When you braze copper sheet metal, you are using a filler rod to adhere the two pieces of copper together. The most common filler rod used is Stay Silv silver solder. This filler rod is around 15 per cent silver, 5 per cent phosphorous and the balance is copper. The silver acts to lower the melting point of the filler rod so that it melts before the copper sheet metal does. Silver solder is great for joining copper because you do not need to wire brush the metal to clean it or apply any flux like you would with regular soldering.
Join the two pieces of copper sheet metal tightly, using metal clamps if necessary to hold the metal together.
Turn on acetylene torch and hold it so the blue part of the flame is touching the area for brazing.
Allow the copper sheet metal to heat up until it just begins to turn cherry red.
Take silver solder rod and slowly brush it across the seam of the two copper pieces. It should begin to melt and the silver solder will flow between the two pieces of copper through capillary action. Apply enough silver solder to thoroughly seal the seam but not so much that it drips down your metal.
Move the torch to the next section of copper you want to join and repeat the above brazing process.
Turn the torch off when finished and remove any clamps you have on your work. The sheet metal should now be firmly joined.
Thin copper sheet metal can go from cherry red to melted in a matter of seconds. Take caution so you don't accidentally melt a hole through your work.
Brazed copper sheet metal is extremely hot. Allow to cool for several minutes before handling.