A Moscow mule is three things: vodka, ginger beer and lime. Nothing on that list has anything to do with copper. The mug came out of a deal struck in New York in the 1940s between a bartender and a woman left with a warehouse of unsold copper mugs. If it were only a marketing trick it would not have lasted eighty years. The drink genuinely behaves differently in metal.
Why the mug frosts over
Copper conducts heat roughly four hundred times better than glass. The moment ice and cold ginger beer go in, the cold spreads through the whole wall; the outside drops to five or six degrees within seconds and the moisture in the room condenses on it. The mug goes white from the outside. Pour the same drink into a glass and only the inner face cools, while the outside stays at room temperature.
The same conductivity works the other way round. When your hand wraps the mug, the heat of your palm does not pool in one spot, it spreads across the body. On a glass you can find your fingers by the warm patches. On copper you cannot.
The aroma
In a mule the real work is done by ginger and lime peel. A copper mug has a narrow mouth, a rim rolled outward and no transparent surface anywhere. Drop a sprig of mint in and the oils gather under that rim, so you meet them the moment your nose comes near. In a wide tumbler the same aroma disperses much faster.
450 to 500 millilitres
A standard mule mug holds 450 to 500 ml, roughly 16 ounces. The recipe explains it: 50 to 60 ml of vodka, 120 to 150 ml of ginger beer, the juice of half a lime and ice to the top. Without a full charge of ice the drink falls apart. Buy a 300 ml mug and you end up cutting either the ice or the ginger beer.
The inside must be tinned
A mule is acidic; lime juice and ginger beer together bring it to around pH 3. When acid meets bare copper, copper ions dissolve off the surface, the drink picks up a metallic edge, and that is not something anyone should be drinking. This is why the interior is lined with tin. The outside stays copper, every benefit above is kept, and the surface in contact with the drink is neutral. Every mug we make is tinned inside. Avoid the cheap unlined ones.
Keep hot drinks out of it
With heat the same conductivity turns against you. Pour 80 degree tea into a copper mug and the whole body, handle included, reaches burning temperature in seconds. You will not be able to pick it up. A copper mug is a cold drink vessel.
Looking after it
Never put it in the dishwasher; the alkaline detergent takes the shine off the outside and eats the tin inside. Rinse it in warm water after use and dry it straight away, and do not let lime juice sit in it overnight. If the outside darkens, rub it with half a lime dipped in salt, then rinse and dry.
