Warped

Dude, what happened to my pan?

A fat-free dome, rising out of a moat of melted butter

It’s morning. While you’re making coffee you pull out the egg pan, lob a dollop of butter in it, dial in some heat and start cracking the eggs. As you’re finishing with the whisk you hear the sizzle of the butter subside, and then a gentle ping. Turning to the stove you see your pan has birthed a fat-free dome, rising out of a moat of melted butter. A moment ago it was perfectly flat.

In addition to how such a thing happens to a slab of solid metal, you wonder if the deli has any sesame bagels left…

Why Things Warp

Repeated heating and cooling changes the hardness of any metal. Warping is essentially the differential expansion/contraction of a metal’s crystalline structure, meaning one part of a pan subtly changes its shape more often or at a different rate from another, such as when a pan base gets hotter than its walls.

Another factor is how walls act as both practical and structural containment for a pan.

As a practical matter, pan walls keep food contained over heat. Structurally, a solid ring (i.e., the wall of a circular pan) contributes to rigidity. This is why thin-walled pans frequently have a hardened steel bead rolled under the top edge.

The base of your pan cannot expand horizontally beyond the ring, so the only direction it has to go on heat is up or down. More often than not, straight-sided pans will “cup” downward (concave), while slope-sided pans tend to “crown” upward (convex). This has to do with the inclination of crystalline stresses at the “knee” or the curve where the base ascends into the wall.

Thin pans are generally much more likely to warp than thickly constructed ones because the more material there is within its thickness the more a metal can differentially expand and contract. Types of metal play a big role too, as does their method of manufacture (e.g., stamped pans, owing to stretching at the knee, warp much more readily than spun pans). Given enough use, soft metals (such as copper and aluminum) almost always warp if thinly constructed. Harder, more resistant metals, such as carbon or stainless steel, resist warping but will still do so under the right circumstances. They are concomitantly much more difficult to true (i.e., flatten).

Generally, if a pan has food in it before heating it’s protected from warping by never getting hotter than the food within it (nominally about the boiling point of water).

Different metals, owing to their ingrained thermal resistance, will store or discharge heat more or less quickly (e.g., copper moves energy at a quick 400 kilowatts per meter per second, while most stainless steel clocks ~16kW/m). With food in a pan acting as a heat sink, energy can slowly and uniformly harden the metal and, with time, the pan becomes less likely to warp.

Hardening and Annealing

Hardening, or “toughening” happens when a (usually ferrous) metal is heated and cooled rapidly, which seizes its crystalline structure in a relatively open lattice of large grains, or when it is cold-worked, which through pressure and motion deform fine metallic crystals into long, continuous grains. Heat-tempering steel requires controlled rapid heating and cooling, while spinning or hammering copper into hardened shapes exemplify cold-working.

Warping happens when an isolated part (usually the base) of a pan is heated to the point of expansion and then either constrained or cooled quickly, seizing the metal in its new, expanded configuration.

Toughened C11000 Copper, courtesy of www.copper.org

For a given thickness, metals that bank a lot of energy (i.e., move it slowly) are much more inclined to warp if they’ve not been hardened in manufacturing, or by judicious use. A classic way to lose true on a relatively new pan is to heat it empty, sear some foodstuff, and then plunge it into water. Even heavy stainless can warp under such circumstances (see photo above).

The opposite of hardening is annealing, which softens metal, making it more pliable and plastic. As in the case of hardening a lot of heat is involved, but when annealing the metal is cooled slowly rather than seized by quenching in water, allowing finer crystal grains to form (“recrystallization”) and enhancing the metal’s plasticity. Metallic crystals slide against one another (“creep”) in their boundaries, so the more boundaries there are, the more motile crystals can be, resulting in more ductile metal. Copper enjoys even greater advantages here, as its crystalline structure is highly regular (“non-corrugated”), so grains creep with very little friction. Hardened or annealed, copper is very resilient.

Annealed C11000 Copper, courtesy of www.copper.org

Truing a Warped Pan

Annealing means hope for a warped, higher quality pan. If you care to try anything I describe below, you do so, of course, at your own risk, but with care and patience, you have a chance to restore a favorite pan’s true.

Whether you should true your pan or not depends mostly on the value of (or your attachment to) the pan. Even if a pan clears the worthiness bar to re-true it takes a little doing.

I hate to say any pan should be junked, but if it was low-quality junk to begin with, warping is only one of the ways it’s fated to disappoint you.

Better pans start more “true” than others in many regards and reward not only proper use, but extra effort given to their repair when damaged.

You may be aware of folks who have tried to true a warped pan by striking it with a hammer. While this may give the appearance of straightening a hardened curve, what it is in fact doing is opening artificial boundaries within large (toughened) crystal grains, increasing granular complexity at the expense of crystalline integrity. These artificial boundaries are corrugated, and friction between adjacent crystals weakens the metal’s integrity, otherwise known as metal fatigue. Not only will the pan likely re-warp soon, the metal will be permanently damaged.

Weight + Heat + Time

Provided your pan is sufficiently valuable to you to merit the effort, “crowned” pans can be trued by heating with a heavy, metallic weight in the pan.

A preferred method is the use of flat iron gym weights, with the diameter of the weight in the bottom of the pan covering as much surface as possible. Heating the pan with the weight in it for an extended period and letting it cool slowly where it sits on the stove top can true a warped pan base.

A restaurant chef I know here in NYC (and from whom I first learned of this trick) needed 200 pounds and over two hours of steady low heat to flatten a heavy four-burner cast iron griddle. On the first pass the griddle relaxed suddenly and two of the four weight stacks were disrupted. Fortunately, they tipped only onto the range top, but the exercise had to be rerun since the griddle did not anneal (cool slowly) under steady weight. The second pass worked quickly and thoroughly as the griddle was already partly annealed and close to true from the first effort. After a little re-seasoning the griddle was flat and good as new (however, I’m told the kitchen did smell like a metal shop for some time after). This is similar to the technique our smiths use in the BCC retinning and restoration program.

Crowning most readily befalls skillets and fry surfaces owing to their mode of use and their curved walls. Fortunately these are relatively easy to stack with weight. The thinner the metal, the faster the effect, which you can check by simply looking at the pan base during the process (be careful!) to see if it’s settled down. Once the pan is flat, simply turn off the heat and let everything cool slowly. If at first you don’t succeed, try again.

For concave, or cupped, pans the same basic technique works, except the weight(s) rest on the wall of the pan at the upper rim, rather than in the bottom of the pan. This applies pressure downward at the outer margin of the base, which will gently work cupping back to level. Because there is no weight resting on the surface over heat, there is no mass into which to sink the energy, so the pan bottom will heat much faster. Be sure to go low and slow with your heat, and don’t over-weight the pan wall — remember, you’re softening metal. Too much time on heat or weight on wall will cause the wall to soften and “belly” outward.

Some additional caveats are called for here, such as if your pan is non-stick (Teflon and many so called “ceramic” linings contain polymer bases) it’s not a candidate for repair since heating it empty not only out-gasses toxic compounds, but the lining will be irreparably damaged. If your pan is tinned copper, the tin lining will be likewise damaged, but fortunately it’s completely reparable, and a copper pan body is probably valuable enough to merit the effort. Enameled linings will often “dinnerplate” (fracture) under weighted heat, while “sandwiched” or bi-metal pans can delaminate.

Stainless over copper beginning to delaminate in the “knee”.

Of course, the iron weights themselves must be unpainted / uncoated.

As the total weight required varies by pan (thicker pan, more weight), due care should be taken in balancing lots of hot metal — no inverted pyramids! It goes without saying that your range top should be as flat (and level) as possible. Also, use only a gas range, which will distribute energy load out and up the stack sides until the metals come up to temperature. Electric ranges might work, but there’s risk to the coil elements from concentrating heat beneath the stack before it begins to flow uniformly. No matter what metal you need to true (including ferrous), induction ranges will not work for this technique.

Bear in mind, too, that this method is applicable only to warped, not bent, metal. Bending weakens a metal’s crystalline structure, while warping does not affect metal integrity, just its shape (see “metal fatigue” above).

If you incline toward heavy-bottomed cookware you’re already less likely to need a pan trued because of the robust nature of the pan’s construction (and because you’re likely the kind of person who is thoughtful about good tools and their proper use).

Apart from pure metal construction, a key unseen indicator of high-quality are the many stresses such pans are put through just in their making; the resilience that results is one of the attributes they bring to your table.

Heavy pans stay true.

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How to use and care for pure metal cookware.

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