What is Rust? - Oxidation Reactions
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What is Rust?

Rusty fence
(Thanks to Wikipedia)

February 2017 - Fire isn't the only kind of oxidation reaction. Some kinds of metal, if you leave them in contact with oxygen, will also react with the oxygen, but much more slowly, and without making a flame. Because it's slower and doesn't make a flame, we call that an oxidation reaction and not a combustion reaction. We call the result rust.

Like burning grass, the combustion reaction between iron and oxygen that produces rust, or iron oxide (a molecule containing both iron and oxygen atoms), also produces heat. In fact, it produces the same amount of heat as the fire, but spread out over a longer time, so that the iron oxide or rust also cools off at the same time that it is heating up.

When iron is exposed to oxygen in the air, the two combine very slowly to make iron oxide, or rust. But if the iron and oxygen molecules are also touching water molecules, the water molecules speed up this reaction and make the iron rust a lot faster.

Exekias vase
Black-figure vase by Exekias (Athens, ca. 540 BC)

One special kind of rust reaction happens inside a pottery kiln when you heat up a clay pot and there is some iron in the clay. The iron in the clay reacts with the oxygen inside the kiln and turns red. This is how Greek vase-painting worked.

Learn by Doing - Rust
Another slow kind of oxidation reaction: digestion
A faster kind of combustion reaction: fire

Bibliography and further reading about oxidation and combustion reactions:

Fire
Iron
Steel
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Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram or Twitter.
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