Material Editor > Reset Map/Mtl to Default Settings

Reset Map/Mtl to Default Settings resets the values for the map or material in the active sample slot.

The material colors are removed and set to shades of gray. Glossiness, opacity, and so on are reset to their default values. Maps assigned to the material are removed.

If you are at a map level, this button resets the map to default values.

Reset changes the name only when this field names a material used in the scene.