Materials and forging
As mentioned above, the earliest springs, from the Roman Era on, were made of cold-hammered iron. Without iron, there would be no springs. The Etruscans began manufacturing iron in blast furnace in Europe about 3,000 years ago. Initially the iron was used for weapons, but later it was put to more everyday use in tools for farming and in buildings (hinges, window grates, locks and keys, fittings, etc.).
Blast furnaces
Expertise on manufacturing iron passed from the Etruscans to the Romans and Greeks. In the 7th century BC, the knowledge also spread north, coming into use in Scandinavia 400–500 years later.
A blast furnace was built directly on the ground, and was used to produce malleable iron using one or two bellows that were operated by hand or by foot. The furnaces were fired with charcoal, and a variety of substances were used for the ore, including red earth, limonite and bog ore. Blast furnaces were common throughout Sweden and were used well into the 19th century. They produced Osmund iron, a ball of pig iron weighing just under 300 g, which were sold individually or by the barrel. Osmund iron was usually then made into bar iron. In the 16th century, during the Vasa Renaissance, there were three types of iron available for smiths to use, whether guild smiths, locksmiths or farmer-blacksmiths – bloom iron, Osmund iron, pig iron and bar iron.
Steel from blast furnaces
In the 13th century, Sweden began mining iron ore and using puddling furnaces to produce iron. These new puddling furnaces were housed in buildings – foundries. A puddling furnace was designed to smelt iron ore. The furnace consisted of a bricked pipe that was open at the top and surrounded by timber walls. The space between the pipe and the walls was filled with mud and stone. At the bottom were openings for removal of slag and ore, and for blowing in the air using water-powered bellows. The bottom of the pipe was filled with charcoal, followed by a blend of crushed and roasted ore and more charcoal. Crushed limestone was also used to make the slag more fluid. Once the furnace was lit, air was provided through the bellows, increasing the heat and speeding up combustion. The oxygen in the ore was reduced when it merged with the carbon and gases and rose out of the pipe. Some of the carbon dissolved into the iron (making up about 4% of the content), lowering the melting point of the metal. The smelt iron collected at the bottom of the pipe, with the more fluid slag on top. The iron and the slag were drained off alternately. More charcoal and limestone were continuously fed through the top of the pipe throughout the process. Refined pig iron could be forged into bar iron.
Even in the Vasa Renaissance of the 16th century, springs were made solely of iron; but it was carefully selected iron that was hammered to a specific hardness. The smiths were trained and skilled in judging the properties of the iron by its appearance. They were also aware of methods of changing the carbon content of iron if necessary.
In the early 17th century, Swedish blacksmiths had learned to manufacture iron springs that could be hardened into steel. The plate springs that had been used up to now, single or double (V-shaped), were replaced with coiled helical springs. All iron that was alloyed with carbon and could be hardened was called steel. The carbon serves as a binding material in the process. To harden sprung steel, the iron was heated to light blue, about 315–320°C, then immersed in cold water or oil.
The quality was regulated by an ordinance from 1637 issued by Queen Kristina’s government, which the smiths had to follow to the letter. Bar iron had to be “good” and bear a manufacturer’s stamp. Those who forged “bad” iron, or used someone else’s stamp, would be punished as thieves.