Reduction and oxidization

This post is from my series on building a backyard foundry.

We all have a basic understanding of “oxidization” I think: when a metal like iron is exposed to oxygen, either in the air, or dissolved in water, the metallic iron turns into iron oxide, which has quite different properties. In particular, iron oxide is brittle, flaky, and expands away from the underlying metal, which means that oxidization destroys the metal. Copper produces a green oxide. Aluminum oxide does not flake off; it actually forms a protective “passive” layer on top of the aluminum protecting it from further oxidation. Same with the chromium that is in stainless steel; the surface is actually chromium oxide, which does not flake off. 

That chemical reaction can of course be driven in the other direction; it is possible to turn an oxide back into a pure metal by a variety of means. My favourite technique for doing so is to put a tarnished silver object loosely wrapped in aluminum foil in a plastic tub full of boiling water with baking soda dissolved in it. The silver oxide is reduced to metallic silver, and the aluminum is aggressively oxidized far more than it would be when exposed to gaseous oxygen. Essentially the oxygen is moving from the surface of the silver to the surface of the aluminum. The cheap aluminum can then be discarded. This technique is not only less work than polishing by hand, it saves the silver. (Conventional silver polish is simply a chemical which dissolves and removes the silver oxide, destroying the silver.)

Now, that description of polishing silver with chemistry should be making you scratch your head (provided that you do not remember your grade twelve chemistry): what’s the baking soda for?

It’s to help move electrons around; it make the water more conductive.

But wait, what do electrons have to do with it?

Oxidation is in these specific cases the reaction of a metal with oxygen to form an oxide; 18th century chemists knew that, and hence gave it the name “oxidation”. And those same 18th century chemists also observed that when oxidized metals were turned by chemical means back into pure metals, they appeared to lose mass, and were hence “reduced”. Of course the “lost” mass was nothing more than the oxygen leaving the metal and going into solution or into the atmosphere, which those same clever chemists did manage to work out for themselves as well.

We now know that the hallmark of an oxidation is that the oxidized substance must give up some electrons in order to bond with the oxygen, and the reduced substance must accept those electrons. The key fact for the chemist to understand is that the number of electrons is always conserved. If the oxidized substance is giving up, say, two electrons per oxidized molecule then some reduced substance must be accepting those two electrons. There is no oxidation without reduction. (*)

I said last time that what I wanted to talk about the air blast into the charcoal furnace. A charcoal fire is essentially oxidation of the fuel at a tremendous rate, and the thing oxidizing the fuel is in fact oxygen.

Suppose the air blast is set so high that the rate of oxygen arriving is outstripping the speed at which the oxidation reaction can occur. What characteristics will such a fire have?

First off, it will be incredibly hot; the heat-producing chemical reactions are happening as fast as they possibly can.  Second, the atmosphere, despite having been reduced enormously by the oxidation of the fuel (remember, if something is oxidized then something else has to be reduced, and that would be the atmosphere) there will still be oxygen available for more oxidation of something else. Such as the metallic body of the crucible itself, or the metal in the melt. Third, there will be a strong flow of air out the chimney, carrying the heat with it. Fourth, the exhaust gasses will be extremely hot and quite “clean” for a charcoal fire, because more of the combustion will be “complete” combustion.

Now suppose the rate of oxygen arriving is just barely enough to run the oxidation reaction, or even slightly less. What characteristics does such a fire have?

It will be less hot because the chemical reactions are not running at peak speed. The atmosphere will be entirely reduced, leaving no oxygen to oxidize the crucible or melt. In fact, the atmosphere could be so reducing that it starts pulling oxygen out of the crucible and melt, unrusting them. The airflow will be lessened, and the smoke is likely to be dirtier.

What I think will work, and I’ve been trying to do, is to run the furnace so that the air blast produces a reducing environment until the metal melts. Once I’ve melted as much as I want, I turn the air blast up to rapidly increase the temperature from the melting temperature of 1220F to the pouring temperature of 1400F.

When it achieves this temperature the melt should be glowing with a red heat, and it will not stick to a steel rod used to stir the melt.

Lacking a pyrometer, it’s going to take some practice to figure out exactly what the right moment to pour is. I’ll continue to report my results as I practice.


(*) There can be oxidation without oxygen; perhaps the oxidized substance is giving up its electrons in order to get together with sulphur, rather than oxygen.

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