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3D3DCopper vacuum gasket.
This ring is a used vacuum gasket. A gasket is a soft material placed between two mating surfaces with the idea that, being soft, it will deform itself to fill in any small gaps or irregularities in the surfaces, creating an air-tight seal. A common rubber washer is an example of a gasket, and indeed most gaskets you see in common use are made of rubber or rubber-like materials.
But for high-vacuum applications rubber or plastic is completely unsuitable. They may be air-tight according to ordinary, every-day standards, but they are quite porous by the standards of high vacuum work. Any sort of organic matter absorbs gas that comes back out when it's placed under vacuum, and allows oxygen, water vapor, and other gases to pass through slowly.
Metal makes a much better gasket material because it really is completely air-tight. Copper may not sound very soft, but compared to steel it is. See the groove in the center of the ring? That's created by a steel edge when the joint is assembled: By squeezing the joint together so tight that the two parts being joined actually dig into the copper gasket, a completely tight joint is created. It also means the gasket can be used only once, which is why I have several of these: After one use, they are junk.
Source: Max Cane
Contributor: Max Cane
Acquired: 23 December, 2007
Text Updated: 29 January, 2009
Price: Donated
Size: 2.5"
Purity: >95%