pressed down hard and rubbed quickly against the groove of the second piece in a "plowing" motion, to produce hot dust which creates an ember. A split is often made down the length of the grooved piece, so that oxygen can flow freely to the coal/ember. A fire-saw is a method by which a piece of wood is sawed through a notch in a second piece or pieces to generate friction. The tinder may be placed between two slats of wood with the third piece or "saw" drawn over them above the tinder so as to catch a coal, but there is more than one configuration. A fire-thong uses a non-melting cord, ratan, or flexible strip of wood to 'saw' the wood creating friction. On the board, opposite side the cord, is a well with a hole through the board to gathered the charred, soon-to-smoke, wood dust. The Rudiger roll friction fire method, also known as the "fire roll" method, is believed to have been invented by World War 2 POWs. A German survival expert named Rüdiger Nehberg wrote about this method in one of his books. A small amount of wood ash is rolled up in a piece of cotton like a cigar. The cotton is then placed between two boards and rolled back and forth. Pressure and speed are both gradually increased. With proper technique ignition can occur in seconds. Percussion A fire striker or firesteel when hit by a hard, glassy stone such as quartz, jasper, agate or flint cleaves small, hot, oxidizing metal particles that can ignite tinder. The steel should be high carbon, non-alloyed, and hardened. Similarly, two pieces of iron pyrite or marcasite when struck together can create sparks. The use of flint in particular became the most common method of producing flames in pre-industrial societies (see also fire striker). Travelers up to the late 19th century would often use self-contained kits known as tinderboxes to start fires. Some fire-starting systems use a ferrocerium rod and a hard scraper to create hot sparks by manually scratching the ferro rod with a knife or sharp object to ignite man-made or natural tinder. Fire starters based upon ferrocerium are popular with bushcraft hobbyists and survivalists. Similar sparking devices have a built-in striking blade which provides an easy method for sparking with one hand. Another common type has the ferro rod attached to a magnesium bar that