Aluminium reacts with iron oxides, particularly ferric oxide, in highly exothermic reactions, reducing the iron oxides to free iron, and forming a slag of aluminium oxide.

3Fe₃O₄+ 8Al = 4Al₂O₃+ 9Fe (3088°C, 719.3kCal↑)

3FeO + 2Al ⇒ Al₂O₃ + 3Fe (2500°C, 187.1kCal↑)

Fe₂O₃ + 2Al ⇒ Al₂O₃ + 2Fe (2960°C, 181.5kCal↑)

The various iron oxides are used in appropriate proportions so as to get the correct resultant quantity and temperature of molten steel. Approximately equal quantities of molten steel and liquid aluminium oxide are separated at about 2400°C, after a few seconds of the exothermic reaction. The iron obtained from such a reaction is soft and unusable as a weld metal for joining rails. To produce an alloy of the correct composition, alloys like ferro-manganese are added to the mixture along with pieces of mild steel, both as small particles, to allow rapid dissolution in the molten iron, to control the temperature and to increase the "metal recovery". Complete slag separation in a short time and better fluidity of the molten metal is achieved by adding compounds like calcium carbonate and fluorspar, etc.

Pre-heating the rail ends (to about 1000°C) is required to help the poured molten metal in washing away the surface oxidation on the rail ends, as otherwise, the molten metal may chill and solidify immediately on coming in contact with cold rail ends, without washing off the surface oxidation.

The procedure of "controlled localized reaction" to keep the thermit mixture ignition under control was invented by Dr Goldschmidt and hence the process is sometimes also known as the Goldschmidt process. Individually patented processes have led to different trade names such as "Thermit", "Boutte", "Argothem", etc.

Thermit material is a mechanical mixture of metallic aluminum and processed iron oxide.

Molten steel is produced by the reaction in a magnesite-lined crucible.

At the bottom of the crucible, a magnesite stone is burned, into which a magnesite stone thimble is fitted.

This thimble provides a passage through which the molten steel is discharged into the mold.

The hole through the thimble is plugged with a tapping pin, which is covered with a fire-resistant washer and refractory sand.

The crucible is charged by placing the correct quantity of thoroughly mixed material in it.

In preparing the joint for welding, the parts to be welded must be cleaned, alined, and held firmly in place.

If necessary, metal is removed from the joint to permit a free flow of the metal into the joint.

A wax pattern is then made around the joint in the size and shape of the intended weld.

A mold made of refractory sand is built around the wax pattern and joint to hold the molten metal after it is poured.

The sand mold is then heated to melt out the wax and dry the mold.

The mold should be properly vented to permit the escape of gases and to allow the proper distribution of the metal at the joint.

Thermit Welding (GB) Ltd.

87 Ferry Lane, Rainham

Essex, RM13 9YH

England

Telephone: +44 1708 522626

Facsimile: +44 1708 553806

Email: thermitwelding.demon.co.uk

Ignition of a thermite reaction normally requires supervision by a trained technician, and may require persistent efforts, as ignition can be unreliable and unpredictable. Thermite reactions require very high temperatures for initiation. These temperatures cannot be reached with conventional black powder fuses, nitrocellulose rods, detonators, a suitable pyrotechnic initiator, or other common igniting substances. Even when the thermite is hot enough to glow bright red, it will not ignite as it must be at or near white-hot to initiate the reaction. It is possible to start the reaction using a propane torch if done correctly. The torch can preheat the entire pile of thermite which will make it explode instead of burning slowly when it finally reaches ignition temperature.

Often, strips of magnesium metal are used as fuses. Because metals burn without releasing cooling gases, they can potentially burn at extremely high temperatures. Reactive metals such as magnesium can easily reach temperatures sufficiently high for thermite ignition. However, this method is notoriously unreliable: Magnesium itself is difficult to ignite, and in windy or wet conditions the strip may be extinguished. Also, magnesium strips do not contain their own source of oxygen so combustion cannot occur unless the magnesium strips are exposed to air. A significant danger of magnesium ignition is the fact that the metal is an excellent conductor of heat; heating one end of the ribbon may cause the other end to transfer enough heat to the thermite to cause premature ignition. Despite these issues, magnesium ignition remains popular amongst amateur thermite users, mainly because it can be easily obtained.

The reaction between potassium permanganate and glycerine or ethylene glycol is used as an alternative to the magnesium method. When these two substances mix, a spontaneous reaction will begin, slowly increasing the temperature of the mixture until flames are produced. The heat released by the oxidation of glycerine is sufficient to initiate a thermite reaction. However, this method can also be unreliable and the delay between mixing and ignition can vary greatly due to factors such as particle size and ambient temperature.

Apart from magnesium ignition, some amateurs also choose to use sparklers to ignite the thermite mixture. These reach the necessary temperatures and provide enough time before the burning point reaches the sample. However, this can be a dangerous method, as the iron sparks, like the magnesium strips, burn at thousands of degrees and can ignite the thermite even though the sparkler itself is not in contact with it. This is especially dangerous with finely powdered thermite.

Similarly, finely-powdered thermite can be ignited by a regular flint spark lighter, as the sparks are burning metal (in this case, the highly-reactive rare-earth metals lanthanum and cerium). Therefore it is unsafe to strike a lighter close to thermite.

A stoichiometric mixture of finely powdered iron(III) oxide and aluminium may be ignited using ordinary red-tipped book matches by partially embedding one match head in the mixture, and igniting that match head with another match, preferably held with tongs in gloves to prevent flash burns.