We often get asked why the solder at the hardware store cannot be used in stained glass. The solder stained glass stores sell is different than common solder you might find in your garage or at the hardware store. Normally that solder is rosin core or acid core solder. If you use these you will end up with a scum mess.
You want solder that is 1/8" in diameter. Most are a mixture of tin and lead. The mix is designated by two numbers. The first number always refers to the % of tin, the second is the % of lead. The most used solders in stained glass are 60/40 (60% tin/40% lead) 50/50 and 63/37. "Lead-free" solders have no number designation and are a mix of tin and small amounts of other metals. Also look for "pure" solders--free of impurities. You will spend less but you will get scum as you solder and ruin your iron tip. It also interferes with patina.
60/40 melts at 374 degrees and becomes solid when it cools to 361 degrees. That means you have a working range of 13 degrees. This is your best choice for copper foil work. The liquid temperature and narrow working range make it easy to form and maintain consistent high, rounded, beaded seams.
50/50 melts at 421 degrees and is solid at 361 so has a working range of 60 degrees. This solder will produce a much flatter bead than 60/40. Because of its higher melting point, 50/50 is often used on the back (or inside) of a stained glass project to protect against "melt through" when soldering the front. Because it spreads and flattens out, 50/50 is often used when soldering lead joints.
63/37. Melts at 361 degrees and becomes solid at 361. No working range! This solder is also known as decorative or quick set solder. It is great to bead up the ourside rim of copper foiled pieces. It is used to create dimensional effects in the solder itself and can be manipulated to produce a vaiety of textures and designs.
Lead-Free melts and cools depending on the mix of metals. It performs similar to 50/50. This should be used for pieces that will contact food, that will be handled frequently or will be where a child could touch it.