A circular, usually cone-shaped, metallic 'cap' that is soldered to the top of a stained glass lamp shade and used to connect the shade to the lamp base or hanging apparatus, a vase cap fits exactly into the hole at the top of the stained glass shade. They often have decorative holes cut into them that also vent the heat that builds up in a lamp shade, and a hole in the centre that is used to attach cap to the lamp base.
Brush flux on the vase cap, and melt a thin layer of solder onto the vase cap with the soldering iron. You can either apply solder to the entire piece or just to the area that will be connected to the lamp with solder. This is called 'tinning' the vase cap.
Set the vase cap into place on the top of the shade and make sure it is level.
Brush a dab of flux to a couple of spots along the seam where the vase cap connects to the top of the lamp shade and use the soldering iron to melt a drop or two of solder where the flux was applied. This will hold the vase cap in place while the rest of the solder seam is applied.
Brush flux onto the entire seam where the vase cap connects to the lamp shade. Melt solder onto this seam with the soldering iron. Build the line of solder up so that it forms a nicely rounded bead around the edge of the vase cap.
To solidify the lamp shade seam, repeat these steps on the inside of the shade where the vase cap connects with the top of the lamp shade.
The seam will take the weight of the entire lamp shade, so make sure the solder is strong.
Make sure the work area is well ventilated so you do not breathe hazardous fumes from the solder, and do not touch your mouth or face when working with lead.