Hardened Glass is an item from the Thermal Expansion modpack. You can make it with the Tinkers' Construct Smeltery. It is similar to glass except highly blast resistant. If mined, it breaks like glass, not dropping an item, unless a silk touch pick or buildcraft wrench/crescent hammer is used.
Hardened Glass is made by putting lead in the Smeltery, and then pouring that over an Obsidian block you have placed yourself in a Casting Basin. It is used to make the Redstone Energy cell, the Redstone energy conduit, and liquiducts.