The basic thing to remember here is that Lead Came stretches. That really is the base fact in deciding when to use what.
1. Lamp Shades should always be done in foil. All those beautiful Tiffany Studio lamp shades were done with foil. He was the master so just do what he did! If you make a lamp with lead, over time it will collapse unless you reinforce the lead. That just adds to the work and to the weight.
2. Lead is heavy. If weight is an issue, use foil.
3. Items like jewelry boxes should be foiled. It is way easier to deal with the angles with foil.
4. When you have little pieces that will be lost in the lead, use foil. You can even mix. A lead came panel can have a section that is foiled. Most little projects are done with foil.
1. When you are making a project that will be in the weather, use came. Came can be puttied/cemented to make it more weather tight.
2. Geometrical patterns lend themselves nicely to lead came.
3. Really large piece, like Church window are nice with lead came because of the size. The boldness of the came fits the size. To get the bold width with foil would be difficult and probably irregular. Came stays nice and uniform.
4. Even though lead stretches, it takes time. Most Church windows do not need to be releaded for 100 years. When you see that a window is budging at the bottom, you can usually bet that it is old and the lead has expanded and contracted over time and the weight of the glass has been pushing down during this unseen activity. If you used foil on a window that is exposed to weather and weight, you may see stress cracks in the glass.