Copper foil is one method for holding the glass securely in place. Copper foil tape comes in a variety of widths, allowing for more or less "overhang," which translates into a deeper or shallower channel and appears in the final artwork as a thicker or thinner leadline.
Because each piece of glass is surrounded with copper foil separately, all of the copper must be covered with a "bead" of solder, i.e., the entire leadline is covered, front and back, with solder. If there are gaps between one piece of glass and the next (due to imprecise cutting of the glass), these gaps become filled with solder, too, resulting in leadlines that are thicker than where the glass pieces fit precisely next to one another. Too much variation is a sign of poor cutting of the glass and is especially detracting with geometric shapes, straight lines, etc.
Here is a list of what to look for in a well crafted piece of copper foiled stained glass.
1) The copper foil "leadlines" display a uniformity of width.
As per the discussion of the copper foil method above, for Tiffany technique need be precision cutting of the glass and precision laying down of the foil onto the glass. Straight lines should be perfectly straight and show almost no variance in leadline width... curves should be smoothly curved... circles should be perfectly circular, and the variance between the thinnest and the thickest leadlines should be minimal unless the artist has obviously chosen to create different widths for artistic effect.
2) The presence of very thin leadlines.
One of the biggest advantages of using copper foil over lead as a method of holding the glass is that with copper foil the artisan can create very thin leadlines.
3) A solder bead that is smooth.
This may not apply where the artist has meant to add some decorative soldering. However, beware of artisans claiming that poor soldering is "meant to be decorative"... once you've seen a good example of smooth soldering, you'll soon learn to tell the difference.
4) A uniform appearance of the patina (if one is applied).
Poor craftsmanship here appears as a blotchy or uneven look in the coloration of the leadlines.
5) There are no copper foil "ends" showing.
Since the copper foil tape is applied to the entire edge of each piece of glass, it must slightly overlap itself where it begins and ends its circumnavigation of each piece of glass. The absence of visible ends means that the copper foil is exactly lined up where this overlap occurs. The presence of visible ends is a sure sign of hurried craftsmanship. These can occur anywhere along the leadlines, as well as at the corners of a piece of glass.