Chemistry of Cast Iron Seasoning: A Science-Based How-To
I wanted to understand the chemistry behind seasoning so I’d know how
to fix this, but there is nothing that addresses this issue directly. A
Web page on cast iron
posted by someone similarly obsessed with the science gave me two
crucial clues, the phrases “polymerized fat” and “drying oil”. From
there I was able to find the relevant scientific literature and put the
pieces together.
The pictures below are both of the same antique cast iron skillet. The “before” close-up on the left is from a
picture of the skillet in my previous blog post
on making German Pancakes. I stripped the pan with oven cleaner and
reseasoned it based on my new understanding. The “after” close-up on the
right shows the result.
Griswold skillet closeups: old seasoning on left, new seasoning on right
Start With the Right Oil (It’s Not What You Think)
I’ve read dozens of Web pages on how to season cast iron, and there
is no consensus in the advice. Some say vegetable oils leave a sticky
surface and to only use lard. Some say animal fat gives a surface that
is too soft and to only use vegetable oils. Some say corn oil is the
only fat to use, or Crisco, or olive oil. Some recommend bacon drippings
since lard is no longer readily available. Some say you must use a
saturated fat – that is, a fat that is solid at room temperature,
whether it’s animal or vegetable (palm oil, coconut oil, Crisco, lard).
Some say never use butter. Some say butter is fine. Some swear by Pam
(spray-on canola oil with additives). Some say the additives in Pam
leave a residue at high temperatures and pure canola oil is best. Some
say it doesn’t matter what oil you use.
They are all wrong. It
does matter what oil you use,
and the oil that gives the best results is not in this list. So what is
it? Here are some hints: What oil do artists mix with pigment for a high
quality oil paint that dries hard and glassy on the canvas? What oil is
commonly used by woodturners to give their sculptures a protective,
soft-sheen finish? It’s the same oil. Now what is the food-grade
equivalent of this oil?
The oil used by artists and woodturners is linseed oil. The
food-grade equivalent is called flaxseed oil. This oil is ideal for
seasoning cast iron for the same reason it’s an ideal base for oil paint
and wood finishes. It’s a “drying oil”, which means it can transform
into a hard, tough film. This doesn’t happen through “drying” in the
sense of losing moisture through evaporation. The term is actually a
misnomer. The transformation is through a chemical process called
“polymerization”.
The seasoning on cast iron is formed by fat polymerization, fat
polymerization is maximized with a drying oil, and flaxseed oil is the
only drying oil that’s edible. From that I deduced that flaxseed oil
would be the ideal oil for seasoning cast iron.
As a reality check of this theory, I googled “season cast iron with
flaxseed oil” to see what came up. The very first hit is a page written
by a guy who seasons his cast iron cookware with linseed oil from the
hardware store because it gives the hardest surface of anything he’s
tried. (I’m not sure how safe that is; I don’t recommend it.) Below that
were several sites selling traditional cast iron cookware from China,
which they advertise as being
“preseasoned with high quality flax oil”.
I don’t know whether they really use food-grade flaxseed oil (which is
expensive) or linseed oil from a hardware store. What’s significant is
the claim. Seasoning with high quality flaxseed oil is something to brag
about.
With this encouragement, I stripped one of my skillets and reseasoned
it with flaxseed oil. As you can see in the picture above, the result
was a dramatic improvement. The finish is smooth, hard, and evenly
colored.
Seasoning Is Not Cooking: Different Principles Apply
The first time I seasoned a pan I chose avocado oil because it’s
monounsaturated and doesn’t easily go rancid. It also has the highest
smoke point of any edible oil, 520°F, so I could heat it in a 450°F oven
without passing the smoke point. I knew that when cooking, you should
never heat an oil past its smoke point because that causes the release
of “free radicals”, which are carcinogenic. I was careful not to choose a
polyunsaturated oil – and especially not an oil high in omega-3 fatty
acids – because these are especially vulnerable to breakdown with heat
and the release of free radicals.
Ironically, it’s for exactly these reasons that the best oil for
seasoning cast iron is an oil high in omega-3 fatty acids – in
particular, alpha-linolenic acid (ALA). Free radicals are actually what
enable the polymerization. Drying oils, which produce the hardest
polymers, are characterized by high levels of polyunsaturated fatty
acids, especially the omega-3 fatty acid ALA.
The lard that was traditionally used for seasoning 100 years ago was
much higher in ALA than fat from pigs today, because back then pigs ate
their natural diet. Today they are raised on industrial feedlots and
forced to eat grain, making their fat low in omega-3s.
Since lard is traditional but no longer readily available, many
people substitute bacon drippings, but this is a bad idea. If it’s
conventional bacon, you’re baking in carcinogenic nitrates. But even
organic bacon is not good for an initial seasoning because it’s filled
with salt.
The reason that Pam seems to work well in seasoning is that its main
ingredient is canola oil, which is relatively high in ALA (10%), making
it a “semi-drying oil”. Flaxseed oil, a drying oil, is 57% ALA. But it’s
not a good idea to use a spray oil, no matter what oil it’s made with,
because of its additives. You’re doing chemistry here. If you want good
results, use pure ingredients.
Fat polymerization can be triggered or accelerated in a variety of
ways. As best I can tell from my reading, the cast iron seasoning
process is an example of “radical polymerization”. The process is
initiated when something causes the release of free radicals in the oil.
The free radicals then “crosslink” to form the tough, hard film you see
in a well-seasoned pan.
So what is the “something” that initiates the release of free
radicals in fat? Iron, for one thing. High heat, light, and oxygen, for
some others. To prevent cooking oils from going rancid – i.e., breaking
down and releasing free radicals – you need to store them in dark,
tightly sealed containers in a cool location. To initiate or accelerate
the release of free radicals, put the oil in contact with bare iron and
heat it above its smoke point, which will cause even non-drying oils to
release free radicals.
I haven’t defined “free radical” or “crosslink” because that gets
into details of chemistry that you don’t need to understand to season a
cast iron pan. All you need to know is that the molecular structure of
the oil changes and becomes something else, something tough and solid.
The process is initiated with the release of free radicals, which then
become crosslinked, creating a hard surface.
Free radicals are carcinogenic inside your body, and also a cause of
aging. So don’t ever heat oil you’re going to eat above its smoke point.
If the oil starts to smoke, toss it out and start again. When you’re
seasoning a pan, you’re not cooking food. By the time the seasoned pan
comes out of the oven, there are no more free radicals.
The Recipe for Perfect Cast Iron Seasoning
The basic idea is this: Smear a food-grade drying oil onto a cast
iron pan, and then bake it above the oil’s smoke point. This will
initiate the release of free radicals and polymerization. The more
drying the oil, the harder the polymer. So start with the right oil.
Go to your local health food store or organic grocery and buy a
bottle of flaxseed oil. It’s sold as an omega-3 supplement and it’s in
the refrigeration section because it goes rancid so easily. Check the
expiration date to make sure it’s not already rancid. Buy an organic
flaxseed oil. You don’t want to burn toxic chemicals into your cookware
to leach out forever more. It’s a fairly expensive oil. I paid $17 for a
17 ounce bottle of cold-pressed, unrefined, organic flaxseed oil. As it
says on the bottle, shake it before you use it.
Strip your pan down to the iron using the techniques I describe in my
popover post.
Heat the pan in a 200°F oven to be sure it’s bone dry and to open the
pores of the iron a little. Then put it on a paper towel, pour a little
flaxseed oil on it (don’t forget to shake the bottle), and rub the oil
all over the pan with your hands, making sure to get into every nook and
cranny. Your hands and the pan will be nice and oily.
Now rub it all off. Yup – all. All. Rub it off with paper towels or a
cotton cloth until it looks like there is nothing left on the surface.
There actually is oil left on the surface, it’s just very thin. The pan
should look dry, not glistening with oil. Put the pan upside down in a
cold oven. Most instructions say to put aluminum foil under it to catch
any drips, but if your oil coating is as thin as it should be, there
won’t be any drips.
Turn the oven to a baking temperature of 500°F (or as high as your
oven goes – mine only goes to 450°F) and let the pan preheat with the
oven. When it reaches temperature, set the timer for an hour. After an
hour, turn off the oven but do not open the oven door. Let it cool off
with the pan inside for two hours, at which point it’s cool enough to
handle.
The pan will come out of the oven a little darker, but matte in
texture – not the semi-gloss you’re aiming for. It needs more coats. In
fact, it needs at least six coats. So again rub on the oil, wipe it off,
put it in the cold oven, let it preheat, bake for an hour, and let it
cool in the oven for two hours. The picture above was taken after six
coats of seasoning. At that point it starts to develop a bit of a sheen
and the pan is ready for use.
If you try this, you will be tempted to use a thicker coat of oil to
speed up the process. Don’t do it. It just gets you an uneven surface –
or worse, baked on drips. Been there, done that. You can’t speed up the
process. If you try, you’ll mess up the pan and have to start over.
The reason for the very hot oven is to be sure the temperature is
above the oil’s smoke point, and to maximally accelerate the release of
free radicals. Unrefined flaxseed oil actually has the lowest smoke
point of any oil (see
this table). But the higher the temperature the more it will smoke, and that’s good for seasoning (though bad for eating –
do not let oils smoke during cooking).
I mentioned earlier there’s a myth floating around that vegetable
oils leave a sticky residue. If the pan comes out of the oven sticky,
the cause is one of three things:
- You put the oil on too thick.
- Your oven temperature was too low.
- Your baking time was too short.
It’s possible to use a suboptimal oil for seasoning, like Crisco or
bacon drippings, and still end up with a usable pan. Many (most) people
do this. But the seasoning will be relatively soft, not as nonstick, and
will tend to wear off. If you want the hardest, slickest seasoning
possible, use the right oil: flax seed oil.
