Lava furnaces are functional and easy to create in survival or creative. They can be used as incinerators or for starting massive fires without flint and steel.
Creating Lava Furnaces is fairly simple. You need:
A non flammable block (to build the furnace)
A lava bucket
Flammable Blocks (optional)
First to create a lava furnace you must build a chamber made of your non flammable blocks. If you want to start a fire put your flammable blocks in a tower form in the chamber. Then you can optionally build pipes leading to the chamber. Pour lava from the pipes or directly into the chamber.