Before artificial freezers, lake/river ice was transported all over the world (including to the tropics) in large blocks. Sufficiently large blocks of ice melt slowly, compared to small blocks, because their surface area (where melting occurs) is smaller in proportion to their volume. Storing ice over summer has been going on for centuries. In Persia (Iran) ice-storing structures still remain. Yakhchals were a type of evaporative cooler, and ice was brought down from the mountains in winter and could be stored over summer, even in that climate.
Lake-ice is clear ice. The ice we have in our freezer compartments is cloudy. Why is freezer-ice cloudy? Mostly, it is trapped air. Liquid water contains dissolved air. Water freezes in ice-cube trays from the outside in. As ice takes on its crystalline form on the outside, it forces dissolved air out, and this air migrates further into the centre of the cube where the water is still liquid. Eventually, this water freezes too, but now the air has nowhere left to go (it can’t get out through the frozen exterior) so it is incorporated into the ice structure in tiny bubbles that diffract light and make the ice cloudy.
As the ice freezes it expands, but to begin with it can expand evenly and the ice-cube holds its shape. However, the water in the centre freezes last and has nowhere to expand to, placing pressure on the ice surface that bulges it out. That’s a modern ice cube — cloudy and deformed. This frustrates cocktail makers looking for perfection.
Why does ice expand on freezing? The H2O molecule has a ‘V’ shape with the oxygen in the middle and hydrogens on the two arms. The oxygen end is slightly negatively charged, and the hydrogen ends correspondingly positively charged (making the whole molecule neutral). When water is above 4C, the thermal energy easily overcomes any electrical repulsion or attraction between water molecules, and they intermingle closely. As the water cools below 4C and freezes, the electrical poles want to line up (positive to negative), and the v-shape means that under normal conditions the ice takes on a hexagonal form like a beehive. This introduces extra empty space into the structure, meaning the ice has expanded and is less dense than water. The hexagonal structure is the most common, however scientists now know of at least 14 other forms that ice can take, for example it can be cubic at high altitudes.
Why is lake ice clear? For the same reason that ice cubes tend to be clearer near their surface — the air was forced out during ice formation. Because lake ice freezes from the top down, there is a continual process of forcing air out of the crystalising ice and into the underlying water. Harvesting the surface ice yields clear ice.
Why is ice slippery — why can people skate? Urban legend says the pressure under the skates lowers the freezing point and melts a thin sheen of water to skate on. However, the effect of pressure is trivial, it lowers the freezing point by 0.02C for an average skater. A more likely candidate is friction-heating, however this does not explain why ice is slippery when standing still. The actual explanation may be that ice is slippery because it is slippery by nature. While water molecules bond to each other tightly when in solid ice, on the surface they experience ice-like bonds on one side and are exposed to air in the other. This weakens bonding and puts them in a quasi-fluid state. This is not melted ice per se — it exists even if air temperature is below zero. As far as I know, the slipperiness of ice is still and open question.
Cloudy ice due to dissolved gas is a clathrate hydrate. In areas of decomposing vegetation (bottom of lakes, deep ocean sediments, peat bogs etc) the predominant gas is methane (CH4) and methane clathrates form. There are enormous methane clathrate deposits in the permafrost, particularly in the continental shelf of northern Siberia. As a greenhouse gas, methane is about 30 times more potent than carbon dioxide (actually about 100 times, but it has a shorter half-life in the atmosphere). If the permafrost melts, the methane bound in the clathrate can escape, at some point triggering the feared tipping-point in global heating: more methane > temperature rise > more melting > runaway heating. There are signs the process has begun.
It is thought that this process last occurred during the end-Permian extinction, an event ~250 million years ago which saw the extinction of up to 95% of all species (and known as the Great Dying). At that time, global temperature was increased by 6C. Current models indicate that 6C is quite achievable by the end of this century if we continue with business as usual. Rather than be worried, some see receding ice fields as a fossil-fuel mining opportunity. Instead of scrambling to do something about it, nations are scrambling to claim sovereignty.
Sometimes I wonder if the term ‘homo sapiens’ isn’t an oxymoron. Or at least, a vanity. No surprise that we named ourselves.
But, since this is a cooking blog, is there anything to learn? The lake-ice suggests two possibilities — how to make clear ice-cubes at home and how to serve food on thin ice.
Dave Arnold describes how to make clear ice. Part-freeze water in a small esky (cooler). The water will freeze from the top down (because the sides and bottom are insulated) and be clear. Stop the process when about half the water is frozen, remove the ice block and cut into perfect cubes of the desired size.
The master-innovator, Ferran Adria, was intrigued by the fact that we like crunching ice as we eat. He started incorporating ice into aspects of his dishes, for example as small cubes (ice croutons) in a salad — providing an unexpected textural and temperature contrast.
He also refined lake-ice as a dish. Placing a bowl of water in the freezer, he waited until the surface froze over. Then removing it, he would pierce the surface with two holes on opposite edges and pour out the underlying unfrozen water. It was now a thin ice sheet suspended across the top of the bowl with nothing beneath. The surface of the pond was dusted with flavourings (e.g. mint essence, green tea powder and sugar). The diner cracks off large shards and enjoys the flavour of the topping with the icy and brittle shards. Ferran was also interested in the aromas carried to the diner by the water vapour trapped under the ice sheet. There’s a video here.