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I'm not trying to be dismissive of the minds behind the 25% figure. Just pointing out that it is at best an average. Or as Dave the Cook suggests, a goal with certain recipes. 

The Cooking Issues link (Thanks Mitch!) shows much of what D.A. talks about in his book. 

A few takeaways:

• Size and shape of the ice cubes matter, but not in the ways that many people assume. More/smaller ice cubes chill faster, but not more. They also tend to dilute more, but for a specific reason: bar ice is wet (it's always hanging out at 0°C) and more surface area means there's more water clinging to it. That water adds to dilution without contributing to chilling. Dave recommends shaking the water off of wet ice using a strainer.
• Temperature of the ice is irrelevant. Virtually all of ice's chilling power comes from its latent heat of fusion (the energy required to melt it). Making it 10 or 20 degrees colder will have little effect. Except, curiously ...
• Ice colder than 0°C will chill your drink more slowly than 0°C ice. Because science! It will take a bit of time to warm to 0°, during which it does some very inefficient chilling. Only when it hits 0 and starts to melt does the powerful chilling begin. However ...
• Chilling is asymptotic—you'll eventually reach a fixed minimum temperature when shaking or stirring.
• But stirred drinks chill slowly enough that ice cubes matter. Because no one stirs long enough to hit that asymptotic low temperature with big ice cubes. To hit the temperature you want consistently when stirring, you need to be consistent about ice cube size and shape, stirring speed, and time.
• The nuances of shaking technique might have an effect on drink texture, but they don't affect the temperature or dilution—provided you shake long enough ... but not much too long. But we were talking about Martinis, and Sean Connery is dead. 
• Anything that adds significant heat to the drink from the outside will cause extra melting and dilution. This includes a hot ambient temperature, spending way too much time stirring / shaking / hanging out, or using an unchilled vessel that has a lot of thermal mass, like a classic stirring glass. 

I'm not trying to be dismissive of the minds behind the 25% figure. Just pointing out that it is at best an average. Or as Dave the Cook suggests, a goal with certain recipes. 

The Cooking Issues link (Thanks Mitch!) shows much of what D.A. talks about in his book. 

A few takeaways:

• Size and shape of the ice cubes matter, but not in the ways that many people assume. More/smaller ice cubes chill faster, but not more. They also tend to dilute more, but for a specific reason: bar ice is wet (it's always hanging out at 0°C) and more surface area means there's more water clinging to it. That water adds to dilution without contributing to chilling. Dave recommends shaking the water off of wet ice using a strainer.
• Temperature of the ice is irrelevant. Virtually all of ice's chilling power comes from its latent heat of fusion (the energy required to melt it). Making it 10 or 20 degrees colder will have little effect. Except, curiously ...
• Ice colder than 0°C will chill your drink more slowly than 0°C ice. Because science! It will take a bit of time to warm to 0°, during which it does some very inefficient chilling. Only when it hits 0 and starts to melt does the powerful chilling begin. However ...
• Speed of chilling isn't that important, provided you shake / stir long enough. As Dave's graphs show, the chilling is asymptotic, and you'll reach the same temperature (more or less) in a reasonable amount of time.
• Anything that adds significant heat to the drink from the outside will cause more melting and dilution. This includes a hot ambient temperature, spending way too much time stirring / shaking / hanging out, or using an unchilled vessel that has a lot of thermal mass, like a classic stirring glass. 
• All the nuances of shaking technique might have an effect on drink texture, but they don't affect the temperature or dilution—provided you shake long enough ... but not much too long. But we were talking about Martinis, and Sean Connery is dead. 

I'm not trying to be dismissive of the minds behind the 25% figure. Just pointing out that it is at best an average. Or as Dave the Cook suggests, a goal with certain recipes. 

The Cooking Issues link (Thanks Mitch!) shows much of what D.A. talks about in his book. 

A few takeaways:

• Size and shape of the ice cubes matter, but not in the ways that many people assume. More/smaller ice cubes chill faster, but not more. They also tend to dilute more, but for a specific reason: bar ice is wet (it's always hanging at 0°C) and more surface area means there's more water clinging to the surface. That water adds to dilution without contributing to chilling. He recommends shaking the water off of wet ice using a strainer.
• Temperature of the ice is irrelevant. Virtually all of ice's chilling power comes from its latent heat of fusion (the energy required to melt it). Making it 10 or 20 degrees colder will have little effect. Except, curiously ...
• Ice colder than 0°C will chill your drink more slowly than 0°C ice. Because science! It will take a bit of time to warm to 0°, during which it does some very inefficient chilling. Only when it hits 0 and starts to melt does the powerful chilling begin. However ...
• Speed of chilling isn't that important, provided you shake / stir long enough. As Dave's graphs show, the chilling is asymptotic, and you'll reach the same temperature (more or less) in a reasonable amount of time.
• Anything that adds significant heat to the drink from the outside will cause more melting and dilution. This includes a hot ambient temperature, spending way too much time stirring / shaking / hanging out, or using an unchilled vessel that has a lot of thermal mass, like a classic stirring glass. 
• All the nuances of shaking technique might have an effect on drink texture, but they don't affect the temperature or dilution—provided you shake long enough ... but not much too long. But we were talking about Martinis, and Sean Connery is dead.