All-Clad D5 vs Copper Core-- the Physics

As someone who enjoys cooking and has experience as an engineer, I thought I might try to clarify some of the design theory that might cause one to choose a particular All-Clad collection over another.

It is helpful to consider the units used to measure different things as they will often help to clarify. I will try to avoid making this too geeky, so that if you only slept through class at the average rate you will be able to understand.

Thermal conductivity is the rate at which thermal energy moves through a material, all else being equal. The units of Watts per meter*Kelvin sound confusing, but it’s really just a fraction with power on top (in watts) and on bottom the material thickness or length times the temperature. To put some numbers to this, Aluminum is about 235. Stainless Steel (grade 304, the proper name for 18-8) is about 16.2.

It would be tempting to say that higher thermal conductivity is always better, right? But that’s simply not the case. Aluminum foil is very conductive because it’s so thin. But if you tried to cook something on hot foil on your hob, you’d be pretty frustrated. Why? Because cookware must not only distribute heat, it must store it as well. The ability to store energy is important, and a key attribute of Cast Iron cookware that has kept it popular for many decades, even though cast iron does not conduct heat that well at all. Our aluminum foil cools very quickly when we touch it (which is why even out of the oven it often won’t burn you.)

The problem with designing good cookware is that there is some degree of mutual exclusivity between conducting heat and storing it. If you think about it, those are the only two things a cookware item can do with heat applied to it. It can either conduct it to the thing it’s touching (another cookware layer, or the air perhaps) or it can store it internally.

This is where another material property comes in: specific heat. This is the amount of energy it takes to raise or lower a given mass (usually a gram) of a material by a certain temperature. Very high specific heat values store a lot of heat—and take a long time to heat up as a result. But they also take a long time to cool down—like cast iron. Specific heat is specified in Joules per gram- degree (Celsius). In other works how much energy (in Joules) flows in or out when a gram of that material changes temperature by a single degree Celsius.

I will also draw your attention to the fact that specific heat is listed by mass, not by volume. That means the specific heats are only comparable in cookware if the layer thicknesses are adjusted so that the overall mass of the layer is comparable. Consider these densities (note: water is 1.0) and the corrected Heat Capacity (see table above) based on assuming a constant layer thickness in the cookware:

We can put all of these variable together into something called thermal diffusivity—which is thermal conductivity divided by density and specific heat capacity. This is important because a material that stores more heat would take more energy to raise a given area in temperature, and will appear to have worse conductivity as a result, even if conductivity is the same. All the heat can either be stored (capacity) or passed along (conductivity). The more that’s stored, the less there is to pass along, and vice versa.

With a diffusivity of 23, cast iron is pretty low. At 111, copper is very high. Aluminum has a value of 88, and stainless steel is just 3.4

Water analogies are often used to help explain things in engineering, so I will make use of one here. The Heat capacity is like the total quantity of water that can be stored—like a tank size. The thermal conductivity is like the size of the inlets and outlets that would fill the tank and drain it. It tells us how quickly we can flow water into or out of the tank. (Heat from the hob to the pan, or from the pan to the food).

What differences should we forecast between the All-Clad D5 and Copper Core collections?

The only real construction difference is the middle layer material being stainless in the former and copper in the latter. If we assume they layers are equally thick, we can generalize about the performance difference between them.

The D5 should have marginally higher heat storage capacity, similarly to how cast iron does a great job a storing heat.

But the Copper Core will also store a good bit of heat, just less than the D5—about 85% as much capacity in the middle layer, which is only a small part of the total storage. Through the whole cookware item, the difference in heat capacity between them is tiny and not likely a practical significance.

The disadvantage of the D5 comes in how accessible that heat is to the food in the pan. If you sear a steak in the D5, the initial burst of heat flow comes only from the top stainless and secondary aluminum layers, as the middle stainless layer will act like a washed out bridge that isolates the lower layers from the top ones. The Copper Core will instead act like a solid bridge to connect the top layers to the bottom, making more heat available to the surface in a faster amount of time. The sear in the copper core will be harder as heat flows faster into the food.

This is a more significant difference than may first be apparent. Heat capacity itself cannot cook food. Your average backyard swimming pool stores many times more heat than is necessary to turn premium beef to char. But I cannot burn steaks by throwing them in the pool because the heat of the water is not accessible to the meat—the relative temperatures won’t allow it to flow.

The Copper Core will better deliver the entire heat capacity of the pan to the food surface in less time, and overall would be a superior performer. Is it enough to justify the price? It’s up to you.

I personally think the Copper Core would be worth considering for situations where heat response is super important. Skillets come to mind. I don’t think it would be worth it for a saucepan, stir-fry  pan or many daily use applications.

 The Ultimate Pan Design.

Since we are doing this analysis, we might as well ask what the ultimate pan design would be. I think you’d want to separate the layers by function.

The top layer that touches the food should be stainless for food resistance and durability. It should however be as thin as we think we can make it, as its thermal properties are undesirable. It’s basically just here for corrosion resistance and durability.

The layer underneath it should be heat-transfer layer of highly conductive material with good heat capacity. Copper is ideal here.

The middle layer is the main heat storage layer, this should be thick aluminum. It takes about half as much weight of Aluminum to store the same amount of energy as in a layer of copper, so using aluminum here allows a lighter pan to perform much like a heavier copper pan.

 Underneath that Aluminum core we’d want a distribution layer of silver. Silver has both excellent conductivity and low heat storage capacity. That means that much of the heat that goes into it shows up as higher temperature and it excels therefore in feeding heat to the main aluminum core.

Finally, outside this thin silver layer we would use a thin layer of magnetic stainless for induction compatibility.

A pan of this design would optimize the performance for a given weight, and also demonstrate superior thermal response because the layers underneath the main heat reservoir don’t store much heat themselves, they just pass the heat from the source to the core.

So from top to bottom, here’s our “ideal” pan: Super-thin stainless, medium copper, thick aluminum, thin silver, super-thin magnetic Stainless. It wouldn't cost as much as you might think, as the expensive Silver isn't thick, and the copper is reduced to a single layer of copper.