If, like us at Agricamping Sophia, you are fond of pizzas in the pan and focaccia, you have certainly received the news or have accidentally read the miraculous cooking skills of the legendary blue iron trays . Out of curiosity, also thanks to the lockdown due to the Covid-19, we purchased a couple, for private use, from different manufacturers. Being aware, through the attendance of specialized forums and the reading of a myriad of articles on the net, of the mysterious burning which should be a preliminary operation necessary to give the green light to their use, we are here uncritically lavished in following to the letter one of the guides found among the first results of Google. The outcome? A lot of smoke, the smell of burnt oil throughout the house and two apparently ruined trays. Immediately consulted the forums we mentioned, the diagnosis is that we would probably have used too much oil ... Something did not return: how is it possible that a little extra virgin olive oil, whose temperature does not exceed 190 ° during the boiling, is it enough to ruin the trays made to be inside an oven (in this specific case, domestic) if the bluing process would be obtained at about 310 °?

Let's take a step back. What should the burn be for?

To eliminate the residual substances (mineral oil) produced and to create a natural non-stick by crystallizing or vitrifying the burnt oil layer, on several occasions and at relatively high progressive temperatures.
First consideration: go for the elimination of mineral oil but this non-adherence should not already be one of the results of bluing? At this point, if we had to produce the non-stick by means of this other phantom crystallization or vitrification of edible oil, what sense would it make to buy a blue iron pan?
Second consideration: what is the difference between the encrusted oil of a pan or fryer or of the oven glass and the crystallization / vitrification of the oil that should be burned in a blue pan?
Third consideration: what if many blue iron trays on the market were common steel trays with a layer of non-stick synthetic?

What is blue iron?

In technical jargon it is called rolling blue or bluito steel . It is a mild steel sheet (i.e. with low carbon content) subjected to a temperature of about 310 ° and rapidly cooled, a process by which the steel oxidizes in magnetite (Fe3O4) instead of in red oxide, i.e. hematite (Fe2O3), thus obtaining blue and purple shades. This oxidation is stimulated by a jet of steam at very high temperature. In addition to blue iron it is often also called flamed iron, shaded iron, burned iron or black iron calamine. Blue is therefore the iridescence deriving from an oxide layer which in turn acts as an antioxidant against rust: being already oxidized it cannot further oxidise.
What is it for in trays?

Theoretically it is used to reach the temperature set by the thermostat very quickly, due to the reduction of reflectance , i.e. the mirror effect (which occurs not only for light but also for heat): by reflecting less heat, absorbs it more quickly. The bluing therefore gives the blue iron trays a superior heat absorption compared to stainless steel or aluminum trays . The practical result is a pizza or focaccia with a thicker and crisper crust at the base.
Never wet the blue iron with water?

bluing is a passivation protection: the metal protects itself with its own oxide. However, a structural defect (for example a scratch) of the magnetite layer is sufficient to trigger hematitic oxidation. The oxidation of anodized aluminum (artificially passivated) is instead much more stable: it corrodes only in contact with other metals due to the ignition of galvanic currents but, if scratched, the oxide layer recreates immediately. For example, anodized aluminum trays do not fear contact with water. Those who made the scout or the military certainly remember the aluminum mess boxes with their typical opaque gray color: that is anodized aluminum. In other words, the problem is not wetting the blue iron pan but wetting it in the presence of scratches through which the water can infiltrate and trigger a hematitic oxidation. Even letting a leavened product sit on an iron pan for a long time before cooking (in slang, sizing) can, due to the intrinsic humidity of the leavened product, be problematic. The alternative to risky washing with water is the safest anointing with edible oil which, acting as an additional antioxidant, protects it from rust. From a hygienic point of view, however, the high cooking temperatures and the removal of dirt with kitchen paper are sufficient. In any case, if an oxidation start occurs, simply wet the part with food vinegar (for example, wine or apple vinegar), let it act and finally rub gently with kitchen paper.

In conclusion, probably the vast majority of pans sold as blue iron on the market are very common mild steel pans with a non-stick synthetic layer (the same as the frying pans you can buy at the supermarket). The so-called burning process has no scientific justification except to eliminate residual mineral oil. During burning, it is useless to grease the pan with oil because either the pan is really blue iron or has a non-stick synthetic layer. In our specific case, it is impossible for an oxidation produced at 310° to alter between 190° (boiling temperature of oil) and 250° (maximum temperature of a domestic oven with a functioning thermostat). It is not true that blue iron, if structurally intact, should not see water and detergent. The important thing is to dry it immediately and treat any signs of oxidation with vinegar. In case, like us, you fall for the urban legend of burning blue iron pans ruining the non-stick coating (since it is made of very common mild steel), know that the pans can be recovered by eliminating the ruined non-stick layer using fine sandpaper or a steel abrasive sponge, taking care to patiently eliminate all the ruined part and not to damage the unruly part, proceeding with the patient creation of a layer of burnt oil (or, if you prefer, "crystallized"), spreading a little oil with a kitchen paper before each use and taking care to clean the pan with only vinegar or lemon juice. Over time, use after use, the pan will acquire a layer of encrusted oil that will act as a non-stick.

We thank our dear friend Eng. Filippo Cucinotta for the technical-scientific advice and Dr. Corrado Rampulla for the stimulating avant-garde tips.