Two ways to change a shape
Metal’s most useful property for a maker is that it can be deformed without breaking, up to a point. Two forming methods grew out of that: squeezing it between rollers to make it flatter and longer (rolling), and pulling it through a tapered hole to make it thinner and longer (drawing). Both work the metal while it is hot or cold, and both change its internal structure, often making it harder as it is worked.
Early wire
The earliest wire was probably made without drawing. Craftspeople in the ancient world cut thin strips from hammered sheet, or hammered gold rods, and twisted or rolled them into thread-like forms for jewelry. Drawn wire, made by pulling a rod through a hole in a hard plate, appears to be a later development. Exact dates are uncertain, but drawn wire was in use in the Mediterranean world and Asia well before the Middle Ages, particularly for jewelry and decorative thread.
The draw plate
The key tool is the draw plate, a piece of hard metal pierced with a series of tapered holes of decreasing size. A smith pointed the end of a rod, pushed it through the largest hole, gripped it with pliers or tongs and pulled. Repeating the process through ever smaller holes produced finer wire. Because drawing hardens the metal, wire often had to be reheated and softened (annealed) between passes to prevent cracking.
For fine work, such as gold and silver thread used in embroidery, hand drawing remained common for centuries. For heavier iron wire, hand pulling was slow and tiring.
Power arrives
By the late Middle Ages, workshops in parts of Europe, including German cities such as Nuremberg, used water wheels to drive drawing machines. Water power gave stronger and steadier pulling than people could, making it practical to produce long lengths of iron and steel wire. The demand was considerable. Wire went into chain mail in earlier eras, and later into needles, pins, cards used to comb wool, fence wire, nails and strings for musical instruments. Piano makers, for example, pushed for stronger steel wire.
In the nineteenth century, wire took on new roles. The rise of the electric telegraph required thousands of miles of iron wire for lines, and copper wire later carried electric power and telephone signals. Cables for suspension bridges were built from many strands of steel wire. Later still, the filament inside incandescent bulbs and the cord used in countless household devices relied on drawn wire.
Sheet and bar: the rolling mill
Early sheet metal was made by hammering. Rolling mills replaced much of that labor. A rolling mill passes metal between two heavy cylinders, called rolls, that turn in opposite directions and squeeze the metal thinner. A sketch from Leonardo da Vinci’s notebooks shows a rolling device, and rolling mills for soft metals such as lead, gold and silver were in use in Europe by the sixteenth century. These were often used, for example, to flatten metal for coins.
Rolling hot iron was a bigger task. Slitting mills, which used rolls with cutting edges to slice flat iron into narrow rods, appeared in Europe in the sixteenth and seventeenth centuries and supplied nail makers. Their output of nail rod fed a huge household trade.
Industrial rolling
Rolling advanced rapidly during the Industrial Revolution. Henry Cort, in Britain in the 1780s, patented the use of grooved rolls for shaping hot wrought iron into bars, a step that made iron production much faster than hammering. Grooved rolls could turn out bars, angles and eventually rails. Powered first by water wheels and then by steam engines, mills could shape long lengths of consistent metal. Rails for railroads were one of the largest products of rolling mills, and large steel plate later served for ships and boilers.
Later developments included:
- Cold rolling, which produces smooth, thin, hard sheet suitable for items such as tinplate for cans and body panels for cars.
- Continuous mills, in which metal passes through a series of roll stands in sequence without stopping.
- Mechanical drawing machines, which pull wire through multiple dies in a row and can run at high speed.
Dies, lubricants and limits
Modern drawing dies are made from very hard materials such as tungsten carbide and diamond for the finest wire. Lubricants reduce friction and heat. Both rolling and drawing have limits: too much reduction in one pass can crack the metal, which is why multiple passes and intermediate annealing are standard.
Why it matters
Rolling and drawing turned metal from something that smiths shaped one piece at a time into a material available in standard forms, such as plates, bars, rails and wire, that other industries could build on.