Getting an induction base truly flat

Gas does not care much about the shape of your pan's base. A flame heats what it touches and what it surrounds. Induction is different, and it exposed a manufacturing problem we had not previously had to solve.

Why flatness matters so much here

Induction heats by inducing eddy currents in the base of the vessel, and the coupling between coil and vessel falls off sharply with distance.

A base that is convex by even a small amount sits on its centre with a gap at the edges. The consequences:

• Reduced power transfer. The hob works harder for less heat.
• Uneven heating, concentrated where contact is closest.
• Detection problems. Some commercial hobs will not recognise a poorly seated vessel.
• Rocking, which is a safety issue with a full pan.

The two problems

Cold flatness is a machining question and it is comparatively straightforward: grind the base flat and check it.

Hot flatness is the real problem. When a vessel heats, the base expands. If the base heats faster than the wall — which it does, since the heat arrives there first — it expands against a cooler ring of metal that constrains it, and the base bows.

So a vessel can be perfectly flat cold and convex at working temperature, which is precisely when flatness matters.

What we worked on

• Base thickness. Thicker plate resists bowing but takes longer to heat and costs more. Our induction kadai settled at 3 mm as the point where the base is stiff enough without becoming sluggish.
• The transition from base to wall. The radius where the flat base turns up into the side determines how much the wall constrains base expansion. Getting that geometry right does more than adding thickness.
• Grinding after forming. The base is ground flat after the body is formed, not before, so any distortion introduced by forming is removed rather than built in.
• Stress relief. Formed metal carries residual stress from the forming operation. Left in, it relaxes unevenly the first time the vessel gets hot, and the base moves.

Why the shape changed

A traditional kadai has a rounded bottom, and that shape is correct for a gas ring — the flame wraps around the curve and heat enters over a large area.

On induction, a rounded bottom is close to useless: contact is limited to a small central patch. So the induction kadai has a genuinely flat ground base while keeping the bellied profile above it that makes a kadai useful for frying.

That is a compromise, and it is worth being clear about it: the induction kadai is slightly less good on gas than a traditional round-bottomed one. It is designed for a hob.

Made to size

Because commercial induction hobs vary in coil diameter, base size should match the hob. The induction kadai is made to the size you specify rather than to a fixed catalogue — tell us your hob and we will match the base to it.