From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to cool drinks.
Both are frozen, but when ice is placed in a drink it slowly melts, using energy from the drink, thus cooling it.
But whisky stones don't melt, so how do they cool the drink?
Ice cubes have three distinct cooling effects:
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:
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 ?
But what business have you torturing perfectly fine Whisky with unnatural coolness ?
$\endgroup$
A fundamental principle of thermodynamics is that heat flows from warm places to cold ones, through either convection, conduction or radiation, and it will continue to do so until the temperature equalizes across the system.
The stones are colder than the whiskey when you put them in the glass, so as the system heads towards equilibrium, the whiskey gets colder while the stones get warmer.
Assuming no environmental influence, the equilibrium temperature will be somewhere between the two initial temperatures (the whiskey and the stones).
At what point on that scale the equilibrium temperature is exactly depends upon;
The time taken to reach equilibrium also depends on the thermal conductivity of the stones.
TL;DR: Whiskey stones work by absorbing heat from the whiskey in an attempt to reach thermal equilibrium1.
As Thomas mentioned, ice has three cooling effects:
Note that specific heat works both ways; the same number applies for an increase of temperature/inflow of heat and a decrease of temperature/loss of heat
When you chill a whiskey the normal way, ice absorbs heat in different ways, cooling the whiskey (which itself has a specific heat capacity of 3.4 kJ/kgK) till the system is at the same temperature. Usually, what happens is this 2: The whole of the ice reaches 0 Celsius, and the whiskey is significantly cooled. Once this happens, the ice melts and drops the temperature of whiskey significantly. Finally, the ice(now water) heats up a bit.
Now, looking at soapstone (what they use in whiskey stones): Soapstone is thrice as dense as ice, but only half as effective in absorbing heat (specific heat capacity 0.9). So it will be a 50% more effective than the same number of ice cubes during the "reach 0 degrees" phase. After this, ice wins by a huge margin -- the same back-of-the-envelope calculation (where I take the same volume of soapstone, giving a 3:10 ratio for mass) gives me 28C as the final temperature. Of course, this can easily be remedied by using more whiskey rocks (I think that my estimate for the 1:10 ratio in the ice case may be wrong though -- I don't drink and I don't know how much ice typically goes into a glass of whiskey).
Note that another important factor is thermal conductivity. Ice has a thermal conductivity of 2.18 W/(m·K) and water has 0.56 W/(m·K), while soapstone has 6.4 W/(m·K), approximately thrice as much as ice.
I guess that it takes a while for the ice to reach the 0C state uniformly. Instead, just the outer layer will be at 0C, and there will be a drastic temperature gradient on going inwards. The outer layer will slowly melt (this is a negligible effect since it's a very thin layer that is melting3). On the other hand, soapstone is able to absorb heat at a faster pace (and it is able to absorb more heat than non-melting ice). Which may partially balance out the drastic differences mentioned above -- after all, one doesn't wait for thermal equilibrium to be reached before drinking a whiskey. So the faster chilling from soapstone may level the playing field here -- as long as ice isn't allowed to melt in bulk, it ought to be fine.
Conclusion:
As far as chilling effects go, ice cubes are more effective in the long run, but whiskey stones will initially chill faster than ice. And of course, the stones have the additional benefit of not diluting your drink.
1. Thermal equilibrium is when there is no longer any net heat exchange between components of a system. This usually occurs when all the temperatures are equal.
2. Assuming that the mass of ice in a glass of whiskey is a tenth of the mass of whiskey itself, and that ice started off at -15C and whiskey at 30C, we need 3.4*30*10=1020 units of energy to bring whiskey down to 0C. We get 2.11*15=31 units from the warming up of ice, and we have a potential 334 more from the melting. Clearly, this is not enough, so the water formed must also heat up above 0 degrees C. A back-of-the-envelope calculation gives me 17 degrees as the final temperature for this mix.
3. I may verify this later; not in the mood to solve differential equations now :)
Whiskey stones aren't necessarily designed to keep the drink cold, instead they are designed to allow flavor profiles to come out in the drink that might not be present at room temperature. Some whiskeys open up at a slightly cooler temperature and using stones allows you to experiment without diluting the flavor of the beverage. There are better math answers already so I won't go into that.
As lynks explained, all that matters is the temperatures of the whiskey and the stones. Additionally,
The short answer is they don't cool nearly as well as ice
The harvard food and science team did an experiment for this exact scenario. The results were:
As the ice melts it drops the temperature of Whiskey to nearly -4C, where as the whiskey stones barely got the drink close to zero.
The reason has already been explained nicely in Manishearth's answer.
