Do you know why copper mugs are commonly used to keep drinks cold? It’s the same reasons why copper bottom pots and pans are used in cooking. Copper is a metal that conducts heat extremely efficiently. When heat is applied to copper it spreads evenly throughout the copper surface very quickly and retains that heat. Similarly when cold is applied to the same surface it gets cold very quickly and retains that chill across the entire surface.
Metallic surfaces also feel colder to the lips when you drink out of them and when a cold beverage fills a mug halfway to the top the copper spreads that cold feeling all the way to the rim where you actually drink out of. Unlike glass mugs that only spread part of that chill to the rim copper will make your drink feel colder even when it really isn’t.
When you go to make a Moscow Mule remember that the copper mug the drink is served in doesn’t really insulate your drink but it does make it seem colder while you drink it meaning you need less ice to keep the drink feeling cold to the tongue and that means your drink is less likely to get watered down by melting ice.
You don’t have to drink your mules out of copper mugs but it is traditional and it does add a bit of style to the drink which is one of only a small set of drinks that can be identified across the room just by the cup is is being consumed out of.
One piece of advise I’d like to give all fans of this cocktail is that if you are planning on owning a set of your own copper Moscow Mule cups you should always hand wash the mugs otherwise the nickel lining may start failing early. You can read more about putting copper mugs in the dishwasher here.
You should also see this page for some best practices for cleaning copper kitchenware.