MATERIALS, PART II: IRON

by Editorial Staff | May 25, 2020 | Quality & Operating Procedures | 0 comments

MATERIALS, PART II: IRON

Iron (carbon steel and cast iron)

The importance of iron (₂₆Fe) to the development of technology—and human civilization itself—surpasses that of any other material. It is no exaggeration to say that without a metal as available, workable, versatile, durable, and strong as iron, many of humanity’s advances in every field, especially engineering and architecture, would never have occurred. Just think of the Industrial Revolution and its symbol: the steam engine, simply impossible to build in the 18th century from any metal other than iron.

In the culinary field too, iron has been used since ancient times and still holds a place of honor in professional kitchens, both Western (French-based) and Far Eastern (Chinese-based). We stress professional because, at home, cookware made from ferrous alloys has been almost entirely replaced by stainless steel and nonstick pans. However, much like copper now has a small but growing fan base, iron is also making a comeback among those who want to reclaim classic techniques as well as cooks who dislike nonstick and see iron as an effective “vintage” alternative.

Before examining cookware characteristics, a clarification: strictly speaking, the term “iron” isn’t accurate. In practice, iron is never used in pure form but alloyed with at least carbon (₆C)—also owing to the steelmaking process, which uses the heat generated by burning coke.

Cookware sold as “iron” is therefore, more correctly, carbon steel if the carbon content does not exceed 2.1%—above that threshold the alloy is called cast iron. Both materials are used successfully in the kitchen, though with slightly different performance profiles.

Thermal properties and how the tools are built

The thermal conductivity of iron–carbon alloys decreases as temperature rises; for simplicity, consider that in the 50–200 °C range conductivity settles around 60–50 W·m⁻¹·K⁻¹ for steels and somewhat less for cast irons. On the other hand, cast-iron cookware is made by casting into molds and therefore tends to have thick walls—also because cast iron is relatively brittle, so thin walls would not provide sufficient strength.

Carbon-steel tools, by contrast, are made from sheet that is shaped by stamping, or from a small billet forged under a hammer. In both cases the material is more malleable and elastic than cast iron, which allows for relatively thin, lightweight pieces such as certain inexpensive woks and the classic household “iron pans” once used for sautéing and frying. Therefore, at the same diameter and heat source, a carbon-steel pan—thanks to its lesser thickness—will respond more quickly than cast iron to changes in heat, while still offering excellent heat retention.

The hallmark of “iron” is precisely its ability to hold heat. This is advantageous in two cases: very short, high-temperature cooking, or very long, medium–low-temperature cooking with intermittent heat input. The apparent contradiction is easily resolved with two examples.

From steak to braise: when iron makes the difference

Imagine searing a steak a few centimeters thick, cooked rare. The culinary goal is to quickly sear the exterior while cooking the interior just enough to keep the center rare or nicely rosy and juicy. It is therefore imperative that the metal stays well above 100 °C so the liquids inevitably released by the meat vaporize instantly on contact with the cooking surface, avoiding the unpleasant “puddle” that typically forms when using thin pans and inadequate burners. Once a crust has formed, you can continue in the oven at a lower temperature, also taking advantage of the air’s heat. Carbon-steel or cast-iron cookware excels here too, as it transitions to the oven with no issues—provided any handles or knobs made from other materials are oven-safe. Cast-iron pieces usually have short, cast-on handles (due to the alloy’s brittleness), making them the most suitable for moving seamlessly from stovetop to oven and back.

Now consider cooking a Neapolitan ragù or a traditional braise with tough second- and third-cut meats. These preparations simmer for hours: ragù to extract and concentrate flavor to the utmost; braise to render tough, fibrous cuts palatable. Fine-tuned heat control or 30-second searing isn’t the priority; rather, you want to stay within a temperature band as steadily as possible, without constantly tending the flame or worrying about drafts that might perturb it (it may sound fussy, but cook with copper a few times and you’ll see how much an open or closed window changes things).

In the kitchens of yesteryear, a cast-iron pot—or a cheaper clay one—would be left overnight on the woodstove (hence “stewing”) or set in the hearth, nestled among the still-glowing embers (hence “braising”). Cooking proceeded gently for a long time with minimal fluctuation; even after the last coal died out, the vessel stayed warm for hours, so that by dawn the dish was done—and the pot likely still lukewarm.

Cook at work – Giddings estate, Greenfield Village, The Henry Ford Museum.

Induction, food safety, and protective barriers

Such thermal stamina may be less necessary today given our pace of life and changed eating habits, but it is still widely exploited in professional settings to keep culinary bases warm—rustic sauces, grain and legume soups, and various stews. Note that unlike common stainless steel, carbon steels and cast irons are naturally ferromagnetic and thus work perfectly on induction tops—the best option for holding foods at controlled serving temperatures.

As for food safety, iron will rust when exposed to air, especially in humid environments, and is easily attacked by acids and salt. For iron–carbon alloys as well, a protective barrier between metal and food is needed. This can be achieved either by seasoning with fats or by enameling (in the past, tinning was also used, though only marginally). As we will see, the two solutions have different advantages and applications depending on the alloy and the intended use.

Seasoning

Seasoning consists of coating the cookware surface with a thin layer of fat and then heating it to trigger oxidation and polymerization reactions. When properly executed, the fat becomes a hard, compact patina: besides preventing rust, it limits metal migration into food and, as a bonus, renders the cooking surface partially or even fully nonstick.

There are various techniques and no consensus on the “best.” A simple method is to pour in a high-heat vegetable oil (e.g., peanut), heat to its smoke point, turn off the flame, and discard the oil. Then wipe thoroughly with paper towel or a clean cloth to remove as much oil as possible, leaving only a thin film of now-polymerized fat. This patina is fragile and assumes frequent use so that repeated heat cycles consolidate the coating and reinforce it with small additions of fat each time.

To speed up the process and obtain a more uniform patina, more elaborate approaches prescribe repeated heat/cool cycles, each adding one more polymerized layer until the cooking surface turns dark and glossy. Some suggest seasoning with food-grade linseed (flaxseed) oil, which has a low smoke point (about 105 °C vs. >230 °C for peanut oil) and marked drying properties—i.e., it oxidizes and polymerizes even at room temperature. Advocates say five or six cycles yield a particularly tough, nonstick patina.

Strictly speaking, “seasoning” is a misnomer since the treatment does not alter the alloy’s intrinsic properties; by convention we accept it as it gives the tool a “lived-in,” seasoned look. The paradox is that seasoning aims to preserve the tool’s “eternal youth,” not to age it.

Of course, even the best seasoning does not last forever. The patina can wear away with acidic foods and mechanical stress (scraping with spatulas, etc.) as well as washing with regular dish soap. In that case, resign yourself to re-seasoning the piece, or keep using it bare until a new patina rebuilds with use.

Enameling and final considerations

Enameling is an ancient technique, but it has been applied to cookware only since the mid-19th century. The glaze, applied by dipping or spraying, is fired at high temperature and turns into a hard, glossy coating similar to porcelain vitrification. High-end products typically feature multiple enamel layers for greater resistance to wear, scratches, and impacts.

Food safety depends, of course, on the glaze composition and proper processing. As with all cookware—especially for professional use—it is generally wise to favor brands whose products have stood the test of time and earned consumer trust. Enameled cookware today is very much a niche; European consumers effectively choose among just a couple of well-known brands in the upper and mid-upper price ranges.

Assuming the glaze is safe, there are many advantages over fat-based seasoning, albeit with some limits: enamel resists salt and acidic foods as well as detergents and humidity. Enameled pieces therefore do not oxidize or rust and can be hand-washed like any other tableware.

Manufacturers, however, advise against adding salt off-heat, as salt deposits on the bottom could dull the enamel. Likewise, dishwashers are discouraged because their detergents can be too aggressive. Metal spatulas and utensils should also be avoided to prevent scratching, and unlike seasoning, enamel cannot be “touched up.”

Compared to seasoning—which is elastic by nature and follows the base metal’s expansion—enamel is rigid and sensitive to thermal shock. Thus, whereas an enameled tea kettle can be made from thin carbon steel or even plain sheet iron, pieces destined for high temperatures and especially oven cooking are better made from cast iron, which expands less than carbon steel (1 × 10⁻⁵ vs. 1.2 × 10⁻⁵) and deforms very little even at high heat.

Two important caveats: enamel is not nonstick and therefore does not allow fat-free sautéing (boiling and toasting excepted); and manufacturers recommend not heating empty enameled pieces—a practice that is perfectly acceptable with bare or simply seasoned iron.

In conclusion, “iron” deserves a place of honor in the kitchen: robust, versatile, naturally induction-ready, widely available, eco-sustainable, and fully recyclable. If made and maintained properly, it poses no health risk and lasts a lifetime—indeed, several lifetimes. Its high density makes pieces less handy than aluminum, but in return they better resist warping from heat and mechanical stress. Finally, while carbon steel and cast iron are not great heat conductors, they excel at heat storage and retention.

Carmine F. Milone
Food Technologist