To save mixing my gin or vodka with ice and over-diluting them I pre-mix them and put them in my freezer. When I take them out they are a sludgy mixture of randomly aligned ice sheets which melt quite quickly once in a glass. How do these sheets form? I tend to make my martinis about 8 parts vodka or gin to 1 part dry vermouth.
鈥� Practically all ice is crystalline; the crystals grow as cold molecules packing onto their outer surfaces, rather like infinitesimal Lego blocks.
When water molecules fit together, certain joints are stronger than others, so the crystals grow faster in some directions than others. Molecules of pure water pack easily in almost any direction, so ice forms nondescript blocks unless freezing is slow. But foreign molecules, such as sugar or ethanol, interfere strongly with packing in certain directions, so needles and plates result.
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Such crystals also interfere with each other鈥檚 growth and create temperature and concentration gradients that favour alignment, leading to the packed crystals found in ice lollies for instance, which grow from the lolly stick that acts as a nucleation site.
Ice crystals tend to dissolve in the surrounding liquid, with melting happening largely at the crystal surface; arrays of thin crystals present a large surface, and consequently melt rapidly. This explains why the sheets in the martini melt quickly. The surrounding solution also lost water when the ice froze, increasing its concentration, and that accelerates melting, much as salt melts snow on roads. The result is a very cold martini, but drink it quickly; it will take up heat correspondingly rapidly.
Jon Richfield, Somerset West, South Africa