Bonding copper to steel: the development story

Putting copper on the base of a wide vessel is an obvious idea. Copper conducts heat more than twenty times better than steel, and a wide vessel over a single burner has an obvious heat-distribution problem.

Making the join survive commercial service is the difficult part.

Why the bond is hard

Copper and stainless steel have different coefficients of thermal expansion. Heat them together and they try to grow by different amounts.

Constrained by being bonded, that mismatch turns into shear stress right at the interface — every time the vessel heats, and again every time it cools. In a commercial kitchen that is hundreds of cycles a year.

A poor bond shows up as:

• Separation. A gap opens between copper and steel. Because heat now has to cross an air gap, the copper stops working — while still looking perfectly fine from outside.
• Blistering. Localised lifting, usually starting at the edge.
• Edge peeling. The rim of the copper lifts and then catches on things.

The failure that worries us most is the first, because it is invisible. The vessel looks right and cooks badly, and the cook blames the burner.

What the work involved

The variables that had to be settled:

• Surface preparation. Both surfaces must be genuinely clean and correctly textured. Oxide or oil at the interface is where separation starts.
• Copper thickness. Too thin and there is not enough conductive cross-section to spread heat. Too thick and the expansion mismatch generates more stress than the bond can carry, besides adding weight and cost.
• Coverage area. The copper needs to extend well past the burner ring to do its job, but stopping it short of the vessel's edge protects the bond edge from knocks.
• Edge treatment. How the copper terminates decides whether peeling can start.

Getting these right took building variants and running them in kitchens, because a bench test does not reproduce a burner, a scrubbing brush and two years of daily use.

Why the body is hammered

The copper-base vessels are hand-hammered, and it reads as decoration. It is structural.

A wide, shallow panel of metal wants to flex and drum. Hammering work-hardens the surface and introduces a texture that stiffens it, so the vessel holds its shape under heat and load. A vessel that flexes puts cyclic stress straight into the copper bond — so the hammering protects the bond as well as the shape.

Where we apply it, and where we do not

Only three products carry copper: the biryani degda, the lagan and the mahi tawa.

All three are wide, shallow, and used over a single burner — the exact geometry where steel alone leaves a hot centre and a cool rim.

We do not put copper on the patila range, because a tall vessel full of liquid distributes heat by convection and does not need it. Adding copper there would add cost and another bonded interface for no gain. Declining to do something is also a development decision.

Living with it

The copper will darken with use. That is oxidation and it has no effect on conductivity. If you want it bright, tamarind or lemon with salt will restore it — but there is no functional reason to bother.