On the rocks – how to cool Whiskey adequately

Today’s post is about Whiskey. And Whisky. More precisely, about how to cool it adequately as a geoscientist. I received a great Christmas gift from Andreas – ice cubes made up of “Nordic Rocks”. The manufacturer promises a perfect way of cooling drinks by adding one to three pieces of 400 million year old swedish bedrock. No thinning of the drink, no pollution. Firstly, I have to mention that I do not want to judge if cooling whiskey or whisky (in the following simply referred to as “whiskey”) is adequate at all. I know that some people would never put anything else than pure whiskey in their tumblers and nosing glasses (in the following simply referred to as “glas”). Others do sometimes mix their whiskey with 30-50% of water (20°C) as long as it is the same water that was used for the destillation. I heard people saying that whiskey only tastes good on the rocks. I heard people saying that they enjoy their whiskey with water from the tap. I heard people saying that they only drink whiskey with coke. I even heard people saying they didn’t drink whiskey at all. The following considerations are for those who can imagine to cool their whiskey. I prefer it pure in most cases.

Loch Lomond with two pieces of Nordic Rocks.

Let’s start with a self-experiment. I put two “Nordic Rocks” in a glas (4cl) of Loch Lomond. Loch Lomond is a good one for this experiment – tasty, but not too expensive. Just for the case that the experiment goes wrong. The whiskey cooled down slowly. I did not measure the exact time series, but it felt like it cooled significantly slower than with ice cubes. The minimum temperature did by far not reach the values known from regular ice-cooling. My whiskey was neither thinned nor did I observe any pollution by the rocks. It definitely looked much nicer than any ice-cube-containing whiskey glas.

Let’s do a reality check now. The manufacturer says: “Those tiny rocks keep your drink cool longer than regular ice cubes. Nordic rock is a 400 million years old type of rock. It coms from the mountains of Sweden where nature still is clean, far away from any pollution. Nordic rocks need to be put in the fridge for at least one hour. For cooling your drink, put 1-3 rocks into your glas carefully. The cold will then be transferred to your drink slowly. Your drink won’t be thinned, because Nordic Rocks do not melt. […]” (own translation).

The german description of the product.

While at the first glance the rocks looked like granite pieces, they turned out to be soapstone (soap rock, steatite, german: Speckstein). So I guess there is the chance that they can pollute your drink if you stir to hard (hardness: 1). I don’t want to judge on the statement that they come from a non-polluted area, since I have no idea where they are mined. There is a stealite deposit in Linnajavri, that’s all I know. Anyone any idea? The next thing is the term “cold”. That’s what they wrote and what will not be accepted by geoscientists. There is no “cold”. There is only “heat” that will be transferred from your drink to the rocks, heating them. The decrease in whiskey temperature is because the whiskey’s heat goes to the colder rocks until both have the same temperature.

Nordic rocks - they look nice, it's a great idea - but why didn't they cool my whiskey well?

So why didn’t they cool my whiskey more effectively than ice as the company promised? Let’s have a look at the physical properties. First, heat conductivity. This property describes how fast heat is transported in a medium, it is given in watts per meter kelvin. Ice has a value of ~2 W/(m*K), soapstone of 6.4 W/(m*K). So, in soapstone heat can more easily be transported from its surface to its interior. This should help with cooling things fast. The feeling that my whiskey cooled down more slowly must have been wrong. Physics say so. Next: specific heat capacity. This property describes how much heat is stored in a certain amount (mass, volume, or mol) of a material. It is given in joule per gramm kelvin. Ice has a specific heat capacity of 2.11 J/(gK), soapstone of 0.98 J/(gK). My rocks are cubic and have an edge length of 1.5 cm. With a density of 2.98 g/cm³, they can store 749.0826 J

[3.375 cm³ x 2.98 g/cm³ x 38 K (-18°C to 20°C) x 0.98 J/(gK) x two pieces = 749.0826 J]

until they have room temperature. If I take ice cubes with the same dimensions, the heat necessary to bring them to 20°C is more difficult to be calculated. First, I heat my ice from -18 to 0°C:

3.375 cm³ x 0.918 g/cm³ x 18 K x 2.11 J/(gK) x two pieces = 235.34307 J.

Then, I need to melt my ice! This is what will not happen to the Nordic Rocks in normal use. For melting ice, I need a specific melting heat (or Enthalpy of fusion) of 334 J/g.

3.375 cm³ X 0.9162 g/cm³ x 334 J/g x two pieces = 2065.5729 J.

Then I can heat the water to 20°C:

3.375 g x 4.1813 J/(gK) x 20 K x two pieces = 564.4755 J.

So, in total I can store 235.34307 J + 2065.5729 J + 564.4755 J = 2865.39147 J of heat in my ice cubes until they melt and reach 20°C. This is ~ 3.8 times the amount of heat to be stored in the Nordic Rocks. That’s why they do not cool as effective as ice.

Of course I made only rough calculations, not taking into account warming/cooling of the glas and so on, but: Sorry, Nordic Rocks. They look nicer (much nicer!) and they do not thin your whiskey, but they do not cool that effectively. Decide on your own, which is more important to you. However, Andi, this was a great gift! Thanks so much.

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