Tin
by Chris Woodford. Last updated: August 3, 2011.
Have you ever been to Cornwall, that romantic coastal county in the southwest of England? If so, you've probably seen the remains of tin-mine smokestacks poking up from the landscape. Cornwall was one of the world's leading tin producers until the world market collapsed in the mid-1980s, largely due to the switch from using tin in food cans and packages to alternatives such as aluminum and plastics. Despite this, tin remains an important metal in many other products and industrial processes, from welding and soldering to coating roofs and making insecticides. Let's take a closer look!
Photo: A ruined tin mine in Cornwall, England. After the last Cornish tin mine closed in 1998, the industry is now just a romantic memory. Photo by courtesy of Strange Ones, published on Flickr in 2008 under a Creative Commons licence.
What is tin?
Tin is a silvery white metal that lives in group IV of the periodic table of chemical elements. To look at it, you'd never know that it was (according to archaeologists) one of the earliest and most important metals in human civilization!
It was the discovery of how tin and copper could be combined in an alloy called bronze (a much stronger material than either metal alone) that ushered in one of the major eras of civilization: the Bronze Age. The earliest evidence we have of people using tin is in bronze finds from 3000–3500BCE, though it was apparently not widely used as a pure metal until several thousand years later (probably c.600BCE).
Photo: Cats on a hot tin roof. Two US Marine Corps engineers put a new tin roof on a school in Kenya. Photo by CPL Bryant V courtesy of Defense Imagery.
Where does tin come from?
Although we think of tin as an everyday material, it's actually much less common than comparable metals such as copper or zinc (according to the US Geological Survey, copper is over 30 times and zinc about 50 times more common than tin). In terms of abundance, tin is roughly halfway down the list of chemical elements: the 49th most common in Earth's rocky crust, existing in concentrations of about 2 parts per million (0.0002 percent). In other words, if you dig up a tonne of rock, a measly 2 grams of it will be tin!
Photo: A satellite photo of a tin mine in New South Wales, Australia taken from the International Space Station. Photo by courtesy of NASA Johnson Space Center – Earth Sciences and Image Analysis (NASA-JSC-ES&IA).
There are tin deposits right across the world, though most tin now comes from the southern hemisphere—and chiefly from south-east Asia. The most important producer countries are now China, Indonesia, Peru, Bolivia, and Brazil. England (Cornwall) is no longer the important producer it once was and the United States, despite being the world's biggest consumer of tin, has not (according to the US Geological Survey) mined any of the metal since 1993.
Most tin is produced from an ore (raw rocky mineral) called cassiterite, which is turned into tin by smelting. First, the ore is crushed to a powder and washed free of impurities before being heated with carbon (in the form of coal) and limestone in a giant furnace. Other metals, such as iron, copper and zinc, separate out. Molten tin sinks to the bottom of the furnace and is shaped into solid blocks known as ingots. Like most other metals, tin can also be separated or purified using electrolysis (an electrical-chemical process that works in the opposite way to a battery).
World mine production of tin, 2010. Five countries (China, Indonesia, Peru, Bolivia, Brazil, and Congo [Kinshasha]) mine about 95 percent of the world's tin. Source: USGS Mineral Commodity Summaries: Tin, January 2011.
What is tin like?
There are two common forms (allotropes) of tin that look and behave differently because of their quite different internal crystalline structures. One is the familiar, silvery-white form called white tin or beta tin, which has a body-centered tetragonal structure and predominates at everyday temperatures. The other form, gray tin or alpha tin, is powdery, about two thirds as dense, and appears spontaneously at low temperatures. It's less useful because it's weaker and more brittle, with a face-centered cubic crystalline structure. The sudden degradation of white tin into gray tin is called tin pest.
Photo: This low-temperature oxidation catalyst made from tin oxide and platinum is designed to convert carbon monoxide to less harmful carbon dioxide. Photo by CPL Bryant V courtesy of NASA Langley Research Center (NASA-LaRC).
Physical properties
Tin is a typical metal inasmuch as it's extremely malleable (easy to work in many different ways), ductile (easy to draw into wires), and readily forms a grayish protective oxide on its surface, but it's much weaker than metals such as iron so it's not used as a construction material. It has a relatively low melting point (one of the reasons it's used as a component of solder), but a relatively high boiling point, which means it's a liquid over a wide range of temperatures and can be usefully employed as such in a number of industrial processes.
Chemical properties
Tin has a valency (chemical combining power) of either two (II) or four (IV) and accordingly forms two different forms of compounds: tin (II) compounds (which used to be called "stannous") and tin (IV) compounds (formerly called "stannic"). Notable tin compounds include tin (II) chloride used in galvanizing, dyeing, and perfume production; tin (II) fluoride, which provides the fluoride in some toothpastes; and tin (IV) oxide, an industrial catalyst. Compounds of carbon and tin include a number of important insecticides and disinfectants.
