Blocks old, unused thimble holes in masonry chimneys. Provides a thermal break from combustibles and prevents downdrafts from entering the room. Constructed of a ceramic based fireproof insulating material. The black outside plate covers up to a 10" diameter hole. Installs easily, simply insert the plug into the old thimble opening and tighten the expansion screw on the face of the plug. The side expand and create a tight fit inside the thimble opening. To remove simply reverse the process.