True. The science did somewhat break down... that's the problem with trying to math and logic at the end of a long holiday.
If I'm understanding properly, since soapstone has a higher density and specific heat than water, the ice will have a max cooling of (80+temperature difference between ice and liquid) multiplied by the amount of ice, so 16 in our example. With the 45 degree difference, ice would be (80+45)*16=2000 or 20 degrees. Soapstone should do (1.12*2.65*45)*16=2,136.96 or 21.3 degrees.
That means it takes 41 degrees F of temperature difference between liquid and cubes before soapstone gets better than ice. Which should still happen, but... extra degree down, 6 instead of 5.
Okay... found this as the top result for "do soapstone ice cubes work?" and so even though it's a necro, figured I'd respond to make this (at least appear) a bit more accurate.
Trying to sort out the FlyFish math for granite, using the Wolfram Alpha information at http://www.wolframalpha.com/input/?i=granite, we find that granite has a density of 2.6 g/cc and specific heat of .28 joules per gram degree Celsius. So, we multiply our total volume of 16 by specific heat and density... so 16*.28*2.6=11.648 as our total. Supposing 35 degrees of temperature difference, we're looking at 407.68 giving a total temperature drop of 4 degrees before the granite cubes become the same temperature as our water.
But that wasn't the question... the question was about soapstone ice cubes. Using Wolfram Alpha again, from http://www.wolframalpha.com/input/?i=..., we find that soapstone has a density of 2.65 g/cc and specific heat of 1.12 joules per gram degree Celsius. Using the same math we just did, we multiply 16*1.12*2.65=47.488 as our total. Again supposing 35 degrees of temperature difference, we're looking at 1,662.08 giving a total temperature drop of 16.6 degrees before the soapstone cubes become the same temperature as the water.
Let's summarize. If our 1 CI ice cube actually does give 18.4 degrees of cooling, this makes a fully melted ice cube at equivalent temperature about 11% better than soapstone cubes for cooling. That being said, it waters the drink and the ice cube always starts at 32 degrees F or 0 degrees C. Our sandstone cube may, depending on the freezer, get lower, giving it more degrees of cooling possible. If your freezer cools to 22 degrees F, instead of 35 degrees of temperature difference we're looking at 45 degrees on the soapstone, giving us 2,136.96 calories for 21.4 degrees before the cubes become the same as water, and making the stone cubes actually better. In fact, at anything 5 degrees Fahrenheit or greater below freezing, the soapstone will cool more than ice.
Simple math. Right?
Anyways, summary time: