Why do new pennies feel cooler than older pennies?

Consider that the new shiny penny has not been handled or touched much. And that old dark brown penny has been in people’s hands, their pockets, their purses. How much of that dark brown is oxidation, and how much is sweat, grease, and old skin cells? I imagine that would retain heat longer than clean metal.

New pennies have more zinc and less copper and weigh less. I would guess that something that weighs less will probably chill to room temperature quicker.

The composition of the penny changed in 1982 from almost all copper to almost all zinc, but chances are that most of the older looking pennies in your sample would also have been zinc alloy. Also, copper is much more thermally conductive than zinc, so a copper penny at 70 degrees should feel cooler to the touch than a zinc penny at 70 degrees, not vice versa.

The difference in your case is probably more about emissivity, the rate at which a material gives off absorbed heat in the form of infrared radiation. A polished metal surface has a much lower emissivity than a dull, oxidized surface of the same metal. Emissivity is rated from 0 to 1; polished copper has an emissivity of .03, while oxidized copper can be close to .90.

Any discoloration is corrosion, however slight and it acts as an insulation. So the transfer of heat to a new penny is faster than an old penny.

copper has more than triple the thermal conductivity of zinc. It’s likely that body heat from your hands warms the old pennies much faster than the new ones making them seem warmer.

old pennies are 95% copper 5% zinc as an alloy new pennies are copper plated zinc.