All about thermite
What is thermite?
Thermite is an easy-to-make compound that creates an explosive exothermic reaction, producing a brief burst of extremely high temperature – enough to weld metal, explode steam, and even burn through concrete. Thermite, typically made from aluminum and iron oxide (rust), differs from gunpowder which is composed of sulfur, charcoal, and potassium nitrate (saltpeter). Although similar in form of action, gunpowder does not burn nearly as hot as thermite.
Chemically, thermite requires two components with specific characteristics. One component, a metal such as aluminum, must have a chemical makeup with stable bonds. The second component, a metal oxide such as iron oxide (rust), must have less stable bonds. The metal (e.g., aluminum) acts as a reducing agent to extract metal (iron in this example) from the metal oxide (e.g., iron oxide). During this reaction, aluminum captures oxygen atoms from the iron oxide to become an oxide (aluminum oxide) while the metal oxide loses oxygen atoms to form a pure metal (iron).
Aluminum + Iron Oxide -> Aluminum Oxide + Iron
Ultimately the aluminum and iron oxide are converted to aluminum oxide and iron. The reaction is simple and indeed, thermites are characterized by an almost complete absence of gas production. The reaction that occurs ensures the resulting aluminum oxide and iron products will emerge as molten liquids with temperatures over 4,500 degrees Fahrenheit.
During the reaction, a tremendous amount of heat is released. A half-pound of thermite placed on the hood of your car would melt through the hood, engine block, and concrete below. This enormous burst of heat can be used to weld, cut through near impenetrable materials, or create an explosive action from the rapid expansion of other materials (typically water).
Ignition of thermite
Thermite must be ignited to start the reaction. Thermite ignition requires a high temperature but once ignited, the reaction generates enough heat to continue the reaction until all materials have acted.
Black powder fuses, detonators, or pyrotechnic initiators typically do not generate enough heat to ignite thermite. Even a propane torch will only work if done properly (and any ignition source that requires placing your hand close to the thermite when it ignites will be dangerous).
Magnesium ribbon is the most popular and safest method to ignite thermites. Magnesium ribbon can be placed, lit, and will burn like a fuse. It can be purchased from science supply shops or online. Similarly, a fireworks sparkler can also be used to ignite thermite.
A chemical reaction, such as a mixture of potassium permanganate and glycerol, may be used to ignite thermite. Potassium Permanganate is a filter cleaner and dye and can be found in many hardware stores. When potassium permanganate and glycerol are mixed, the reaction will begin as a slow burn before bursting into a white-hot flame hot enough to ignite thermite. An example of an ideal thermite mixture ignited with potassium permanganate/glycerol would be:
55g iron(iii) + 15g aluminum powder + 25g potassium permanganate + and 6ml glycerin
Potassium chlorate and sugar will work too but may burn too quickly. A layer of potassium chlorate and sugar can be placed around and on top of the thermite mixture. The reaction can be initiated by placing a drop of concentrated sulfuric acid on the starter mixture.
An ideal thermite reaction
When thermite burns optimally, the reaction will be hot enough to melt both the metal and the fuel oxide. If the temperature does not reach its maximum hotness, the resulting product will be sintered metal and slag. If the temperature is too high (above the boiling point of the reactant of the products that are produced), gas can be produced resulting in a low-yield explosion.
The burn rate of thermite can be adjusted by varying the particle size of the reactant metal (e.g., aluminum). Since coarser particles burn slower than finer particles, the finer the reactant metal, the more reactive the metal. For example, extremely fine aluminum powder can spontaneously combust simply when exposed to air.
The thermite recipe can also be adjusted by including additional “binders” in the recipe. Binders can be used to alter the chemical composition for a variety of purposes.
Cautionary note on properties of thermite
Thermite contains its own oxygen (the “oxide” of a metal oxide) which of course, is required for a burn. As a result, thermite does not need air to burn. This means thermite cannot be smothered, blown out, or watered to extinguish the reaction. In fact, thermite will even burn underwater (it is often used for underwater welding).
How to make thermite
The traditional aluminum/iron oxide “thermite” recipe
Most thermite is composed of aluminum and rust (iron oxide) to form iron thermite. Aluminum acts as the reactive metal and iron oxide acts as the oxidizer, the oxygen-rich component needed to produce an explosive reaction. Preferably the oxidizer used is iron(iii) oxide but iron (ii,iii) oxide will work too. Iron(iii) oxide produces more heat while iron(ii,iii) is easier to ignite. If iron(ii,iii) must be used, copper and manganese oxides can be added to make it easier to ignite (called a “first fire mix”)
To make thermite, first, make sure the aluminum and iron oxide components are ground into fine particles.
Make a mixture of aluminum and iron oxide in a strong container. Use a ratio of about 3 parts iron oxide and 2 parts aluminum powder.
2 parts Aluminum + 3 parts iron oxide = iron thermite
A clay flowerpot works well as a container but may shatter during the reaction. Cast iron containers work too but will likely melt. Placing the container between two bricks or into a hole dug into the ground will help guard against potential debris.
The ratio of components can be adjusted to increase or decrease heat as well as the amount of molten iron that is produced from the reaction. For instance, a ratio of 1-part aluminum and 3 parts iron oxide will work (e.g., 2 grams aluminum and 6 grams iron oxide).
Insert an ignition source (e.g., magnesium strip) into the mixture and light the strip to start the thermite reaction. Get *far* away and preferably behind a protective barrier in case molten debris is ejected.
Alternative thermite recipes – substitutes for aluminum
Reactive metal substitutes for aluminum
Aluminum is the ideal reactive for thermite. It is easy to obtain, cheap, and safer to handle than most other reactive metals. Its low melting point allows the thermite reaction to occur quickly while its high boiling point allows the reaction to produce extremely high temperatures without generating explosive gaseous byproducts. However, reactive metals can be substituted for aluminum, including lithium, sodium, potassium, calcium, magnesium, and carbon.
Alternative thermite recipes – substitutes for iron oxide
Metal oxide substitutes for iron oxide (rust)
Other metals can be substituted for iron oxide and act as the oxidizer in the thermite reaction. Ideal metal oxide substitutes should contain at least 25% oxygen, be high density, and have a low heat of formation to produce a metal with low melting and high boiling points (so the energy released is not consumed by the resulting metal).
Aluminum and copper oxide produce a copper thermite reaction which generates enough heat to weld electric joints (in a process called cadwelding). Since the melting point of copper is relatively low, the reaction may be extremely violent – whereas traditional thermite burns in a flash, copper thermite may explode. Copper(i) oxide or copper(ii) oxide are commonly used.
1 Aluminum + 4 parts Copper(ii) oxide = Copper Thermite
Silicon dioxide and aluminum form silicon thermite. A common source for silicon dioxide is sand. Sand can be ground down with a mortar and pestle and mixed with aluminum to create silicon thermite. The result of the silicon thermite reaction is aluminum oxide, aluminum sulfide (slag), and pure silicon metal.
Silicon Dioxide thermite is difficult to ignite. Sulfur can be added to assist.
10 parts Aluminum + 9 parts Sulfur dioxide + 12 parts Sulfur
Dry ice thermite
Dry ice (frozen carbon dioxide) and aluminum form the same chemical reaction as traditional thermite. Called cryo-thermite, the reaction can be ignited with a simple flame. If finely divided and confined in an enclosed pipe, cryo-thermite can form diamond particles.
A salt-based oxidizer (e.g., barium nitrate or peroxides) can be substituted for iron oxide used in the traditional thermite reaction. This recipe is easier to ignite and produces a gas that will spew molten slag. This type of thermite is called thermate.
Thermate is better suited for incendiary purposes and explosive devices. It is commonly used by the military as a device to destroy sensitive equipment.
A typical and highly dangerous military-grade thermate recipe requires 69% thermite, 29% barium nitrate, 2% sulfur, and a .3% binder such as PBAN (polybutadiene acrylonitrile).
69% thermite + 29% barium nitrate + 2% sulfur + .3% binder = thermate
This mixture requires a lower temperature to ignite and produces higher heat than the traditional thermite recipe.
First Fire thermite
A First Fire Mix is a recipe that provides easier ignition than traditional thermite. A first fire mix can be lit with traditional ignition methods such as fuses, matches, and electrical igniters.
5 parts Potassium Nitrate (KNO3) + 3 parts Aluminum + 2 parts Sulfur
Potassium Nitrate can be obtained from common stump remover products.
Other thermite variations
Other thermite variations are possible but not as commonly used. Chromium oxide and aluminum oxide form chromium thermite. Manganese and aluminum form manganese thermite. Both lead and zinc have very low melting points and thus can be added to thermite recipes to produce powerful explosions.
How to make “hard”, solid thermite
Thermite can be molded into a solid to form hard thermite. The thermite gel, formed with thermite and plaster of paris (or play dough) can be placed into any type of mold, including bottles or cans, to shape hard thermite as needed. To create hard thermite, follow the traditional thermite recipe but mix with a ratio of 3 parts iron oxide, 2 parts aluminum, and 2 parts plaster of paris.
2 parts aluminum + 3 parts iron oxide + 2 parts plaster of paris = hard thermite
Obtaining materials needed to make thermite
Iron oxide (rust) can be purchased from the paint department of a hardware store, pottery stores, or created from common iron. Steel wool is an excellent source of iron. Use pure steel wool without soaps or other cleaning/degreaser additives. The best steel wool will be labelled as “0000”.
Tear apart the steel and grind if possible. Rubbing steel wool over a metal screen can help “powder” the steel wool into very fine iron particles. Steel wool can also be powdered in a ball mill (e.g., rock tumbler) or with a mortar and pestle.
Exposing stool wool to heat will produce a dark substance – this is iron oxide. Alternatively, you can soak steel wool in a 1:1 mixture of bleach and vinegar for a day to create an iron oxide paste that can then be dried to extract iron oxide.
Iron oxide can also be made by electrolyzing a metal steel rod (e.g., a nail). A DC power source should be used and can be obtained from transformers in toys, phone chargers, and DC battery chargers.
In a well-ventilated area, fill a jar with water. Using a DC power source, wrap the positive lead around the rod. Place the rod and negative end of the DC power source into the jar of water. Add a teaspoon of salt to the water to ensure it conducts well. Let sit overnight.
Note that the process produces chlorine gas and thus, should only be done outdoors or in a well-ventilated area.
The dark substance in the jar is iron oxide. It can be filtered using traditional filtering methods. For a more concentrated solution, pour off some of the water, add more water, and repeat the electrolytic process.
The final iron oxide “paste” can be heated to dry.
Aluminum foil can be ground using a coffee grinder to make flakes suitable for thermite. The powder in an Etch-a-Sketch toy is pure aluminum powder. Aluminum can also be ground from solid aluminum bars.
For a hotter, more explosive ignition, aluminum powder can be purchased from hobby shops or online. Note that aluminum powder’s “mesh” value specifies its fineness. 325 mesh or finer works well in thermite recipes.
Uses of thermite
Thermite can be used for thermite welding (i.e., the Goldschmidt process). Railroads use the Goldschmidt process to join rail tracks together. Since thermite does not require air to burn (it contains its own oxygen), it can be used for underwater welding. Thermite welding works especially well because molten iron produced from the reaction can seep into cracks in metal that would otherwise be difficult to reach using a typical welding method.
Generation of elemental forms of metal
Thermite recipes can be used to extract pure metal from its oxide form. In a traditional thermite reaction using aluminum and iron oxide (rust), elemental iron is generated. Similar results occur with other types of metal oxides. For instance, copper thermite reactions (using aluminum and copper oxide) produce pure elemental copper.
Other uses for thermite
Thermite can also be used to cut through solid metal containers, as incendiary weapons, as explosive devices in demolitions, and as a pyrotechnic signaling device.
Due to the high temperatures created and the difficulty of smothering a thermite reaction, thermite can be especially dangerous. The white-hot molten material produced can be ejected during the reaction and will melt through most containers.
Depending on the materials used in the thermite recipe, dangerous gases can be released.
If thermite is preheated, for instance, a new batch is poured over a hot, recently ignited batch of thermite, it can ignite accidentally at a much lower temperature.
Especially dangerous is pouring water over a thermite reaction (or including damp, wet materials in the recipe). Superheated water can easily produce a powerful and dangerous steam explosion.