We continue to review some of the most important materials in heat treatment and metallurgy.
Lead (chemical Symbol: Pb)
Lead is a soft, malleable, ductile, bluish-white, dense metallic element, extracted chiefly from the ore referred to as galena (Fig. 1). Galena is a combination of lead and silver, which was originally mined for the silver content of the ore. Lead is also found in ores combined with zinc and copper.
Lead is one of several elements that were known from ancient times and, therefore, was never formally discovered in modern times. The oldest lead artifact is thought to be an object discovered by archaeologists in a cave in Israel (the Ashalim cave) that prehistoric people used as a burial ground around 6,000 years ago. The object, thought to be a mace or a spindle whorl, is disk-shaped with a hole in it (Fig. 2). It was found with a 1.5-inch-long wooden stick inserted into it. The stick was in amazingly good condition, probably due to the dry conditions in the Negev Desert where it was discovered.
Lead is malleable and corrosion-resistant. As many of us may recall, the Romans used lead for water pipes and lead-based paints were used for many years (both before the understanding of the environmental hazards associated with lead). The word "plumbing" is related to the Latin word for lead, which is "plumbum" and also explains its atomic symbol, Pb. The reason for lead's widespread use in ancient times is its low melting point and unusual softness, which makes it much easier to work than other metals. The Romans also used lead in aqueducts, tank linings and cooking pots among other uses.
Lead is the heaviest nonradioactive element and extremely dense. This property makes it valuable as a barrier against X-ray and gamma-ray radiation, and it is used in X-ray machines and nuclear reactors. Due to its corrosion-resistance, lead is useful as a covering on wires and cables and in vessels that contain sulfuric acid. It is also used to absorb vibration and sounds. In the manufacture of ammunition, lead's density makes it ideal as a bullet point, and its malleability makes it deadly as it mushrooms into a larger shape upon impact (Fig. 3). Most of the lead used today is for the production of lead-acid batteries for automotive and other applications.
Lead is alloyed with tin to make solder – a material with a low melting point – widely used to join electrical components, pipes, heat-exchanger components and other metallic items. Another alloy of lead, tin and antimony referred to as "type metal" is used to make the type used in printing presses and plates. Babbit metal, another lead alloy, is used to reduce friction in bearings.
Lead is also added to steels to improve their machinability. Here are a few scientific and engineering facts about lead.
- Atomic number: 82
- Atomic weight: 207.2
- Melting point: 600.61 K (327.46°C or 621.43°F)
- Boiling point: 2022 K (1749°C or 3180°F)
- Density: 11.342 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal