|This article relies on references to primary sources. (February 2011)|
Seasoning is desirable on cast-iron cookware and carbon steel cookware, because otherwise they are very sticky to foods and rust-prone. It is generally not desired on other types of cookware either for cosmetic reasons or because the chemical composition of the pan already results in a non-stick surface.
Methods of seasoning
A new pan (bare metal) is porous and will grab food tightly, and must either be oiled before cooking or seasoned. This base coat is initially created by a process of layering a very thin coat of oil on the pan. Then, the oil is polymerized to the metal's surface with high heat for a duration. The base coat will eventually develop a more refined coating through use, e.g., frying or searing, and darken over time. This entire process is known as "seasoning". The colour of the coating is commonly known as its "patina".
To season a pan (e.g., to season a new pan, or to replace damaged seasoning on an old pan), the following is a typical process: (a) cleaning the cookware to expose the bare metal, (b) applying a thin layer of animal fat or vegetable oil, and (c) heating the cookware to generate the seasoned coating. If it is not pre-seasoned, a new cast iron skillet or dutch oven typically comes from the manufacturer with a protective coating of wax or shellac, otherwise it would be rusted. This must be removed before the item is used. An initial scouring with hot soapy water will usually remove the protective coating. Alternatively, for woks, it is common to burn off the coating over high heat (outside or under a vent hood) to expose the bare metal surface. For already-used pans that are to be re-seasoned, the cleaning process can be more complex, involving rust removal and deep cleaning (with strong soap or lye, or by burning in a campfire or self-cleaning oven) to remove existing seasoning and build-up. Once the pan has been heated, dried, and thinly layered with oil or fat, it is placed in an oven, grill, or other heating enclosure for the oil to be polymerized onto the metal's surface. The process of polymerization is dependent on the oil, temperature of the enclosure, and the duration. The precise details of the seasoning process differ from one source to another, and there is much controversy regarding the correct oil to use. There is also no clear consensus with the correct temperature and duration. Lodge Manufacturing uses a proprietary soybean blend in their base coats as stated on their website. Others use lard, or animal fats. Some advocate the use of food-grade flaxseed oil (a drying oil). Likewise, the recommended temperature for seasoning varies to high temperatures above 260 °C (500 °F), while some recommend a lower temperature below 150 °C (302 °F). Some say that a temperature around the smoke point of the oil or fat should be targeted since this will allow vaporization of impurities from the oil, and polymerization and carbonization to occur. And, there is also no clear determination of the correct duration of heat to use. Anywhere from half an hour to an hour is often recommended.
Seasoning a cast iron or carbon steel wok is a common process in Asia and Asian-American culture. While the vegetable oil method of seasoning is also used in Asia, a traditional process for seasoning includes the use of Chinese chives or scallions as part of the process.
The process of heating a pan to cause the oil to oxidize is analogous to the hardening of drying oil used in oil paints, or to varnish a painting. But whereas the curing of oils is the result of autoxidation at room temperature for a painting, for a pan, the thermoxidized oil undergoes a conversion into the hard surface of the seasoned pan at the high temperatures of cooking.
When oils or fats are heated in a pan, multiple degradation reactions occur, including: autoxidation, thermal oxidation, polymerization, cyclization and fission. Often seasoning is uneven in a pan, and over time the distribution will spread to a whole pan. Heating the cookware (such as in a hot oven or on a stovetop) facilitates the oxidation of the iron; the fats and/or oils protect the metal from contact with the air during the reaction, which would cause rust to form. Some cast iron users advocate heating the pan slightly before applying the fat or oil to ensure that the pan is completely dry and to open "the pores" of the pan.
The surface is hydrophobic, and oils or fats for cooking will spread evenly. The seasoned surface will deteriorate at the temperature where the polymers break down. This is not the same as the smoke point of the original oils and fats used to season the pan because those oils and fats are transformed into the plasticized surface. (This is analogous to how the smoke point for crude oil and plastic are different).
As with other cast iron vessels, a seasoned pan or dutch oven should not be used to cook foods containing tomatoes, vinegar or other acidic ingredients. These foods will damage the new seasoning. Instead, newly seasoned ovens should be used to cook food high in oil or fat, such as chicken, bacon, or sausage, or used for deep frying. Subsequent cleanings are usually accomplished without the use of soap. Because modern cleaning methods (detergent soaps, dishwashers) will destroy the seasoning on cast iron, manufacturers and cookbook authors recommend only wiping the pans clean after each use, or using other cleaning methods such as a salt scrub or boiling water.
Other surface types
There are other surfaces on cookware which are stick-resistant. See also Non-stick pans.
In the process of bluing, an oxidizing chemical reaction on an iron surface selectively forms magnetite (Fe3O4), the black oxide of iron (as opposed to rust, the red oxide of iron (Fe2O3)). Black oxide provides minimal protection against corrosion if also treated with a water-displacing oil to reduce wetting and galvanic action.
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