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7b. Thermite (Pyrotechnics)


This article is from the Pyrotechnics FAQ, by Hans Josef Wagemueller zoz@cs.adelaide.edu.au with numerous contributions by others.

7b. Thermite (Pyrotechnics)

The thermite reaction is a redox reaction that produces a lot of heat and
light. In its usual configuration, temperatures can exceed 3000 degrees C,
and molten iron is produced. It is therefore mainly used for welding, and
by the Army in incendiary grenades.

There are many possible configurations - basically it is the reaction
between a reactive metal and the oxide of a less reactive metal. The most
common is as follows:

Aluminium powder, Al (coarse) 1 volume part or 3 weight parts
Iron (III) Oxide, Fe203 1 volume part or 1 weight part

A stoichiometric mixture will provide best results.

The powders are mixed together and ignited with a suitable fuse. Many
people use magnesium ribbon - I don't recommend this, as magnesium ribbon
is not all that easy to light, and quite prone to going out due to oxygen
starvation. A much better fuse for thermite is a common sparkler. The
mixture should be shielded with aluminium foil or similar to prevent
sparks from the sparkler igniting the thermite prematurely.

Reaction:       2Al    +  Fe O     --->  Al O     +  2Fe    +  lots of heat
                   (s)      2 3(s)         2 3(s)       (l)

The mixture can be varied easily, as long as the metal oxide you are
using is of a less reactive metal than the elemental one you are using,
e.g. copper oxide and zinc. Adjust the ratios accordingly.

Safety aspects:

Reaction: Make sure you no longer need whatever you are igniting the
thermite on - the reaction will melt and/or ignite just about
anything. If you ignite the thermite on the ground, make sure
the ground is DRY and free of flammable material. If the ground
is wet a burst of steam may occur, scattering 3000 degree metal

Be careful when igniting the thermite - use adequate shielding
to prevent premature ignition. Don't get close to the mixture
once ignited - it has been known to spark and splatter. Don't
look at the reaction directly. It produces large amounts of
ultraviolet light that can damage the eyes. Use welder's
goggles, 100% UV filter sunglasses or do not look at all.

Aluminium: Chemical dust in the lungs is to be avoided. As always, wear a
dust mask. Make sure the environment you are working in is
dry - aluminium powder can be dangerous when wet. Fine
aluminium dust is pyrophoric - this means it can spontaneously
ignite in air. For this reason aluminium powder with a large
particle size is recommended.

Iron Oxide: This is not directly toxic, but any particulate matter in the
lungs is not good. Again, the dust mask is important.


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