Theory Thursday #5 - Rust

by Massive Voodoo

Welcome everyone to the fifth Theory Thursday!

If you are not interested in understanding how the world works, especially light and shadow, color and harmony, you should better skip reading this post and future Theory Thursday editions.
Opposed to the very direct and practical tutorials you will usually find here, these series of posts will go in depth to answer questions that many painters didn't even ask themselfes.

Be reminded that these posts are written by Raffa alone and reflect his understanding of the topics.
He is not always right and if you found an error or you want to discuss, use the comment section!

This week the topic will be:

This weeks topic will be slightly different than the weeks before.
Light, lightwaves and electromagnetic radiation will have a week off for something more "real" and quite interesting.

Rust, everyone knows it. It looks cool as inspiration and it's annoying when it's on places where it shouldn't be.
Photo by Raffa taken in Spain

Rust is basically a mix of Iron Oxide and Iron Oxide-Hydroxide.

Iron Oxide (Fe2O3) is a very common chemical as iron reacts very happily with oxygen.
It's very hard to find pure iron in nature.
Iron Oxide has a typical red color that is and was used as a pigment very often to make colors.
By the way, many people know the typical red houses in countries like sweden, norway, etc.

Photo by Raffa taken in Norway

This color is called Falu Red and originated from a copper mine in sweden, it has more ingredients, but iron oxide is a very important one in giving it the typical color.

Most of the time, rust is not just red, that's because the Iron Oxide is hydrated.
Iron oxide-hydroxide Fe2O3·1 H2O) has a yellow color (also used as a color pigment).
It can be heated (to remove/vaporize the H2O) leaving Iron Oxide (Fe2O3) ...

But why is iron or alloys containing iron (like steel) rusting?
The process is called corrosion - it's an electrochemical process.

To make this possible you need an anode (a piece of metal that give electrons away), a electrolyte (a liquid that helps electrons to move) and a cathode (a piece of metal that accepts electrons).
Does this sound familiar to you?
Maybe now? Ok, doesn't matter anyway :)

In the case of iron corrosion, we need three components.
Iron, water and oxygen.

When water (our electrolyte) hits the iron a lot of stuff starts to happen.
First, the water reacts with the air to for different kinds of acids, especially with carbon dioxide.
This acid is an even better electrolyte than water and will speed up the process of corrosion as it will dissolve the iron.
The water will start to split up into it's core components (H2O = two hydrogen atoms and one oxygen atom) and the oxygen will start to bond with the dissolved iron into .... iron oxide!
Electrons are freed and will go to the cathode which can be another piece of metal less electrically reactive than iron or another part of the iron itself.

The more salt is present in the water (seawater, sweat, acid rain) the faster the rusting / corrosion will happen as it will be a better electrolyte.

The result is always the same, rust in reddish or yellowish hues.

Keep your eyes open and you will see rust on a lot of places, it's very nice to observe how rust works and where iron parts will rust.

You do now know why iron corrodes, so try to look out for places where water collects.
You will see that those places will rust much more.
 Photo by Raffa taken in Spain

And by the way, did you ever try to make your own rust / iron oxide pigments for painting rust?
Try out our tutorial, maybe with the knowledge from this article, you can even improve the process?

I hope you enjoyed this episode of Theory Thursday and read you next week!

Here are some more practical exercises:

An overview about Weathering, including theory and practical use.
Raffa explains different weathering techniques in a series of 4 videos.

How to: Rust Effects by ModelMates
This article explains how to apply rust.


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