It sounds as if your mug is made from Pyrex or some similar low expansion glass. Experiment suggests that if I pour boiling water at 100C into a similar mug in my kitchen (around 15C first thing in the morning) the abrupt 85C temperature change doesn't shatter it. Given this, the 45C difference between room temperature and -25C isn't likely to cause problems, especially since the cooling to -25C will be slow.
You might want to avoid putting the mug down on a very cold surface (which would cause more rapid cooling) but I doubt if even this would have much effect.
Ice cubes have three distinct cooling effects:
- The cube, initially at sub-zero temperature, absorbs some heat to reach fusion point (0⁰C).
- The cube absorbs more heat to switch phase: it takes some energy to turn 1 kg of ice at 0⁰C into 1 kg of liquid water at 0⁰C.
- The water absorbs some heat to become warmer than 0⁰C.
The three effects occur more or less successively, although not necessarily simultaneously throughout the ice cube. But the idea remain the same.
For ice, the bulk of the cooling comes from the melting. Let's put some figures on it. Heat capacity of ice is 2.06 kJ·kg-1·K-1, meaning it takes 2.06 kJ to transform 1 kg of ice at -12⁰C into 1 kg of ice at -11⁰C. For liquid water, that's 4.217 kJ·kg-1·K-1. The latent heat, i.e. energy used for turning ice into liquid water (at constant temperature) is 333 kJ·kg-1. Imagine that you have some beverage at room temperature, which you want to lower to 8⁰C with ice cubes. The ice cubes come from the freezer and are initially at -18⁰C. The three cooling effects amount to, per kg of ice:
- Raising ice temperature to 0⁰C: 18×2.06 = 37.08 kJ.
- Melting the ice: 333 kJ.
- Raising the water temperature to 8⁰C: 8×4.217 = 33.736 kJ.
So, in this example, the melting contributes to about 82% of the cooling.
Non-melting stones work only on heat capacity. So they are effective only insofar as a material with high heat capacity is used -- but, in practice, water (and ice) have a quite high heat capacity, higher than stones, so the cooling effect of such stones is necessarily quite reduced compared to ice cubes. On the other hand, since there is no dilution effect, you can put a lot of stones in your glass.
Reusable ice cubes are actually much better at cooling things, because they do melt -- but they do so internally, in a sealed envelope, thus not spilling into your drink. Since they use latent heat of phase transitions, they are as good as true ice cubes. Although they do lack in aesthetics.
But what business have you torturing perfectly fine Whisky with unnatural coolness ?
Best Answer
The copper will heat up the drink more quickly, but it will also cool your hand and lips better than the cold liquid itself. Perhaps cool hands and lips is the feeling drinkers experience and favor. You could do simple experiment - put equal amount of drink and ice into copper and ordinary glass, and find out in which one the ice melts down first.