[Kampernaut (OP)]

Cookery books and manufacturers of mild steel frying pans recommend "seasoning" them to give the metal a non stick surface.  This involves heating table salt (NaCl) up to a very high temperature in the pan, discarding it and then coating the pan with a thin film of oil.

According to the newbielink:http://www.cesa.org.uk/products/cesa-buying-guides/understanding-cookware/ [nonactive], heating the salt "seals any surface imperfection on the base of the pan".

1. Please can anyone explain the chemistry of this sealing process? 

2. Would this process work with stainless steel too?  Some books advocate it with those as well but manufacturers warn against prolonged contact with salt.  They warn against putting salt in cold water in a saucepan because it can pit the surface on the base - that's true as I've discovered to my cost.
 
Thanks,

K

[JP]

I own a bunch of cast iron pans, and I've tried a few techniques to season them, but I've never heard of salt sealing surface imperfections.  Usually, if a pan is dirty and/or the seasoning is wearing off, you scrub it with a mixture of salt and oil to clean it (as using soap will strip off any of the old seasoning that's left) and then you wipe out the salt and use oil to re-season it.

To actually melt salt and get it to "seal" the imperfections in the pan, you'd have to heat it to ~800 C, which is way beyond what a normal kitchen stove can manage.  Plus, oil's a liquid, so it flows into any imperfections and effectively seals them.

Here's an interesting page on the science of seasoning cast iron: http://sherylcanter.com/wordpress/2010/01/a-science-based-technique-for-seasoning-cast-iron/
A chemist could probably critique it better than me, though.   

[Kampernaut (OP)]

Thanks for replying JP and the link to that article.  Interestingly, the "salt method" is mentioned in the discussion at the end of the article.

[RD]

Heating a pan filled with salt could enable it to reach a higher and more uniform temperature than an empty pan.

[Kampernaut (OP)]

I'm sure that's true so are you suggesting that something is happening to the metal simply because it is being heated and that the salt is not somehow chemically reacting with something in the steel?

There is an interesting newbielink:http://www.cookingcute.com/wok_seasoning.htm [nonactive] along with a more detailed description of the method. 

[CliffordK]

To actually melt salt and get it to "seal" the imperfections in the pan, you'd have to heat it to ~800 C, which is way beyond what a normal kitchen stove can manage.  Plus, oil's a liquid, so it flows into any imperfections and effectively seals them.

The combustion temperature of Methane is around 1957°C

And, an electric stovetop can get hot enough to melt out the bottom of an aluminum pan (yes, I've done it).  Ok, that is only about 660°C. 

But, the link suggests washing the pan with water after salting, so I'm doubting the salt is actually staying in the pan.  Perhaps the theory is that it is to spread heat and catalyse oxidation.

[JP]

To actually melt salt and get it to "seal" the imperfections in the pan, you'd have to heat it to ~800 C, which is way beyond what a normal kitchen stove can manage.  Plus, oil's a liquid, so it flows into any imperfections and effectively seals them.

The combustion temperature of Methane is around 1957°C

True, but it's not the combustion temperature that matters.  It's the equilibrium temperature of your cast iron skillet.  The combustion temperature is 1957 C even if you set the flame to its lowest setting, but it won't bring a pot of water to a boil (on my stove at least).  I've accidentally left cast iron on a high flame for over an hour (all the seasoning burned off), and it wasn't hot enough to glow red (which starts to happen around 500 C), so I seriously doubt you could get a skillet up to 800 C on a conventional stove-top.  Certainly you could with a heavy duty burner, and holding a blow torch to salt would work as well.

Interestingly I have tried melting salt by placing it directly on an electric stove-top element, and it did nothing even on the highest setting, where the element glowed bright red.  So presumably your cast iron/stainless steel pot would have to start glowing red before the salt got hot enough to melt.

[damocles]

I think that what is going on here is that the salt is etching the steel just slightly to prepare surface sites for the better adsorption of oil pyrolysis products. Iron has a very high affinity for chloride ions (it forms very strong FeCl₄^– and FeCl₆^3– complexes, for example). With the steel surface prepared in this way the film of pyrolyzed oil -- actually a polymer -- will stick better to the surface. Of course you wash the salt off before using the oil, replacing the chloride ions with more weakly adsorbed hydroxyl ions.

The oil that will form the best surface film is one with a decent polyunsaturated content -- canola, sunflower, or safflower. Do not use a good quality olive oil, or a saturated oil like coconut oil or palm oil.

I do know a bit about the "non-stick" preparation; I am just speculating about the chloride pre-treatment from my general knowledge of surface chemistry. I am confident that the sodium ions are just spectators to the chemistry.

The process is not unlike using linseed oil on your cricket bat, except that you are using a high temperature to polymerize the oil rather than an oil that slowly polymerizes in atmospheric oxygen, and the wood has ready made sites for strong adsorption.

[Kampernaut (OP)]

Thanks everyone for your replies.  I think we may be getting somewhere towards understanding this.

@damocles  The salt isn't washed out from the pan.  It is merely discarded. 

By etching do you mean that heating the steel and salt to a very high temperature rips iron molecules from the surface of steel in the form of iron chloride so that it becomes more pitted and that the polymerised oil sticks to the surface better?  Or are the chlorine ions bonding onto the iron atoms on the surface to make it bumpy?

Is it the heat that makes the disassociation of the salt's ionic lattice possible or is the heat doing something more with the surface of the metal?

Why does the colour of the steel change after the salt is heated on it for 20 minutes?  Is that iron chloride?

If this is what is happening then it is the complete opposite of sealing imperfections in the surface as the article linked to in my original post suggests.


Before Heating



After The Salt



After The Oil

[CliffordK]

If you read about [url-http://en.wikipedia.org/wiki/Flux_%28metallurgy%29#List_of_fluxes]fluxes[/url], Sodium Chloride is one of the fluxes that can be used.  It provides cleaning and protection of the metal, and may help float off slag.  However, I believe that only occurs at temperatures above which the salt melts. 

What would be the effect of salt if your pan was rusty, or can one get mill scale in new pans?

The salt may also help distribute heat around the pan.

Anyway,
The oil may help create a protective layer in your wok or frying pan.
However, if  you took two identical pans, one with salt pre-treatment.  One without.  After a week of use, would you be able to tell the difference?

[RD]

Why does the colour of the steel change after the salt is heated on it for 20 minutes?  Is that iron chloride?

Those colours also occur in the absence of salt, due to the steel reacting with oxygen in the air  ...

https://en.wikipedia.org/wiki/Tempering#Tempering_colors

The iron oxide layer, unlike rust, also protects the steel from corrosion through passivation

https://en.wikipedia.org/wiki/Tempering#Tempering_colors

[Karen W.]

Hi....I have cooked in cast iron my whole life and my mother her mother and hers before..now passed down with my very old cast iron wear were my grezt grandfathers seasoning technique which was to never use soap... wash out your pan rub it down with oil and then go outside and throw the pan once a year into a very hot fire... in a barrel or right into your fire place add wood and let it burn in the fire for a good long burn.
Move it from the flames.. this removes all buildup on outsde of pan as well as inside. Remove it and cool then when cooled give it a good wash and dry... rub it down with I'll both inside and out and place it back on a burner this time heat up until dry and seasoned.. your pan should then be wiped down with a cloth or
Or these days dry paper towels and stored dry until ready to use... I have never heard of
salting them.. but my grandparents let it burn in the fire for a good two hours.. the wooden
handled cast iron was screwed on snuggly with a bolt to remove handle before firing......
salts or acidic foods left would always pit the pans in my experience but only upon leaving
those items to stand in pans for too long... days etc.... after this yearly seasoning one should not wash the pan but wipe it clean with oil and fire it upon the stove after each use. THIS MAINTAINED THE SEASONING THROUGH THE YEAR UP UNTIL THE YEARLY...NO SOAP AND WATER..!!!

[CliffordK]

I'm wondering if the purpose of the salt is to spread the heat.
And, you could just as well use dry beach sand.

[Kampernaut (OP)]

I think CliffordK is right, the salt is to distribute the heat and 20 minutes is to get the metal hot enough.

I corresponded with a metallurgist who thinks that the heating would cause the pan to develop a thin, continuous layer of iron oxide over the surface.  The oil then fills the gaps and cracks in this oxide film and protects it from getting disrupted.