This is what the inside of a Coffee Joulie looks like. The white stuff is PCM or “phase change material” which is a complicated way of saying.. 'something that melts.' The silver is the stainless steel shell, it's cut in half to show the inside. When Joulies are heated to over 140f (60C), the white stuff turns into a clear runny liquid.
The point of Joulies is to get a material with a high heat of fusion into your coffee cup, and substantially improve the temperature profile of your coffee. We've worked very hard to design the most efficient and effective device to achieve this simple goal, and given the constraints of material selection, design, and usability, we're extremely happy with the result. I mean, look at it, jam packed with sweet temperature regulating PCM!