Put every Glass and Crystal into it to collect a suply of Glass Flasks (for Water Orb/ Orbs and Purified Water/Food).
But if you struggle in early and mid game to upgrade to Improved Furnaces, then better stay with standard Furnaces and just build more of them.
You can use a Furnace, an Improved Furnace and Improved Furnace (Kiln).This Guide gives you an overview over Furnaces and their use.
