Wood's metal
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Wood's metal, named for American metallurgist B. Wood, is also known as Lipowitz's alloy or by the commercial names cerrobend, bendalloy or pewtalloy. It is a fusible alloy with a melting point of approximately 70 °C (158 °F). It is a eutectic alloy of 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight.[1]
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[edit] Applications
Wood's metal is useful as a low-melting solder, low-temperature casting metal, high temperature coupling fluid in heat baths, and as a fire-melted valve element in fire sprinkler systems in buildings. Medical gas cylinders in the United Kingdom have a Wood's metal seal which melts in fire, allowing the gas to escape and reducing the risk of gas explosion.
A common use is as a filler when bending thin-walled metal tubes. For this use the tubing is filled with molten Wood's metal. After the filler solidifies the tubing is bent, and the filler prevents collapse of the tubing. The Wood's metal is then removed by heating, often by boiling in water.
Other uses include making custom-shaped apertures and blocks (for example, electron-beam cutouts and lung blocks) for medical radiation treatment, and making metal inlays in wood.
Wood's metal is also useful for repairing antiques. For example, a bent piece of sheet metal may be repaired by casting a Wood's metal die from a good example. The low melting temperature of Wood's metal makes it unlikely this will harm the original. The damaged piece can then be clamped in the die and slowly tightened to form it back into shape.
[edit] Toxicity
Wood's metal is toxic because it contains lead and cadmium, and therefore contact with the bare skin is thought to be harmful, especially in the molten state. Vapour from cadmium-containing alloys is also known to pose a danger to humans. Cadmium poisoning carries the risk of respiratory, liver, kidney, and bone damage, loss of sense of smell, and cancer. Field's metal is its non-toxic alternative.
[edit] Related alloys
A similar alloy is called cerrosafe. This non-eutectic alloy melts at 165 °F (74 °C). It is mainly used by gunsmiths for making a reference casting of the chamber of a firearm. When it solidifies it first shrinks, allowing easy removal from the chamber. When it cools it expands back to the exact size of the chamber. The casting can then be directly measured with calipers or a micrometer to determine the dimension of the weapon's chamber, which is important for safety.
The eutectic alloys cerrolow 136 and cerrolow 117 have even lower melting temperatures of 136 °F (58 °C) and 117 °F (47 °C) respectively.
Their compositions are as follows:
| Alloy | Melting point | Eutectic? | Tin | Bismuth | Lead | Cadmium | Indium | Thalium |
|---|---|---|---|---|---|---|---|---|
| Roses metal | 98 °C (208 °F) | no | 25% | 50% | 25 % | – | – | - |
| Wood's metal | 70 °C (158 °F) | yes | 13.3% | 50% | 26.7% | 10% | – | – |
| Field's metal | 62 °C (144 °F) | yes | 16.5% | 32.5% | – | – | 51% | – |
| Cerrosafe | 74 °C (165 °F) | no | 11.3% | 49% | 37.7% | 8.5% | – | – |
| Cerrolow 136 | 58 °C (136 °F) | yes | 12% | 49% | 18% | – | 21% | – |
| Cerrolow 117 | 47.2 °C (117 °F) | yes | 8.3% | 44.7% | 22.6% | 5.3% | 19.1% | – |
| Bi-Pb-Sn-Cd-In-Tl | 41.5 °C (107 °F) | yes | 10.7% | 40.3 % | 22.2% | 8.1% | 17.7 % | 1.1 % |
Other low-melting solders and fusible alloys include Field's metal, Rose metal, Galinstan, NaK, and alloys of indium and bismuth.
[edit] References
- Birchon's Dictionary of Metallurgy, London, 1965
- Experimental techniques in low-temperature physics, G. K. White, Oxford University Press, Third Edition
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[edit] External links
- Making your own low-melting point eutectic: Science Toys: A metal that melts in hot water
