Why Isn't Tin Good at Keeping Soup Warm?

Tin, or more accurately, tinplate (thin steel coated with tin), isn't very good at keeping soup warm for a few reasons:

Low thermal conductivity: Tinplate is a relatively poor conductor of heat. This means it doesn't transfer heat very efficiently, so the heat from the soup dissipates quickly.
Thin material: The thinness of tinplate means it has a small thermal mass. This means it doesn't hold much heat itself, so it can't effectively keep the soup warm for long.
Lack of insulation: Tinplate cans don't usually have any insulation, which would help to slow down heat loss.

Here's a Comparison

Metal like stainless steel or aluminum are better conductors of heat, which is why they're often used in thermal containers.
Ceramic or glass are even better at retaining heat, especially with a vacuum between the inner and outer layers.

In summary, while tinplate is great for storing and preserving food, its properties don't make it a good choice for keeping soup warm for extended periods.