechanism of a Graflex Series D camera. The blue material is oxidized and chloride (salt) reacted copper, and it got that way from long exposure to small amounts of moisture and the salts in the contacting surfaces, in other words, in a fairly dry environment. Microphones are subjected to some moisture, and sometimes saliva, which is salty.

I am suspicious of the various brasses used in microphones today. The so-called free machining brass can be heavily loaded with lead, and other unspecified materials. Brass has little magnetic shielding value when compared with steel or stainless steels. Brass varies and although there are grades of brass the brass you get in a product is usually not graded or specified. We never use brass in a contact area for obvious reasons and in just about any area where small amounts of corrosion can affect things it might be best to avoid it. Brass tastes bad which may explain why there are so few brass forks being made, though in the long past, in Roman times, there were plenty of brass and bronze-based eating utensils, which, despite the flavor, probably seemed fine and tasty with a good layer of hardened pudding and mutton juice.

Note that the mouthpieces on trumpets and the like are usually plated, to avoid the aforementioned undesired flavor which can get even worse during an hour-long performance. Bugles, on the other hand, often have plain brass-to-lip contact, but thankfully brief songs, and stoic players.

We do use some brass hardware which should be protected from corrosion for a long time. Brass does have the advantage of being nonmagnetic, which comes in handy when you need a metal that won't be attracted to a strong, nearby magnet.

Do you like bronze sculpture? Carol Feuerman is an artist who works in various materials including bronze, and her work is very entertaining. Go here to see it.

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