Pistons are too weak to push obsidian with vanilla Minecraft. But not with Stronger Pistons! Stronger Pistons allows pistons (and sticky pistons) to push obsidian AND tile entities! That's right! Currently, the items drop out of the tile entity when it is pushed, but hopefully I can fix this in a later update. You can make a one-click ejecting furnace! Just attach a sticky piston to a furnace, then smelt your materials, and pull the lever when your furnace goes off! With this mod you can also make secret obsidian doors instead of the average cobblestone doors. Please read the README if you download.