Question: what is it about BluTac that makes it so hard but not brittle?

RAL Cryogenics answered on 13 Jul 2015:

That’s tricky as blutac’s composition is a trade secret! However, we do know that it contains a sticky polymer (amongst other ingredients) which has similar properties to other polymers, or plastics. One of these properties is that when cooled below a certain temperature it becomes stiffer and harder. This temperature is called the “glass transition”. If you were to keep cooling the plastic it would get more and more stiff, and probably more and more brittle. So, for the “blutac nails” demo, it’s important not to cool the blutac too much. In fact what you probably want is a hard outer skin and a softer core. That’s exactly what you get when you drop it into liquid nitrogen.