I worked at Corning Inc at the time, a division, Erwin Ceramics Plant. The material is actually a ceramic tile with a foam like interior. Very lightweight. About the size of a bar of soap. You can hold it in your hand at one end and get the other end red to almost white hot and not fall the heat.
We are referring to the foam insulation on the center fuel tank. i think you are talking about the heat shield tiles on the bottom of the orbiter.