If you ran your finger over a metal ingot, or piece of metal that's been worked into a specific shape for storing, you might describe its surface as smooth to the touch. If you viewed the same piece under a microscope, you would see the truth -- that the metal's surface is riddled with nooks and crannies. When you bring two pieces of metal together, these imperfections and irregularities create channels along which a liquid can move. In brazing, that liquid is a molten filler metal, and the force that pulls it through the microscopic "pores" is something called capillary action.
You can create a model of capillary action by placing a length of glass tubing into a dish of water. When you do this, you'll see the liquid climb up the tube, higher than the water level in the dish. This occurs because water molecules are sticky: They like to stay close to each other, and they also like to stick to the surfaces of other materials. Scientists refer to the former phenomenon as cohesion, the latter as adhesion. When adhesion is stronger than cohesion, capillary action commences. In other words, the water molecules sticking to the surface of the glass tubing pull up on the water molecules below.
Capillary action depends on the diameter of the glass tubing. As the diameter decreases, the water column rises higher. As the diameter increases, the water column falls to a lower position. This happens because the attractive forces between water molecules exert a stronger pull over shorter distances.
Brazing also depends on capillary action, which means the metal being joined must remain solid so the filler metal, once liquefied, can pull itself along the closely fitting adjacent surfaces. To meet this requirement, the filler metal must have a melting temperature above 840 degrees Fahrenheit (450 degrees Celsius) but below the melting point of the metals being joined [source:Sulzer Metco]. The filler metal can be placed into the joint before heating or fed into the joint as it's heated. But in either situation, the temperature must remain in a certain range to promote good capillary action. A craftsman must also be mindful of the gap between the abutting metal pieces. Brazing works best if the joint clearance falls between 0.001 and 0.005 inches (0.0025 and 0.0127 centimeters) [source: Belohlav]. If the gap is narrower, the movement of the filler may be impeded. If it's wider, the capillary forces will be reduced and the resulting joint may not be as strong.
If you're thinking brazing sounds a lot like welding, you're right. Welding usually uses a filler to join two pieces of metal, but it requires a much higher temperature. In fact, welding actually fuses the two pieces of metal together by melting the "base" metals and, if it's used, the filler. Capillary action can't occur during welding because there is no solid surface along which the filler can move.