Given its importance in infrastructure and manufacturing-worldwide demand for it keeps increasing-steel is a crucial part of everyday life. (For example: A given car will be sixty-six percent steel. The industry itself, as well as the scrap-metal recycling industry it in turn gives rise to, is a major employer, and one of the engines of economic development the world over. It accounts for up to sixty-five percent of the typical household appliance.) Yet many don't realize that there are, in fact, several kinds of steel, each having its own particular molecular arrangements-and hence its own particular qualities and strengths.

Steel has been made for at least three thousand years-bits of it dating from 1400 BCE, or thereabouts, have been found in East African sites. Chemically speaking, it's an alloy-a material made up from two or more elements (materials made up of only one kind of atom), one of which elements has to be, in this case, iron. The temperatures needed to actuate the steel making process are pretty high-in excess of 1370 degrees Celsius-but methods for reaching these temperatures have existed for, it seems, more than six thousand years. What makes steelmaking a more subtle art is the fact that the processes by which it's manufactured have an influence over which elements mix-and to what degree-with steel as it's being made, and the smallest changes in manufacturing method can thus lead to a steel with a very different molecular shape-and very different physical properties-than others.

Today, steel may be made via a process called oxygen steelmaking. It involves molten iron being poured into a heatproofed container called a ladle, then dumped into a furnace, in which a precise ratio of old scrap steel is already inserted (to maintain certain chemical balances that catalyze the reactions desired). Nearly-pure oxygen is blown into the steel and iron, which (a) raises the temperature inside the furnace, (b) burns away the carbon and (c) purifies the steel. (Earlier methods used air rather than pure oxygen, but this didn't allow for the same precision-air is, after all, a compound, in which elements like nitrogen mix with oxygen.) Other chemical cleaning agents are introduced later to create slag, a material that forms on the surface of the molten steel to absorb chemical impurities. Slag is good-it drains impurities from the rest of the steel.

At this point the furnace may be emptied (tapped) into a giant ladle, where it is further refined-and this is where the different varieties of steel may be produced. Here it is alloyed-mixed-with other materials, each of which creates a different molecular arrangement with the steel and, thus, creates different properties in the steel itself.

Most modern steel tends to be carbon steel, made wholly from iron and carbon. Once in a while, though, to create a somewhat stronger steel, manganese (or other elements) may be added. This kind of steel-high strength low alloy steel-is more expensive but, as stated, stronger. Stainless steel-named not because it is stainless but because it stains less than other sorts of steel-is created by introducing chromium and sometimes a bit of nickel. (Officially, this corrosion-resistant metal must be made of at least eleven-and-a-half-percent chromium.)

Or let's say a steel tool that is heat-resistant is needed-even to temperatures beyond those at which steel typically melts. For that, steel leavened with cobalt or tungsten may be necessary, which creates a tough steel called tool steel. The implements in a basement workshop are most likely made from tool steel; it's often used in drills, axes, and anything else that needs a cutting edge that won't dull.

There are many other ways to alloy steel-these are just some of the more common. In any case, steel isn't the name for just one kind of thing-it's an umbrella term for many kinds of iron-based alloys that see every imaginable kind of use.