Minecraft wiki entry for Flint_and_Steel, associated text: When used on top of a solid, fully opaque block or on the sides of a flammable block, the flint and steel set a fire on the surface on which it was used. Flint and steel have 64 uses. If used on a transparent block (such as glass) or an underwater block, it does not start a fire but still consumes durability. Durability is also consumed if it is used on an already existing fire.