Inserts are designed to enhance the operation and appearance of an existing wood burning fireplace, whether masonry or factory-built. Categorized primarily by the fuel burned for operation (natural gas, propane, EPA certified wood, pellet and coal), a fireplace insert is installed into an existing wood burning fireplace. Fireplace inserts are made from cast iron or steel and have self-cleaning glass doors that allow the dancing flames of the fire to be viewed while the insulated doors remain closed, making the fire more efficient. Many manufacturers also augment the operation of fireplace inserts by offering state-of-the-art features such as fans and thermostatic controls (depending on the fuel).