Ice cubes

Ah, ice cubes. Such a simple ingredient. Every cocktail need it, every home cocktail makers curses at the quality of the stuff coming out of their freezer door. Industry has sprung up to deal with the problem, giving people the ability to make ice cubes in every shape, size and density they could ever want. Machines have been developed to compress ice cubes into spheres, grate ice cubes into snow, and chop them into jewel-like shapes. All for one reason...

Dilution. Ice cubes not only chill the alcohol in the shaker tin, but also dilute the drink, which can open up aromatics, and bring the flavor and mouthfeel of alcohol in line with the flavors of the cocktail. While this brief article does not go into great detail about the science behind ice for cocktail usage, people who are interested should start here, but to sum up the article, here are some points about ice:

• Most ice is 0 degrees Celsius, which is fine -- colder than 0 degrees ice doesn't help that much
• Ice at 0 degrees can chill a cocktail to below 0 degrees
• There is a direct relationship between chilling and dilution of a cocktail
• It takes 82 calories to change a gram of ice into a gram of water at 0 degrees, but 1 calorie to move it from 0 to 1 degree. 
• Shaking a cocktail gets it to 0 degrees in 15 or so seconds -- stirring can take 60-75 seconds, depending on stirring speed
• The size of ice used matters quite a bit for stirred cocktails -- larger ice dilutes less but chills less. 
• There is no optimal percentage of dilution -- it varies based on palate fatigue, other drinks consumed, and other factors

Probably most importantly of those bullet points is point 4. Ice has a lot of chilling power, and using lots of good quality ice can make the difference between a properly diluted cocktail and something overdiluted.

Most home icemakers make what is called shell ice, due to its shape and how it's formed (rectangular shapes that are hollowed out). Shell ice shatters quickly with shaking and will shatter moderately with stirring, which can overdilute cocktails. Better is commercial ice (like the stuff found at the grocery store), which makes solid, medium-sized cubes that are resistant to shattering.

There are a few companies that make specialized ice machines (Kold Draft being the best known) who offer different sizes of high quality, dense ice. Some cocktail bars (especially those in Japan) carve ice out of blocks, sometimes into incredibly fancy shapes.