Four basic jobs
Almost every metalworking technique, ancient or modern, does one of four things. It extracts metal from ore (smelting), it pours metal into a shape (casting), it shapes solid metal with force (forging, rolling, drawing), or it changes the metal’s properties by mixing in other elements or by heating and cooling it (alloying and heat treatment). The history of metalworking is mostly the story of people getting better at each of these jobs and combining them.
Dates for the earliest steps are uncertain, and they differ by region. Metalworking was probably invented, or at least independently reinvented, in several places. The sequence below is a general outline, not a single line of descent.
Native metals and hammering
The earliest metal objects were made from metal found in nature in a usable state, mainly native copper and gold, along with small amounts of meteoritic iron. These could be hammered cold into beads, pins and blades. Hammering makes metal harder but also more brittle, so early smiths learned to heat it and let it soften again, a process now called annealing.
Smelting copper and the first alloys
Smelting means heating ore with a source of carbon, usually charcoal, so that the metal is released from its compound. Copper smelting is generally dated to somewhere between the fifth and fourth millennia BCE in parts of the Near East and southeastern Europe, though the exact dates remain debated. Early copper was sometimes alloyed, by accident or design, with arsenic. Later, smiths found that adding tin produced a harder and more castable metal, which led to the bronze alloy and the era named after it.
Casting
Casting lets a craftsperson make a shape that would be very hard to hammer out. Early metalworkers poured molten copper and bronze into open stone molds, then into closed molds made of two halves. The lost-wax method, in which a wax model is covered in clay, melted out and replaced with metal, is very old and is still used today for statues and for jewelry casting. Casting also made it possible to produce many identical pieces, such as axe heads, from one mold.
Iron arrives
Iron was harder to work than copper or bronze. Its melting point (about 1538 degrees Celsius) was beyond what ancient furnaces typically reached. Instead, smiths used a small clay furnace that kept the ore and charcoal hot enough to produce a spongy lump of iron mixed with slag, called a bloom. The bloom had to be reheated and hammered to squeeze out the slag. The bloomery furnace was the standard way to make iron across much of the world for a very long time. Iron became common across the Near East and Mediterranean around the end of the second millennium BCE, although the timing varies by region.
Iron smiths learned that iron containing a little carbon could become steel, which can be hardened by heating and rapidly cooling it (quenching). Controlling that carbon was difficult, and quality varied. Special cases, such as the crucible steel made in India and Central Asia, produced exceptional blades.
Liquid iron and the blast furnace
In China, taller furnaces and strong bellows (sometimes water-powered) allowed iron to be melted and cast, with evidence of cast iron going back well before the common era. In Europe, similar large furnaces appeared much later, in the late Middle Ages. The result, pig iron, was cheap and could be cast, but it was brittle because of its high carbon content. Further refining was needed to make wrought iron or steel. The switch from charcoal to coke fuel in Britain in the early eighteenth century helped expand output, since forests could no longer supply endless charcoal.
Power, rolling and drawing
Water wheels, and later steam engines, changed metalworking by driving hammers, bellows and rollers. Heavy tilt hammers could forge larger pieces. Rolling mills squeezed hot or cold metal between rollers to make flat sheet, bars and eventually rails. Wire was made by pulling metal through a tapered hole; by the late Middle Ages, water power was used to turn the drawing wheels in some European workshops. These forming methods supported railroads, bridges, machinery and household goods.
Cheap steel
Until the middle of the nineteenth century, steel was expensive and made in small batches. Blowing air through molten iron, an idea associated with Henry Bessemer in the 1850s, turned steelmaking into a large-scale industry within a few decades. Open-hearth furnaces and, much later, oxygen-based converters took over in the twentieth century. Cheap steel fed everything from rail networks to skyscrapers and machine tools.
New metals and alloys
The nineteenth and twentieth centuries added metals that older furnaces could not produce. Aluminum, a costly novelty in the mid-nineteenth century, became inexpensive after electrolysis was developed in the 1880s. Chromium-bearing steels led to stainless steel in the early twentieth century. Metallurgists also developed alloys tuned for particular jobs, such as heat-resistant alloys for engines.
Modern shaping
Today, metal is shaped by forging, casting, machining, extruding, stamping and welding. Newer additive manufacturing methods build parts layer by layer, and metal powder printing can produce complicated shapes that would be hard to cast or machine. Even so, the core ideas, heating ore with carbon, pouring into molds, and working solid metal with force, go back thousands of years.