Perhaps you have already noticed this with a new pan: The oil runs from the middle to the edge of the pan, especially with non-stick pans. Or the cold pan wobbles a little on the glass ceramic hob surface (ceramic or induction cookers). Or the base seems to be not completely flat.
Quite a few customers think that these are signs of a manufacturing or material defect. Therefore, customer questions regarding these phenomenons are among the most frequently asked questions.
The reason for what was described at the beginning is the concave shape of the cookware bottom.
Many customers think that a cookware base must be absolutely 100% flat. This is not true, because physics stands in the way: metal expands when heated, as is known. Everyone has seen old tracks running in serpentine lines under the glistening sun of Australia or Africa. The metal of the track expands and if there is no possibility of further expansion in the longitudinal direction, said serpentines are formed.
Well, a pan that gets a totally wavy bottom when heated strongly would be extremely impractical. The bottom would warp convexly, like a bowl and permanent deformations would render the cookware unusable.
The base of pots, pans and roasters is thus given a shape that anticipates the expansion in the opposite direction when heated, so that the cookware is still firmly and securely on the stove even during heavy frying. This is achieved by a concave bottom shape. This form is designed for a maximum operating temperature, so that a certain concave, hollow curvature can remain during normal frying.
This, of course, has the disadvantage, especially with low-fat frying, that the oil can drain noticeably to the edge. Depending on how pronounced the concave shape of the bottom is this can never be completely prevented and must be tolerated.
Both for the magnetic waves of an induction cooktop and for the heat radiation of a radiant heat stove (ceramic / halogen type cooktop), the deviations from perfect flatness that are technically necessary for cookware and the resulting minimal empty spaces between the cooktop and the cookware do not generally pose a problem in terms of hindering functionality.
It should be noted that the thermal expansion of the base and, as a result, the change in the hollow curvature varies according to the type and size of cooker and, last but not least, the base construction of the cookware. For example, in the case of induction-suitable aluminium and cast aluminium cookware, there may be a tendency to an increase in the hollow curvature during heating. Why is that? Well, aluminum is an excellent heat conductor, making it ideal for cookware, but unfortunately it is not ferromagnetic and thus not suitabe for induction cookers. To overcome that and to make aluminum cookware usable on induction cooktops, a plate of ferromagnetic stainless steel is applied to the base of the aluminum cookware. This is done by impacting or modling of a punched stainless steel disc on or in the aluminum body, or, the technologically more sophisticated and more expensive variety, by welding on a full disc stainless steel disc (Full Disc Induction), very rarely also by plasma spraying technology.
However, the two metals of the base (body made of aluminum and base plate made of stainless steel) do expand to different degrees during heating because they have different coefficients of thermal expansion. Consequently, the aluminum expands more than the stainless steel does. But since both metals are connected for good heat transfer, the aluminum basically expands "over" the stainless steel plate that limits its expansion. That causes an increase of the concave curvature and it can let the oil run to the edge more when the pan is hot than when the pan is cold. This effect is purely physical and is under no circumstance a claimable material defect!
But since this effect only occurs with aluminum cookware that is always coated anyway (either a non-stick coating or ceramic coating), and the oil used is therefore usually filled sparingly and not covering the bottom anyway, the bimetal effect does not usually play a significant role for the frying success. Simply turn the food in the oil in the pan so that it is covered all around, and you will have no problems frying. If you are frying something in the pan that requires a closed oil film (deep-frying schnitzels etc.), you will have to add a little more oil as an exception.
If in doubt, choose a model with a particularly thick base (6-8 mm), as this effect will be less noticeable here. Of course such a thick cookware is also noticeably heavier and takes longer to heat up, but even with cookware the following applies: All good things never go together....
If you just cannot accept the inherent bimetal-effect of all-hob-capable / induction-suitable aluminum cookware, choose a stainless steel or cast iron cookware instead.
And if you do not have an induction cooktop and also do not plan to get one, you can simply choose an aluminum cookware that is not induction-suitable, so one that does not have a stainless steel plate on the base, e.g. the Granitica Extra series or Gran Sasso Plus by Barazzoni or the Granito Hard Stone series by Risolì.
Often we have already heard such and similar sentences when explaining the topic of the concave cookware base. Be assured, however, that no modern cookware is 100% flat. The reason why some cookware seems completely flat to the layman at first glance is simply due to the fact that there are two typical courses of the hollow curvature of concavely shaped bottoms: concentric and longitudinal courses.
In case of a concentric course, the base of the cookware lies flat all around and without wobbling; the holow space (YELLOW) in the center between the hob surface and the base of the cookware is not visible.
In case of a longitudinal course, there are usually two edges along a longitudinal axis of the bottom sitting on the hob. A slight wobbling of the cold cookware is particularly noticeable on flat and expansion-neutral glass ceramic surfaces. The slight wobbling usually subsides during heating.
Both types of concave shape are common in industry. Price and brand of the cookware per se are no indication for one way or the other. However, the hollow curvature of a thick base can often be concealed better by clever shaping and is therefore less noticeable.
Things get particularly difficult with cookware that is not round, e.g. oval or rectangular. Since the size of the cooking zone rarely corresponds exactly to the size of the base of such cookware, the thermal expansion in the base of the cookware is correspondingly uneven; this can lead to considerable tension in the structure of the base. The course of the concave shape usually runs longitudinally here and is slightly larger so that the bottom does warp like a bowl when frying on high heat.
Yet still, especially with large oval or rectangular cookware, whether made of cast iron or of aluminum, whether it is suitable for induction or not, whether it may be inexpensive or price-intensive, there will always be a certain amount of unevenness, not only when cold but also at operating temperature, which is of course escpecially noticeable on an expansion-neutral glass ceramic surface.
Please adapt your approach to cooking to account for that and please take special care to heat rectangular / ovale cookware slowly and gradually so that the heat can be better distributed throughout the cookware.
Please note that the presence of a proper concave bottom shape proper does not constitute a defect and therefore we do not accept this as a reason for a complaint.
For customers placing extra value on cookware where the bottom's concave shape is as small as possible and hardly noticeable we recommend round cast aluminium or (especially for induction) stainless steel cookware with a very thick base (> 5 mm).
If you have any further questions in this regard, please contact us if possible before purchasing in order to obtain advice on product selection, as the type and design of the concave bottom shape are in no way dependent on the brand or price of the cookware, but only on model-dependent material and technical production factors.
If the cookware item has already been delivered, please check, if necessary, BEFORE using the cookware, whether the bottom curvature of the item is okay for you. Carefully remove the product packaging so that if you do not like the item, you can return it without any possible deductions as part of the revocation.