In a previous article, we compared common cookware materials along important criteria. Those criteria included thermal properties, health safety, weight, cost and design. In this article, we will explain the concept behind the thermal properties of cookware. The most important factors that we need to understand are thermal conductivity and heat capacity.
Fast and even heating is important for a good cooking experience. We don’t want to wait too long until the pan is hot. We also want our onions to simmer at the same temperature no matter where they are in the pan.
Thermal diffusivity is the variable that tells us how fast temperature transfers through the cookware material and spreads out across the pan. Therefore, it tells us how even and how fast our cookware heats up and cools down. If two fry pans had the same thickness, the pan with the higher thermal diffusivity would heat up faster and more evenly.
Thermal diffusivity is a trade-off between heat capacity and thermal conductivity. This trade-off is important to understand.
Thermal conductivity is a variable that determines how fast a material absorbs and releases energy. However, we are interested in how fast a material spreads temperature across the pan. In order to determine how fast temperature moves across a material, we need to look at both, thermal conductivity and heat capacity. This becomes clear when we imagine an empty bucket which we want to fill with water. The water represents the heat. The rate at which we pour the water into the bucket represents the conductivity. The size of the bucket is represents the heat capacity. As a result, the rate at which the water flows (fast = highly conductive) is equally as important as the the size of the bucket (capacity) in order to determine how fast the temperature moves across the material.
Therefore, a highly conductive material tends to show good thermal diffusivity (good heat transfer and distribution). The exceptionally good conductivity of copper allows it to lead the diffusivity table even though it has a relatively high heat capacity.
Excursion: Heat conduction is the transfer of heat by particles colliding with each other. Metals are good heat conductors because particles are located close to one another. In addition, metal atoms possess free delocalized electrons which can transfer heat from one end of the material to the other end very quickly (they can sort of jump the line). Here is a great video that explains this principle in a little bit more depth and helps to understand why metals are such great heat conductors.
Heat capacity is a function of specific heat multiplied by density. As a result, heat capacity (bucket size) describes how much energy can be stored in a material. In other words, a large bucket takes longer to fill but also longer to empty compared to a small bucket.
If we compare cookware with similar conductivity, pans made from material with high heat capacity will show lower thermal diffusivity. This means that temperature will take longer to move away from the heat source towards the rims. In other words, they will take longer to heat up and to cool down. As a consequence, there will be heat spots in the warming up period. However, once hot, they will stay hot.
In everyday cooking situations, this means that under the above condition of similar conductivity, the rim of the pan with high heat capactiy might still be cold while you are burning your onions in the middle of the pan. In addition, you lose control: imagine you are doing a sauce that should be on point. If you make this sauce in a pan with high heat capacity, it will continue to cook even though you have taken the pan off the heat source.
On the other hand, in case you plan to drop a 4lbs chunk of cold minced meat into the pan, a pan with high heat capacity, once heated up, will not cool down as quickly. In addition, serving food in a pan with high heat capacity means that the food will be kept warmer for a longer period of time.
Copper seems to offer the best of both worlds. It has the highest conductivity while having a capacity similar to stainless steel or iron. Due to its excellent conductivity, its still leading the diffusivity (heat transfer) table. In other words, copper’s bucket is huge but the rate at which water is flowing in and out is even higher. This allows it to have high control and even heating while not cooling down as quickly as aluminum when cold food is dropped into the pan.
Specific heat describes the heat capacity of a material for a given mass. It expresses the amount of heat, in Joules, that is required to raise a specific mass (usually kg) by 1 Kelvin (which is a temperature scale such as Fahrenheit or Celsius).
The density of a material is the amount of mass per volume. This explains why specific heat and density together determine how much energy is stored inside a pan’s material. Specific heat is the capacity per mass and density is the mass per volume.
Density determines the weight of the cookware too (mass per volume). Next to heat capacity and conductivity, weight is an important criterion to consider when buying everyday cookware. However, keep in mind that in order to achieve the same capacity of two materials with similar specific heat, the material with higher density can also store more heat per volume (because for the same volume, it has more mass). Hence, a denser material can be thinner (lighter) and still achieve the same capacity.
We also need to consider food reactivity of different materials when we evaluate cookware. Highly conductive materials (especially copper and aluminum, but also iron) are the ones that are most reactive to acidic and alkaline foods e.g. lemon juice or tomatoes. This means that the food takes up chemical elements from the cookware and can take on a metallic flavor. While iron can be processed quite easily by the body, copper and aluminum cookware is often coated or wrapped into layers of stainless steel. This helps to eliminate food exposure.
Practical explanation based on a comparison of copper and aluminum
We argue that a good pan is characterized by a high thermal diffusivity while having high heat capacity. A pan that has both characteristics has a fast and more even heat transfer. At the same time, it is able to store a lot of heat in the pan. This means that it will not cool out as quickly when cold food is placed inside of it.
Let’s compare copper and aluminum with regards to these two characteristics. While copper conducts temperature significantly better than aluminum, they differ in their specific heat and density (heat capacity). In order to best compare both materials, we will show their performance in two different scenarios.
Let’s imagine we had 2 pans, both consisting of 100% aluminum and 100% copper, respectively. Both pans have a diameter of approximately 10 inch (25.4cm).
Scenario 1: Both pans have the same thickness. In this case 1mm (0.039 inch).
- Copper conducts the temperature 20% better. Therefore, it offers better heat distribution and a shorter warming-up and cooling-down period
- Copper stores appr. 60% more heat (heat capacity). Even though it cools down more quickly due to significantly higher conductivity, it releases more heat while doing so.
- Copper is about 3 times as heavy
Scenario 2: Both pans have the same heat capacity
- In order to achieve the same heat capacity for both pans, the aluminum pan needs to increase its heat capacity. This can be achieved only by adding additional mass to the aluminum pan. If we increase the mass of the aluminum pan by roughly 41% to a 1.41mm thickness, it has the same heat capacity as the 1mm copper pan.
- The increased thickness requires the heat to travel a longer way. Therefore we need to adjust the heat diffusivity by this factor. In our example, the advantage of copper to distribute heat faster and more evenly increases from 20% to roughly 50%. In other words, the copper pan achieves the same temperature than the aluminum pan in nearly half the time.
We have prepared the numbers underlying these calculations here.
The article explained the principles underlying different cookware materials. This allowed us to understand and discuss their pros and cons solely based on their physical properties.
When we compare two materials with the same conductivity, the material with the higher heat capacity will show slower heat transfer and less even heat distribution. Comparing two materials with the same heat capacity, the material with higher conductivity will heat up faster and more evenly. These relationships are a logical consequence of the mathematical equation shown above: increasing heat capacity (divisor) reduces the quotient (diffusivity) and vice versa.
In summary, heat diffusivity is a critical variable. It describes how fast heat will move across the material while taking into account its weight. Copper is leading the table by 20%, followed by aluminum. However solely looking at heat diffusivity does not reveal all the properties of a certain material.
In a direct comparison between copper and aluminum, copper has a significantly better heat distribution and more “power” in terms of heat capacity at the expense of higher weight. While copper cools down faster than aluminum, it releases more heat while doing so. Therefore it offers fast and even heat distribution, high control and more power in terms of total energy stored. Due to its superior heat conductivity, it is also highly sought after and therefore expensive. Both copper and aluminum are not safe to use for cooking which is why they are usually used in cladded or coated constructions where they are not directly exposed to the food.
Less diffusive materials have their place too. While iron, stainless steel and carbon steel pans take significantly longer to heat up and heat up less evenly (depending on the thickness), they stay hot for a long time.