Born from the fire – The history of cast iron
When it comes to high-quality accessories like pots, pans, or even grill grates, many grillers swear by cast iron products. The excellent thermal conductivity, robustness, and easy maintenance of the material have been appreciated for generations. But what are the exact origins of cast iron? How is it processed, and what types of cast iron have been developed over time? FIRE&FOOD investigated these questions, among others, at the iron foundry of the Fondium Group in Singen. The company, located at the Hohentwiel, a volcanic plug about 30 kilometers northwest of Constance, manufactures cast iron products, following a centuries-old tradition in the region.
Knowledge of iron as a forging material dates far back into human history. During the Iron Age, the Celts in Central Europe (800 B.C. to the end of the 1st century B.C.) preferred to settle in regions with ore deposits. As archaeological findings of ancient bloomery furnaces near Neuenbürg in the Northern Black Forest and Liel in the Markgräflerland region prove, the Celts were already masters of iron smelting and blacksmithing in Southwest Germany (presumably around 600 B.C.). According to linguistic research, the word "Eisen" (iron) can be traced back to the Celtic term "isorai." Cast iron, however, was unknown in antiquity. Iron founding is an art whose beginnings date back no more than six hundred years. This may seem surprising at first, because metal casting, especially bronze casting, was already known in ancient times. But if we consider the methods and tools of the old iron smelters, it becomes clear why there were highly skilled iron smiths in antiquity, but no iron founders. What distinguishes iron from other metals? Temperatures of over 1539 °C are necessary to melt it completely. In the early days, iron was smelted in bloomery furnaces (small shaft furnaces made of clay or stone) with the help of weak hand or foot bellows. The furnaces were heated with wood or peat and then filled from the top for smelting in alternating layers with fuel, usually softwood charcoal, and finely crushed iron-bearing ore. The ore was reduced at low heat (max. 1350 °C), and the forgeable iron collected together with slag at the bottom of the melting hearth as a spongy lump.
The furnace was broken open, the so-called "bloom" was pulled out and cleaned and compacted by repeated heating and hammering. This resulted in soft, low-carbon iron, and under particularly favorable conditions, also hardenable steel. Iron was not known in a liquid state, which requires the reduced iron to remain in the furnace for a longer time and a higher temperature for melting. The melting of larger quantities of liquid iron, as required for the production of cast goods, was not possible at that time.
Reconstruction drawing: Longitudinal section through a Celtic bloomery furnace with a forehearth and attached furnace shaft.
Iron scrap: Today, cast iron products are no longer made from iron ore, but from a recycled content of up to 100%.
This possibility only arose when the use of hydropower in the iron industry began in the 14th century. Bellows moved by water power were significantly stronger, the heat was increased, and a larger portion of the reduced iron-slag mixture became liquid. At first, there was little enthusiasm for this, because the resulting hard, brittle product was useless for forging. After some time, however, it was discovered that liquid iron could be poured into molds like other metals and used for cast goods. This was the beginning of iron founding. In the second half of the 14th century, due to the invention of gunpowder, artillery gained great importance in Europe. It is therefore not surprising that cannonballs are the first objects cast from iron to be mentioned in writing. For example, in 1383, the gunsmith Ulrich Beham in Memmingen (Swabia) is said to have cast balls made of lead and iron. From 1400, small cannons also began to be cast from iron. Germany is considered one of the first countries to develop iron foundries for small castings.
Stronger blowers and even taller shaft furnaces ensured over time that cast goods could be produced directly in blast furnace operations. Casting was mostly done directly from the blast furnace until well into the 19th century. In these facilities, also referred to as ironworks or smelting works, liquid pig iron could be produced in a continuous reduction and melting process. For large castings, the iron was allowed to flow into the mold through channels. For smaller castings, the iron was scooped from the blast furnace forehearth with ladles. Sand and flask molding, which was only used on a larger scale for iron casting in the 18th century, was already being used in the mid-16th century for molding smaller castings made of bronze and precious metal. In 1708, flask molding with wet sand was put into practice in England. This made it possible to move away from expensive loam molding processes. While in previous centuries cast iron had been used on a large scale only for cannonballs and artillery, the growing industry in the 18th century ensured that cast-iron stove plates, pots, and pans were also produced on a large scale. Cast iron also became increasingly interesting for construction purposes during the industrialization era. Eventually, even entire bridges could be manufactured from cast iron.
During casting, the liquid metal is introduced into the interior of the mold. Once the casting has solidified, it is unpacked.
The casting process has continuously evolved to this day, so that components with complex geometries can now also be manufactured. Nowadays, various modern melting units are used to liquefy the iron, which are heated by gas, oil, coke, or electricity, depending on the type. These include cupola furnaces (shaft furnaces), electric arc furnaces, and induction furnaces. After a three-dimensional construction of the desired casting has usually been created, a model is made. Molds are produced using this model. A distinction is made between permanent molds, which are made of metallic material and can be used for a high number of casting processes (molds), and expendable molds, which are made of bonded sand and can only be used for one casting. During casting, the liquid metal is introduced into the interior of the mold. For this purpose, metallic and non-metallic components are melted in the foundry. After some time, when the casting has solidified and sufficiently cooled, it is unpacked. Finally, the casting is processed in the so-called cleaning shop. There, burrs and unevenness are removed. The quality of cast iron is demonstrated by the fine-pored nature of the material. Especially with cast iron cookware, a fine-pored material surface is easily visible to the naked eye. A uniform surface without air inclusions prevents the penetration of moisture, which could lead to rust formation or, under the influence of heat, even to stress cracks.
Cast iron contains a high proportion of carbon. A distinction is made between two main types of cast iron. Gray cast iron, also known as gray iron, in which carbon occurs in the form of graphite (gray fracture edge), and white cast iron, in which carbon occurs as carbide in the form of cementite (white fracture edge). The simplest and most common type of cast iron is gray cast iron with flake graphite. This gray iron is mostly used for the production of Dutch ovens & Co. The density of cast iron is lower than that of pure iron and, as mentioned at the beginning, it cannot be forged due to its high carbon content and the resulting high hardness and brittleness. However, cast iron parts are more corrosion-resistant than steel and have very good thermal conductivity, good damping properties, and good dimensional stability. Every Dutch oven enthusiast knows the advantages of their Dutch oven, because nothing conducts and stores heat better when grilling than cast iron. The heat is transferred to the food up to four times faster than, for example, with aluminum or steel, and the food stays warm for a long time even after cooking. Cookware as well as grills made of cast iron are not only extremely durable when manufactured to a high quality, they also guarantee a unique taste of the food. Furthermore, cast iron pots and pans are no longer made from iron ore today, but from a recycled content of up to 100 percent. It hardly gets any more sustainable.
After repeated use, cast iron develops a protective patina that acts like a non-stick coating. Additional fat can therefore be dispensed with during preparation. Cast iron cookware should only be cleaned with clear water or rubbed dry with paper towels after use. The material can be easily maintained with a few drops of cooking oil. Uncoated cast iron products must be "seasoned" before the first use, but most manufacturers today also supply products that are already ready for cooking.
Ductile iron, also known as spheroidal graphite iron, has even better mechanical properties than gray cast iron with flake graphite. Here, the graphite is formed into spheres by treating the melt with magnesium. This so-called desulfurization gives the material mechanical properties similar to steel. Comparable properties also apply to white and black malleable cast iron, which is subjected to an additional annealing treatment after solidification. These types of cast iron can also withstand minor plastic deformations without breaking and are therefore typically used in pipeline construction for screwed connections or for safety-relevant chassis components in the domestic automotive industry. There is now also cast iron with vermicular graphite, whose mechanical properties lie between those of cast iron with flake graphite and those with spheroidal graphite. Material development is therefore far from complete, and further innovations will follow.
Today, iron is the most important utility metal of all, and cast iron in particular is used in very many areas of industry and daily life. The material born from fire is an indispensable part of our present and future. For BBQ fans, grill utensils made of cast iron are usually lifelong companions whose advantages they do not want to do without. FIRE&FOOD can also warmly recommend high-quality cast iron products to anyone who expects the best grilling properties from their BBQ equipment. These products still do justice to the proud tradition of iron founding to this day.
